The Student's Elements of Geology
by
Sir Charles Lyell
Part 4 out of 14
(FIGURE 88. Succinea elongata.)
Although this loam of the Rhine is unsolidified, it usually terminates where it
has been undermined by running water in a vertical cliff, from the face of which
shells of terrestrial, fresh-water and amphibious mollusks project in relief.
These shells do not imply the permanent sojourn of a body of fresh water on the
spot, for the most aquatic of them, the Succinea, inhabits marshes and wet
grassy meadows. The Succinea elongata (or S. oblongata), Figure 88, is very
characteristic both of the loess of the Rhine and of some other European river-
loams.
(FIGURE 89. Pupa muscorum (Linn.).)
(FIGURE 90. Helix hispida (Linn.) (plebeia).)
Among the land-shells of the Rhenish loess, Helix hispida, Figure 90, and Pupa
muscorum, Figure 89, are very common. Both the terrestrial and aquatic shells
are of most fragile and delicate structure, and yet they are almost invariably
perfect and uninjured. They must have been broken to pieces had they been swept
along by a violent inundation. Even the colour of some of the land-shells, as
that of Helix nemoralis, is occasionally preserved.
In parts of the valley of the Rhine, between Bingen and Basle, the fluviatile
loam or loess now under consideration is several hundred feet thick, and
contains here and there throughout that thickness land and amphibious shells. As
it is seen in masses fringing both sides of the great plain, and as occasionally
remnants of it occur in the centre of the valley, forming hills several hundred
feet in height, it seems necessary to suppose, first, a time when it slowly
accumulated; and secondly, a later period, when large portions of it were
removed, or when the original valley, which had been partially filled up with
it, was re-excavated.
Such changes may have been brought about by a great movement of oscillation,
consisting first of a general depression of the land, and then of a gradual re-
elevation of the same. The amount of continental depression which first took
place in the interior, must be imagined to have exceeded that of the region near
the sea, in which case the higher part of the great valley would have its
alluvial plain gradually raised by an accumulation of sediment, which would only
cease when the subsidence of the land was at an end. If the direction of the
movement was then reversed, and, during the re-elevation of the continent, the
inland region nearest the mountains should rise more rapidly than that near the
coast, the river would acquire a denuding power sufficient to enable it to sweep
away gradually nearly all the loam and gravel with which parts of its basin had
been filled up. Terraces and hillocks of mud and sand would then alone remain to
attest the various levels at which the river had thrown down and afterwards
removed alluvial matter.
CAVERN DEPOSITS CONTAINING HUMAN REMAINS AND BONES OF EXTINCT ANIMALS.
In England, and in almost all countries where limestone rocks abound, caverns
are found, usually consisting of cavities of large dimensions, connected
together by low, narrow, and sometimes torturous galleries or tunnels. These
subterranean vaults are usually filled in part with mud, pebbles, and breccia,
in which bones occur belonging to the same assemblage of animals as those
characterising the Post-pliocene alluvia above described. Some of these bones
are referable to extinct and others to living species, and they are occasionally
intermingled, as in the valley-gravels, with implements of one or other of the
great divisions of the stone age, and these are not unfrequently accompanied by
human bones, which are much more common in cavern deposits than in valley-
alluvium.
Each suite of caverns, and the passages by which they communicate the one with
the other, afford memorials to the geologist of successive phases through which
they must have passed. First, there was a period when the carbonate of lime was
carried out gradually by springs; secondly, an era when engulfed rivers or
occasional floods swept organic and inorganic debris into the subterranean
hollows previously formed; and thirdly, there were such changes in the
configuration of the region as caused the engulfed rivers to be turned into new
channels, and springs to be dried up, after which the cave-mud, breccia, gravel,
and fossil bones would bear the same kind of relation to the existing drainage
of the country as the older valley-drifts with their extinct mammalian remains
and works of art bear to the present rivers and alluvial plains.
The quarrying away of large masses of Carboniferous and Devonian limestone, near
Liege, in Belgium, has afforded the geologist magnificent sections of some of
these caverns, and the former communication of cavities in the interior of the
rocks with the old surface of the country by means of vertical or oblique
fissures, has been demonstrated in places where it would not otherwise have been
suspected, so completely have the upper extremities of these fissures been
concealed by superficial drift, while their lower ends, which extended into the
roofs of the caves, are masked by stalactitic incrustations.
The origin of the stalactite is thus explained by the eminent chemist Liebig.
Mould or humus, being acted on by moisture and air, evolves carbonic acid, which
is dissolved by rain. The rain-water, thus impregnated, permeates the porous
limestone, dissolves a portion of it, and afterwards, when the excess of
carbonic acid evaporates in the caverns, parts with the calcareous matter, and
forms stalactite. Even while caverns are still liable to be occasionally flooded
such calcareous incrustations accumulate, but it is generally when they are no
longer in the line of drainage that a solid floor of hard stalagmite is formed
on the bottom.
The late Dr. Schmerling examined forty caves near Liege, and found in all of
them the remains of the same fauna, comprising the mammoth, tichorhine
rhinoceros, cave-bear, cave-hyaena, cave-lion, and many others, some of extinct
and some of living species, and in all of them flint implements. In four or five
caves only parts of human skeletons were met with, comprising sometimes skulls
with a few other bones, sometimes nearly every part of the skeleton except the
skull. In one of the caves, that of Engihoul, where Schmerling had found the
remains of at least three human individuals, they were mingled in such a manner
with bones of extinct mammalia, as to leave no doubt on his mind (in 1833) of
man having co-existed with them.
In 1860, Professor Malaise, of Liege, explored with me this same cave of
Engihoul, and beneath a hard floor of stalagmite we found mud full of bones of
extinct and recent animals, such as Schmerling had described, and my companion,
persevering in his researches after I had returned to England, extracted from
the same deposit two human lower jaw-bones retaining their teeth. The skulls
from these Belgian caverns display no marked deviation from the normal European
type of the present day.
The careful investigations carried on by Dr. Falconer, Mr. Pengelly, and others,
in the Brixham cave near Torquay, in 1858, demonstrated that flint knives were
there imbedded in such a manner in loam underlying a floor of stalagmite as to
prove that man had been an inhabitant of that region when the cave-bear and
other members of the ancient post-pliocene fauna were also in existence.
The absence of gnawed bones had led Dr. Schmerling to infer that none of the
Belgian caves which he explored had served as the dens of wild beasts; but there
are many caves in Germany and England which have certainly been so inhabited,
especially by the extinct hyaena and bear.
A fine example of a hyaena's den was afforded by the cave of Kirkdale, so well
described by the late Dr. Buckland in his Reliquiae Diluvianae. In that cave,
above twenty-five miles north-north-east of York, the remains of about 300
hyaenas, belonging to individuals of every age, were detected. The species
(Hyaena spelaea) has been considered by palaeontologists as extinct; it was
larger than the fierce Hyaena crocuta of South Africa, which it closely
resembled, and of which it is regarded by Mr. Boyd Dawkins as a variety. Dr.
Buckland, after carefully examining the spot, proved that the hyaenas must have
lived there; a fact attested by the quantity of their dung, which, as in the
case of the living hyaena, is of nearly the same composition as bone, and almost
as durable. In the cave were found the remains of the ox, young elephant,
hippopotamus, rhinoceros, horse, bear, wolf, hare, water-rat, and several birds.
All the bones have the appearance of having been broken and gnawed by the teeth
of the hyaenas; and they occur confusedly mixed in loam or mud, or dispersed
through a crust of stalagmite which covers it. In these and many other cases it
is supposed that portions of herbivorous quadrupeds have been dragged into
caverns by beasts of prey, and have served as their food-- an opinion quite
consistent with the known habits of the living hyaena.
AUSTRALIAN CAVE-BRECCIAS.
Ossiferous breccias are not confined to Europe, but occur in all parts of the
globe; and those discovered in fissures and caverns in Australia correspond
closely in character with what has been called the bony breccia of the
Mediterranean, in which the fragments of bone and rock are firmly bound together
by a red ochreous cement.
Some of these caves were examined by the late Sir T. Mitchell in the Wellington
Valley, about 210 miles west of Sidney, on the river Bell, one of the principal
sources of the Macquarie, and on the Macquarie itself. The caverns often branch
off in different directions through the rock, widening and contracting their
dimensions, and the roofs and floors are covered with stalactite. The bones are
often broken, but do not seem to be water-worn. In some places they lie imbedded
in loose earth, but they are usually included in a breccia.
The remains belong to marsupial animals. Among the most abundant are those of
the kangaroo, of which there are four species, while others belong to the genera
Phascolomys, the wombat; Dasyurus, the ursine opossum; Phalangista, the vulpine
opossum; and Hypsiprymnus, the kangaroo-rat.
(FIGURE 91. Part of lower jaw of Macropus atlas. Owen. A young individual of an
extinct species.
a. Permanent false molar, in the alveolus.)
(FIGURE 92. Lower jaw of largest living species of kangaroo. (Macropus major.))
In the fossils above enumerated, several species are larger than the largest
living ones of the same genera now known in Australia. Figure 91 of the right
side of a lower jaw of a kangaroo (Macropus atlas, Owen) will at once be seen to
exceed in magnitude the corresponding part of the largest living kangaroo, which
is represented in Figure 92. In both these specimens part of the substance of
the jaw has been broken open, so as to show the permanent false molar (a, Figure
91), concealed in the socket. From the fact of this molar not having been cut,
we learn that the individual was young, and had not shed its first teeth.
The reader will observe that all these extinct quadrupeds of Australia belong to
the marsupial family, or, in other words, that they are referable to the same
peculiar type of organisation which now distinguishes the Australian mammalia
from those of other parts of the globe. This fact is one of many pointing to a
general law deducible from the fossil vertebrate and invertebrate animals of
times immediately antecedent to our own, namely, that the present geographical
distribution of organic FORMS dates back to a period anterior to the origin of
existing SPECIES; in other words, the limitation of particular genera or
families of quadrupeds, mollusca, etc., to certain existing provinces of land
and sea, began before the larger part of the species now contemporary with man
had been introduced into the earth.
Professor Owen, in his excellent "History of British Fossil Mammals," has called
attention to this law, remarking that the fossil quadrupeds of Europe and Asia
differ from those of Australia or South America. We do not find, for example, in
the Europaeo-Asiatic province fossil kangaroos, or armadillos, but the elephant,
rhinoceros, horse, bear, hyaena, beaver, hare, mole, and others, which still
characterise the same continent.
In like manner, in the Pampas of South America the skeletons of Megatherium,
Megalonyx, Glyptodon, Mylodon, Toxodon, Macrauchenia, and other extinct forms,
are analogous to the living sloth, armadillo, cavy, capybara, and llama. The
fossil quadrumana, also associated with some of these forms in the Brazilian
caves, belong to the Platyrrhine family of monkeys, now peculiar to South
America. That the extinct fauna of Buenos Ayres and Brazil was very modern has
been shown by its relation to deposits of marine shells, agreeing with those now
inhabiting the Atlantic.
The law of geographical relationship above alluded to, between the living
vertebrata of every great zoological province and the fossils of the period
immediately antecedent, even where the fossil species are extinct, is by no
means confined to the mammalia. New Zealand, when first examined by Europeans,
was found to contain no indigenous land quadrupeds, no kangaroos, or opossums,
like Australia; but a wingless bird abounded there, the smallest living
representative of the ostrich family, called the Kiwi by the natives (Apteryx).
In the fossils of the Post-pliocene period in this same island, there is the
like absence of kangaroos, opossums, wombats, and the rest; but in their place a
prodigious number of well-preserved specimens of gigantic birds of the
struthious order, called by Owen Dinornis and Palapteryx, which are entombed in
superficial deposits. These genera comprehended many species, some of which were
four, some seven, others nine, and others eleven feet in height! It seems
doubtful whether any contemporary mammalia shared the land with this population
of gigantic feathered bipeds.
Mr. Darwin, when describing the recent and fossil mammalia of South America, has
dwelt much on the wonderful relationship of the extinct to the living types in
that part of the world, inferring from such geographical phenomena that the
existing species are all related to the extinct ones which preceded them by a
bond of common descent.
CLIMATE OF THE POST-PLIOCENE PERIOD.
The evidence as to the climate of Europe during this epoch is somewhat
conflicting. The fluviatile and land-shells are all of existing species, but
their geographical range has not always been the same as at present. Some, for
example, which then lived in Britain are now only found in Norway and Finland,
probably implying that the Post-pliocene climate of Britain was colder,
especially in the winter. So also the reindeer and the musk-ox (Ovibos
moschatus), now inhabitants of the Arctic regions, occur fossil in the valleys
of the Thames and Avon, and also in France and Germany, accompanied in most
places by the mammoth and the woolly rhinoceros. At Grays in Essex, on the other
hand, another species both of elephant and rhinoceros occurs, together with a
hippopotamus and the Cyrena fluminalis, a shell now extinct in Europe but still
an inhabitant of the Nile and some Asiatic rivers. With it occurs the Unio
littoralis, now living in the Seine and Loire. In the valley of the Somme flint
tools have been found associated with Hippopotamus major and Cyrena fluminalis
in the lower-level Post-pliocene gravels; while in the higher-level (and more
ancient) gravels similar tools are more abundant, and are associated with the
bones of the mammoth and other Post-pliocene quadrupeds indicative of a colder
climate.
It is possible that we may here have evidence of summer and winter migrations
rather than of a general change of temperature. Instead of imagining that the
hippopotamus lived all the year round with the musk-ox and lemming, we may
rather suppose that the apparently conflicting evidence may be due to the place
of our observations being near the boundary line of a northern and southern
fauna, either of which may have advanced or receded during comparatively slight
and temporary fluctuations of climate. There may then have been a continuous
land communication between England and the North of Siberia, as well as in an
opposite direction with Africa, then united to Southern Europe.
In drift at Fisherton, near Salisbury, thirty feet above the river Wiley, the
Greenland lemming and a new species of the Arctic genus Spermophilus have been
found, along with the mammoth, reindeer, cave-hyaena, and other mammalia suited
to a cold climate. A flint implement was taken out from beneath the bones of the
mammoth. In a higher and older deposit in the vicinity, flint tools like those
of Amiens have been discovered. Nearly all the known Post-pliocene quadrupeds
have now been found accompanying flint knives or hatchets in such a way as to
imply their coexistence with man; and we have thus the concurrent testimony of
several classes of geological facts to the vast antiquity of the human race. In
the first place, the disappearance of a great variety of species of wild animals
from every part of a wide continent must have required a vast period for its
accomplishment; yet this took place while man existed upon the earth, and was
completed before that early period when the Danish shell-mounds were formed or
the oldest of the Swiss lake-dwellings constructed. Secondly, the deepening and
widening of valleys, indicated by the position of the river gravels at various
heights, implies an amount of change of which that which has occurred during the
historical period forms a scarcely perceptible part. Thirdly, the change in the
course of rivers which once flowed through caves now removed from any line of
drainage, and the formation of solid floors of stalagmite, must have required a
great lapse of time. Lastly, ages must have been required to change the climate
of wide regions to such an extent as completely to alter the geographical
distribution of many mammalia as well as land and fresh-water shells. The 3000
or 4000 years of the historical period does not furnish us with any appreciable
measure for calculating the number of centuries which would suffice for such a
series of changes, which are by no means of a local character, but have operated
over a considerable part of Europe.
RELATIVE LONGEVITY OF SPECIES IN THE MAMMALIA AND TESTACEA.
I called attention in 1830 to the fact, which had not at that time attracted
notice, that the association in the Post-pliocene deposits of shells,
exclusively of living species, with many extinct quadrupeds betokened a
longevity of species in the testacea far exceeding that in the mammalia.
(Principles of Geology 1st edition volume 3 page 140.) Subsequent researches
seem to show that this greater duration of the same specific forms in the class
mollusca is dependent on a still more general law, namely, that the lower the
grade of animals, or the greater the simplicity of their structure, the more
persistent are they in general in their specific characters throughout vast
periods of time. Not only have the invertebrata, as shown by geological data,
altered at a less rapid rate than the vertebrata, but if we take one of the
classes of the former, as for example the mollusca, we find those of more simple
structure to have varied at a slower rate than those of a higher and more
complex organisation; the Brachiopoda, for example, more slowly than the
lamellibranchiate bivalves, while the latter have been more persistent than the
univalves, whether gasteropoda or cephalopoda. In like manner the specific
identity of the characters of the foraminifera, which are among the lowest types
of the invertebrata, has outlasted that of the mollusca in an equally decided
manner.
TEETH OF POST-PLIOCENE MAMMALIA.
To those who have never studied comparative anatomy, it may seem scarcely
credible that a single bone taken from any part of the skeleton may enable a
skilful osteologist to distinguish, in many cases, the genus, and sometimes the
species, of quadrupeds to which it belonged. Although few geologists can aspire
to such knowledge, which must be the result of long practice and study, they
will nevertheless derive great advantage from learning, what is comparatively an
easy task, to distinguish the principal divisions of the mammalia by the forms
and characters of their teeth.
Figures 93 through 105 represent the teeth of some of the more common species
and genera found in alluvial and cavern deposits.
(FIGURE 93. Elephas primigenius (or Mammoth ); molar of upper jaw, right side;
one-third of natural size. Post-pliocene.
a. Grinding surface.
b. Side view.)
(FIGURE 94. Elephas antiquus, Falconer. Penultimate molar, one-third of natural
size. Post-pliocene and Pliocene.)
(FIGURE 95. Elephas meridionalis, Nesti. Penultimate molar, one-third of natural
size. Post-pliocene and Pliocene.)
(FIGURE 96. Rhinoceros leptorhinus, Cuvier-- Rhin. megarhinus, Christol; fossil
from fresh-water beds of Grays, Essex; penultimate molar, lower jaw, left side;
two-thirds of natural size. Post-pliocene and Newer Pliocene.)
(FIGURE 97. Rhinoceros tichorhinus; penultimate molar, lower jaw, left side;
two-thirds of natural size. Post-pliocene.)
(FIGURE 98. Hippopotamus; from cave near Palermo; molar tooth; two-thirds of
natural size. Post-pliocene.)
(FIGURE 99. Horse. Equus caballus, L. (common horse); from the shell-marl,
Forfarshire; second molar, lower jaw. Recent.
a. Grinding surface, two-thirds natural size.
b. Side view of same, half natural size.)
(FIGURE 100. Deer.
Moose (Cervus alces, L.); recent; molar of upper jaw.
a. Grinding surface.
b. Side view, two-thirds of natural size.)
(FIGURE 101. Ox.
Ox, common, from shell-marl, Forfarshire; true molar, upper jaw; two-thirds
natural size. Recent.
c. Grinding surface.
d. Side view, fangs uppermost.)
(FIGURE 102. Bear.
a. Canine tooth or tusk of bear (Ursus spelaeus); from cave near Liege.
b. Molar of left side, upper jaw; one-third of natural size. Post-pliocene.)
(FIGURE 103. Tiger.
c. Canine tooth of tiger (Felis tigris); recent.
d. Outside view of posterior molar, lower jaw: one-third of natural size.
Recent.)
(FIGURE 104. Hyaena spelaea, Goldf. (variety of H. crocuta); lower jaw.
Kent's Hole, Torquay, Devonshire; one-third natural size. Post-pliocene.)
(FIGURE 105. Teeth of a new species of Arvicola, field-mouse; from the Norwich
Crag. Newer Pliocene.
a. Grinding surface.
b. Side view of the same.
c. Natural size of a and b.)
On comparing the grinding surfaces of the corresponding molars of the three
species of elephants, Figures 93, 94, 95 it will be seen that the folds of
enamel are most numerous in the mammoth, fewer and wider, or more open, in E.
antiquus; and most open and fewest in E. meridionalis. It will be also seen that
the enamel in the molar of the Rhinoceros tichorhinus (Figure 97), is much
thicker than in that of the Rhinoceros leptorhinus (Figure 96).
CHAPTER XI.
POST-PLIOCENE PERIOD, CONTINUED.-- GLACIAL CONDITIONS. (As to the former excess
of cold, whether brought about by modifications in the height and distribution
of the land or by altered astronomical conditions, see Principles volume 1 10th
edition 1867 chapters 12 and 13 "Vicissitudes of Climate.")
Geographical Distribution, Form, and Characters of Glacial Drift.
Fundamental Rocks, polished, grooved, and scratched.
Abrading and striating Action of Glaciers.
Moraines, Erratic Blocks, and "Roches Moutonnees."
Alpine Blocks on the Jura.
Continental Ice of Greenland.
Ancient Centres of the Dispersion of Erratics.
Transportation of Drift by floating Icebergs.
Bed of the Sea furrowed and polished by the running aground of floating Ice-
islands.
CHARACTER AND DISTRIBUTION OF GLACIAL DRIFT.
In speaking of the loose transported matter commonly found on the surface of the
land in all parts of the globe, I alluded to the exceptional character of what
has been called the boulder formation in the temperate and Arctic latitudes of
the northern hemisphere. The peculiarity of its form in Europe north of the
50th, and in North America north of the 40th parallel of latitude, is now
universally attributed to the action of ice, and the difference of opinion
respecting it is now chiefly restricted to the question whether land-ice or
floating icebergs have played the chief part in its distribution. It is wanting
in the warmer and equatorial regions, and reappears when we examine the lands
which lie south of the 40th and 50th parallels in the southern hemisphere, as,
for example, in Patagonia, Tierra del Fuego, and New Zealand. It consists of
sand and clay, sometimes stratified, but often wholly devoid of stratification
for a depth of 50, 100, or even a greater number of feet. To this unstratified
form of the deposit the name of TILL has long been applied in Scotland. It
generally contains a mixture of angular and rounded fragments of rock, some of
large size, having occasionally one or more of their sides flattened and
smoothed, or even highly polished. The smoothed surfaces usually exhibit many
scratches parallel to each other, one set of which often crosses an older set.
The till is almost everywhere wholly devoid of organic remains, except those
washed into it from older formations, though in some places it contains marine
shells, usually of northern or Arctic species, and frequently in a fragmentary
state. The bulk of the till has usually been derived from the grinding down into
mud of rocks in the immediate neighbourhood, so that it is red in a region of
Red Sandstone, as in Strathmore in Forfarshire; grey or black in a district of
coal and bituminous shale, as around Edinburgh; and white in a chalk country, as
in parts of Norfolk and Denmark. The stony fragments dispersed irregularly
through the till usually belong, especially in mountainous countries, to rocks
found in some part of the same hydrographical basin; but there are regions where
the whole of the boulder clay has come from a distance, and huge blocks, or
"erratics," as they have been called, many feet in diameter, have not
unfrequently travelled hundreds of miles from their point of departure, or from
the parent rocks from which they have evidently been detached. These are
commonly angular, and have often one or more of their sides polished and
furrowed.
The rock on which the boulder formation reposes, if it consists of granite,
gneiss, marble, or other hard stone, capable of permanently retaining any
superficial markings which may have been imprinted upon it, is usually smoothed
or polished, like the erratics above described, and exhibits parallel striae and
furrows having a determinate direction. This direction, both in Europe and North
America, agrees generally in a marked manner with the course taken by the
erratic blocks in the same district. The boulder clay, when it was first
studied, seemed in many of its characters so singular and anomalous, that
geologists despaired of ever being able to interpret the phenomena by reference
to causes now in action. In those exceptional cases where marine shells of the
same date as the boulder clay were found, nearly all of them were recognised as
living species-- a fact conspiring with the superficial position of the drift to
indicate a comparatively modern origin.
The term "diluvium" was for a time the most popular name of the boulder
formation, because it was referred by many to the deluge of Noah, while others
retained the name as expressive of their opinion that a series of diluvial waves
raised by hurricanes and storms, or by earthquakes, or by the sudden upheaval of
land from the bed of the sea, had swept over the continents, carrying with them
vast masses of mud and heavy stones, and forcing these stones over rocky
surfaces so as to polish and imprint upon them long furrows and striae. But
geologists were not long in seeing that the boulder formation was characteristic
of high latitudes, and that on the whole the size and number of erratic blocks
increases as we travel towards the Arctic regions. They could not fail to be
struck with the contrast which the countries bordering the Baltic presented when
compared with those surrounding the Mediterranean. The multitude of travelled
blocks and striated rocks in the one region, and the absence of such appearances
in the other, were too obvious to be overlooked. Even the great development of
the boulder formation, with large erratics so far south as the Alps, offered an
exception to the general rule favourable to the hypothesis that there was some
intimate connection between it and accumulations of snow and ice.
TRANSPORTING AND ABRADING POWER OF GLACIERS.
(FIGURE 106. Limestone, polished, furrowed, and scratched by the glacier of
Rosenlau in Switzerland. (Agassiz.)
a a. White streaks or scratches, caused by small grains of flint frozen into the
ice.
b b. Furrows.)
I have described elsewhere ("Principles" volume 1 chapter 16 1867) the manner in
which the snow of the Alpine heights is prevented from accumulating indefinitely
in thickness by the constant descent of a large portion of it by gravitation.
Becoming converted into ice it forms what are termed glaciers, which glide down
the principal valleys. On their surface are seen mounds of rubbish or large
heaps of sand and mud, with angular fragments of rock which fall from the steep
slopes or precipices bounding the glaciers. When a glacier, thus laden, descends
so far as to reach a region about 3500 feet above the level of the sea, the
warmth of the air is such that it melts rapidly in summer, and all the mud,
sand, and pieces of rock are slowly deposited at its lower end, forming a
confused heap of unstratified rubbish called a MORAINE, and resembling the TILL
before described.
Besides the blocks thus carried down on the top of the glacier, many fall
through fissures in the ice to the bottom, where some of them become firmly
frozen into the mass, and are pushed along the base of the glacier, abrading,
polishing, and grooving the rocky floor below, as a diamond cuts glass, or as
emery-powder polishes steel. The striae which are made, and the deep grooves
which are scooped out by this action, are rectilinear and parallel to an extent
never seen in those produced on loose stones or rocks, where shingle is hurried
along by a torrent, or by the waves on a sea-beach. In addition to these
polished, striated, and grooved surfaces of rock, another mark of the former
action of a glacier is the "roche moutonnee." Projecting eminences of rock so
called have been smoothed and worn into the shape of flattened domes by the
glacier as it passed over them. They have been traced in the Alps to great
heights above the present glaciers, and to great horizontal distances beyond
them.
ALPINE BLOCKS ON THE JURA.
The moraines, erratics, polished surfaces, domes, and striae, above described,
are observed in the great valley of Switzerland, fifty miles broad; and almost
everywhere on the Jura, a chain which lies to the north of this valley. The
average height of the Jura is about one-third that of the Alps, and it is now
entirely destitute of glaciers; yet it presents almost everywhere similar
moraines, and the same polished and grooved surfaces. The erratics, moreover,
which cover it, present a phenomenon which has astonished and perplexed the
geologist for more than half a century. No conclusion can be more incontestable
than that these angular blocks of granite, gneiss, and other crystalline
formations came from the Alps, and that they have been brought for a distance of
fifty miles and upward across one of the widest and deepest valleys in the
world; so that they are now lodged on a chain composed of limestone and other
formations, altogether distinct from those of the Alps. Their great size and
angularity, after a journey of so many leagues, has justly excited wonder; for
hundreds of them are as large as cottages; and one in particular, composed of
gneiss, celebrated under the name of Pierre a Bot, rests on the side of a hill
about 900 feet above the lake of Neufchatel, and is no less than 40 feet in
diameter.
In the year 1821, M. Venetz first announced his opinion that the Alpine glaciers
must formerly have extended far beyond their present limits, and the proofs
appealed to by him in confirmation of this doctrine were acknowledged by all
subsequent observers, and greatly strengthened by new observations and
arguments. M. Charpentier supposed that when the glaciers extended continuously
from the Alps to the Jura, the former mountains were 2000 or 3000 feet higher
than at present. Other writers, on the contrary, conjectured that the whole
country had been submerged, and the moraines and erratic blocks transported on
floating icebergs; but a careful study of the distribution of the travelled
masses, and the total absence of marine shells from the old glacial drift of
Switzerland, have entirely disproved this last hypothesis. In addition to the
many evidences of the action of ice in the northern parts of Europe which we
have already mentioned, there occur here and there in some of these countries,
what are wanting in Switzerland, deposits of marine fossil shells, which exhibit
so arctic a character that they must have led the geologist to infer the former
prevalence of a much colder climate, even had he not encountered so many
accompanying signs of ice-action. The same marine shells demonstrate the
submergence of large areas in Scandinavia and the British Isles, during the
glacial cold.
A characteristic feature of the deposits under consideration in all these
countries is the occurrence of large erratic blocks, and sometimes of moraine
matter, in situations remote from lofty mountains, and separated from the
nearest points where the parent rocks appear at the surface by great intervening
valleys, or arms of the sea. We also often observe striae and furrows, as in
Norway, Sweden, and Scotland, which deviate from the direction which they ought
to follow if they had been connected with the present line of drainage, and
they, therefore, imply the prevalence of a very distinct condition of things at
the time when the cold was most intense. The actual state of North Greenland
seems to afford the best explanation of such abnormal glacial markings.
GREENLAND CONTINENTAL ICE.
Greenland is a vast unexplored continent, buried under one continuous and
colossal mass of ice that is always moving seaward, a very small part of it in
an easterly direction, and all the rest westward, or towards Baffin's Bay. All
the minor ridges and valleys are levelled and concealed under a general covering
of snow, but here and there some steep mountains protrude abruptly from the icy
slope, and a few superficial lines of stones or moraines are visible at certain
seasons, when no snow has fallen for many months, and when evaporation, promoted
by the wind and sun, has caused much of the upper snow to disappear. The height
of this continent is unknown, but it must be very great, as the most elevated
lands of the outskirts, which are described as comparatively low, attain
altitudes of 4000 to 6000 feet. The icy slope gradually lowers itself towards
the outskirts, and then terminates abruptly in a mass about 2000 feet in
thickness, the great discharge of ice taking place through certain large friths,
which, at their upper ends, are usually about four miles across. Down these
friths the ice is protruded in huge masses, several miles wide, which continue
their course-- grating along the rocky bottom like ordinary glaciers long after
they have reached the salt water. When at last they arrive at parts of Baffin's
Bay deep enough to buoy up icebergs from 1000 to 1500 feet in vertical
thickness, broken masses of them float off, carrying with them on their surface
not only fine mud and sand but large stones. These fragments of rock are often
polished and scored on one or more sides, and as the ice melts, they drop down
to the bottom of the sea, where large quantities of mud are deposited, and this
muddy bottom is inhabited by many mollusca.
Although the direction of the ice-streams in Greenland may coincide in the main
with that which separate glaciers would take if there were no more ice than
there is now in the Swiss Alps, yet the striation of the surface of the rocks on
an ice-clad continent would, on the whole, vary considerably in its minor
details from that which would be imprinted on rocks constituting a region of
separate glaciers. For where there is a universal covering of ice there will be
a general outward movement from the higher and more central regions towards the
circumference and lower country, and this movement will be, to a certain extent,
independent of the minor inequalities of hill and valley, when these are all
reduced to one level by the snow. The moving ice may sometimes cross even at
right angles deep narrow ravines, or the crests of buried ridges, on which last
it may afterwards seem strange to detect glacial striae and polishing after the
liquefaction of the snow and ice has taken place.
Rink mentions that in North Greenland powerful springs of clayey water escape in
winter from under the ice, where it descends to "the outskirts," and where, as
already stated, it is often 2000 feet thick-- a fact showing how much grinding
action is going on upon the surface of the subjacent rocks. I also learn from
Dr. Torell that there are large areas in the outskirts, now no longer covered
with permanent snow or glaciers, which exhibit on their surface unmistakable
signs of ancient ice-action, so that, vast as is the power now exerted by ice in
Greenland, it must once have operated on a still grander scale. The land, though
now very elevated, may perhaps have been formerly much higher. It is well-known
that the south coast of Greenland, from latitude 60 degrees to about 70 degrees
north, has for the last four centuries been sinking at the rate of several feet
in a century. By this means a surface of rock, well scored and polished by ice,
is now slowly subsiding beneath the sea, and is becoming strewed over, as the
icebergs melt, with impalpable mud and smoothed and scratched stones. It is not
precisely known how far north this downward movement extends.
DRIFT CARRIED BY ICEBERGS.
An account was given so long ago as the year 1822, by Scoresby, of icebergs seen
by him in the Arctic seas drifting along in latitudes 69 and 70 degrees north,
which rose above the surface from 100 to 200 feet, and some of which measured a
mile in circumference. Many of them were loaded with beds of earth and rock, of
such thickness that the weight was conjectured to be from 50,000 to 100,000
tons. A similar transportation of rocks is known to be in progress in the
southern hemisphere, where boulders included in ice are far more frequent than
in the north. One of these icebergs was encountered in 1839, in mid-ocean, in
the antarctic regions, many hundred miles from any known land, sailing
northward, with a large erratic block firmly frozen into it. Many of them,
carefully measured by the officers of the French exploring expedition of the
Astrolabe, were between 100 and 225 feet high above water, and from two to five
miles in length. Captain d'Urville ascertained one of them which he saw floating
in the Southern Ocean to be 13 miles long and 100 feet high, with walls
perfectly vertical. The submerged portions of such islands must, according to
the weight of ice relatively to sea-water, be from six to eight times more
considerable than the part which is visible, so that when they are once fairly
set in motion, the mechanical force which they might exert against any obstacle
standing in their way would be prodigious.
We learn, therefore, from a study both of the arctic and antarctic regions, that
a great extent of land may be entirely covered throughout the whole year by snow
and ice, from the summits of the loftiest mountains to the sea-coast, and may
yet send down angular erratics to the ocean. We may also conclude that such land
will become in the course of ages almost everywhere scored and polished like the
rocks which underlie a glacier. The discharge of ice into the surrounding sea
will take place principally through the main valleys, although these are hidden
from our sight. Erratic blocks and moraine matter will be dispersed somewhat
irregularly after reaching the sea, for not only will prevailing winds and
marine currents govern the distribution of the drift, but the shape of the
submerged area will have its influence; inasmuch as floating ice, laden with
stones, will pass freely through deep water, while it will run a ground where
there are reefs and shallows. Some icebergs in Baffin's Bay have been seen
stranded on a bottom 1000 or even 1500 feet deep. In the course of ages such a
sea-bed may become densely covered with transported matter, from which some of
the adjoining greater depths may be free. If, as in West Greenland, the land is
slowly sinking, a large extent of the bottom of the ocean will consist of rock
polished and striated by land-ice, and then overspread by mud and boulders
detached from melting bergs.
The mud, sand, and boulders thus let fall in still water must be exactly like
the moraines of terrestrial glaciers, devoid of stratification and organic
remains. But occasionally, on the outer side of such packs of stranded bergs,
the waves and currents may cause the detached earthy and stony materials to be
sorted according to size and weight before they reach the bottom, and to acquire
a stratified arrangement.
I have already alluded to the large quantity of ice, containing great blocks of
stone, which is sometimes seen floating far from land, in the southern or
Antarctic seas. After the emergence, therefore, of such a submarine area, the
superficial detritus will have no necessary relation to the hills, valleys, and
river-plains over which it will be scattered. Many a water-shed may intervene
between the starting-point of each erratic or pebble and its final resting-
place, and the only means of discovering the country from which it took its
departure will consist in a careful comparison of its mineral or fossil contents
with those of the parent rocks.
CHAPTER XII.
POST-PLIOCENE PERIOD, CONTINUED.-- GLACIAL CONDITIONS, CONCLUDED.
Glaciation of Scandinavia and Russia.
Glaciation of Scotland.
Mammoth in Scotch Till.
Marine Shells in Scotch Glacial Drift.
Their Arctic Character.
Rarity of Organic Remains in Glacial Deposits.
Contorted Strata in Drift.
Glaciation of Wales, England, and Ireland.
Marine Shells of Moel Tryfaen.
Erratics near Chichester.
Glacial Formations of North America.
Many Species of Testacea and Quadrupeds survived the Glacial Cold.
Connection of the Predominance of Lakes with Glacial Action.
Action of Ice in preventing the silting up of Lake-basins.
Absence of Lakes in the Caucasus.
Equatorial Lakes of Africa.
GLACIATION OF SCANDINAVIA AND RUSSIA.
In large tracts of Norway and Sweden, where there have been no glaciers in
historical times, the signs of ice-action have been traced as high as 6000 feet
above the level of the sea. These signs consist chiefly of polished and furrowed
rock-surfaces, of moraines and erratic blocks. The direction of the erratics,
like that of the furrows, has usually been conformable to the course of the
principal valleys; but the lines of both sometimes radiate outward in all
directions from the highest land, in a manner which is only explicable by the
hypothesis above alluded to of a general envelope of continental ice, like that
of Greenland (Chapter 11.) Some of the far-transported blocks have been carried
from the central parts of Scandinavia towards the Polar regions; others
southward to Denmark; some south-westward, to the coast of Norfolk in England;
others south-eastward, to Germany, Poland, and Russia.
In the immediate neighbourhood of Upsala, in Sweden, I had observed, in 1834, a
ridge of stratified sand and gravel, in the midst of which occurs a layer of
marl, evidently formed originally at the bottom of the Baltic, by the slow
growth of the mussel, cockle, and other marine shells of living species,
intermixed with some proper to fresh water. The marine shells are all of
dwarfish size, like those now inhabiting the brackish waters of the Baltic; and
the marl, in which many of them are imbedded, is now raised more than 100 feet
above the level of the Gulf of Bothnia. Upon the top of this ridge repose
several huge erratics, consisting of gneiss for the most part unrounded, from
nine to sixteen feet in diameter, and which must have been brought into their
present position since the time when the neighbouring gulf was already
characterised by its peculiar fauna. Here, therefore, we have proof that the
transport of erratics continued to take place, not merely when the sea was
inhabited by the existing testacea, but when the north of Europe had already
assumed that remarkable feature of its physical geography which separates the
Baltic from the North Sea, and causes the Gulf of Bothnia to have only one-
fourth of the saltness belonging to the ocean. In Denmark, also, recent shells
have been found in stratified beds, closely associated with the boulder clay.
GLACIATION OF SCOTLAND.
Mr. T.F. Jamieson, in 1858, adduced a great body of facts to prove that the
Grampians once sent down glaciers from the central regions in all directions
towards the sea. "The glacial grooves," he observed, "radiate outward from the
central heights towards all points of the compass, though they do not always
strictly conform to the actual shape and contour of the minor valleys and
ridges."
These facts and other characteristics of the Scotch drift lead us to the
inference that when the glacial cold first set in, Scotland stood higher above
the sea than at present, and was covered for the most part with snow and ice, as
Greenland is now. This sheet of land-ice sliding down to lower levels, ground
down and polished the subjacent rocks, sweeping off nearly all superficial
deposits of older date, and leaving only till and boulders in their place. To
this continental state succeeded a period of depression and partial submergence.
The sea advanced over the lower lands, and Scotland was converted into an
archipelago, some marine sand with shells being spread over the bottom of the
sea. On this sand a great mass of boulder clay usually quite devoid of fossils
was accumulated. Lastly, the land re-emerged from the water, and, reaching a
level somewhat above its present height, became connected with the continent of
Europe, glaciers being formed once more in the higher regions, though the ice
probably never regained its former extension. (Jamieson Quarterly Geological
Journal 1860 volume 16 page 370.) After all these changes, there were some minor
oscillations in the level of the land, on which, although they have had
important geographical consequences, separating Ireland from England, for
example, and England from the Continent, we need not here enlarge.
MAMMOTH IN SCOTCH TILL.
Almost all remains of the terrestrial fauna of the Continent which preceded the
period of submergence have been lost; but a few patches of estuarine and fresh-
water formations escaped denudation by submergence. To these belong the peaty
clay from which several mammoths' tusks and horns of reindeer were obtained at
Kilmaurs, in Ayrshire as long ago as 1816. Mr. Bryce in 1865 ascertained that
the fresh-water formation containing these fossils rests on carboniferous
sandstone, and is covered, first by a bed of marine sand with arctic shells, and
then with a great mass of till with glaciated boulders. (Bryce Quarterly
Geological Journal volume 21 page 217 1865.) Still more recent explorations in
the neighbourhood of Kilmaurs have shown that the fresh-water formation contains
the seed of the pond-weed Potamogeton and the aquatic Ranunculus; and Mr. Young
of the Glasgow Museum washed the mud adhering to the reindeer horns of Kilmaurs
and that which filled the cracks of the associated elephants' tusks, and
detected in these fossils (which had been in the Glasgow Museum for half a
century) abundance of the same seeds.
All doubts, therefore, as to the true position of the remains of the mammoth, a
fossil so rare in Scotland, have been set at rest, and it serves to prove that
part of the ancient continent sank beneath the sea at a period of great cold, as
the shells of the overlying sand attest. The incumbent till or boulder clay is
about 40 feet thick, but it often attains much greater thickness in the same
part of Scotland.
MARINE SHELLS OF SCOTCH DRIFT.
(FIGURE 107. Astarte borealis, Chem.; (A. arctica, Moll. A. compressa, Mont.)
(FIGURE 108. Leda lanceolata (oblonga), Sowerby.)
(FIGURE 109. Saxicava rugosa, Penn.)
(FIGURE 110. Pecten islandicus, Moll. Northern shell common in the drift of the
Clyde, in Scotland. )
(FIGURE 111. Natica clausa, Bred. Northern shell common in the drift of the
Clyde, in Scotland.)
(FIGURE 112. Trophon clathratum, Linne. Northern shell common in the drift of
the Clyde, in Scotland.)
(FIGURE 113. Leda truncata.
a. Exterior of left valve.
b. Interior of same.)
(FIGURE 114. Tellina calcarea, Chem. (Tellina proxima, Brown.)
a. Outside of left valve.
b. Interior of same.)
The greatest height to which marine shells have yet been traced in this boulder
clay is at Airdie, in Lanarkshire, ten miles east of Glasgow, 524 feet above the
level of the sea. At that spot they were found imbedded in stratified clays with
till above and below them. There appears no doubt that the overlying deposit was
true glacial till, as some boulders of granite were observed in it, which must
have come from distances of sixty miles at the least.
The shells figured in Figures 107 to 112 are only a few out of a large
assemblage of living species, which, taken as a whole, bear testimony to
conditions far more arctic than those now prevailing in the Scottish seas. But a
group of marine shells, indicating a still greater excess of cold, has been
brought to light since 1860 by the Reverend Thomas Brown, from glacial drift or
clay on the borders of the estuaries of the Forth and Tay. This clay occurs at
Elie, in Fife, and at Errol, in Perthshire; and has already afforded about 35
shells, all of living species, and now inhabitants of arctic regions, such as
Leda truncata, Tellina proxima (see Figures 113 and 114), Pecten Groenlandicus,
Crenella laevigata, Crenella nigra, and others, some of them first brought by
Captain Sir E. Parry from the coast of Melville Island, latitude 76 degrees
north. These were all identified in 1863 by Dr. Torell, who had just returned
from a survey of the seas around Spitzbergen, where he had collected no less
than 150 species of mollusca, living chiefly on a bottom of fine mud derived
from the moraines of melting glaciers which there protrude into the sea. He
informed me that the fossil fauna of this Scotch glacial deposit exhibits not
only the species but also the peculiar varieties of mollusca now characteristic
of very high latitudes. Their large size implies that they formerly enjoyed a
colder, or, what was to them a more genial climate, than that now prevailing in
the latitude where the fossils occur. Marine shells have also been found in the
glacial drift of Caithness and Aberdeenshire at heights of 250 feet, and in
Banff of 350 feet, and stratified drift continuous with the above ascends to
heights of 500 feet. Already 75 species are enumerated from Caithness, and the
same number from Aberdeenshire and Banff, and in both cases all but six are
arctic species.
I formerly suggested that the absence of all signs of organic life in the Scotch
drift might be connected with the severity of the cold, and also in some places
with the depth of the sea during the period of extreme submergence; but my faith
in such an hypothesis has been shaken by modern investigations, an exuberance of
life having been observed both in arctic and antarctic seas of great depth, and
where floating ice abounds. The difficulty, moreover, of accounting for the
entire dearth of marine shells in till is removed when once we have adopted the
theory of this boulder clay being the product of land-ice. For glaciers coming
down from a continental ice-sheet like that which covers Greenland may fill
friths many hundred feet below the sea-level, and even invade parts of a bay a
thousand feet deep, before they find water enough to float off their terminal
portions in the form of icebergs. In such a case till without marine shells may
first accumulate, and then, if the climate becomes warmer and the ice melts, a
marine deposit may be superimposed on the till without any change of level being
required.
Another curious phenomenon bearing on this subject was styled by the late Hugh
Miller the "striated pavements" of the boulder clay. Where portions of the till
have been removed by the sea on the shores of the Forth, or in the interior by
railway cuttings, the boulders imbedded in what remains of the drift are seen to
have been all subjected to a process of abrasion and striation, the striae and
furrows being parallel and persistent across them all, exactly as if a glacier
or iceberg had passed over them and scored them in a manner similar to that so
often undergone by the solid rocks below the glacial drift. It is possible, as
Mr. Geikie conjectures, that this second striation of the boulders may be
referable to floating ice. (Geikie Transactions of the Geological Society of
Glasgow volume 1 part 2 page 68 1863.)
CONTORTED STRATA IN DRIFT.
(FIGURE 115. Section of contorted drift overlying till, seen on left bank of
South Esk, near Cortachie, in 1840. Height of section seen, from a to d, about
50 feet.
a, b. Gravel and sand.
f, g. Contorted drift.
Till.)
In Scotland the till is often covered with stratified gravel, sand, and clay,
the beds of which are sometimes horizontal and sometimes contorted for a
thickness of several feet. Such contortions are not uncommon in Forfarshire,
where I observed them, among other places, in a vertical cutting made in 1840
near the left bank of the South Esk, east of the bridge of Cortachie. The
convolutions of the beds of fine and coarse sand, gravel, and loam, extend
through a thickness of no less than 25 feet vertical, or from b to c, Figure
115, the horizontal stratification being resumed very abruptly at a short
distance, as to the right of f, g. The overlying coarse gravel and sand, a, is
in some places horizontal, in others it exhibits cross bedding, and does not
partake of the disturbances which the strata b, c, have undergone. The
underlying till is exposed for a depth of about 20 feet; and we may infer from
sections in the neighbourhood that it is considerably thicker.
In some cases I have seen fragments of stratified clays and sands, bent in like
manner, in the middle of a great mass of till. Mr. Trimmer has suggested, in
explanation of such phenomena, the intercalation in the glacial period of large
irregular masses of snow or ice between layers of sand and gravel. Some of the
cliffs near Behring's Straits, in which the remains of elephants occur, consist
of ice mixed with mud and stones; and Middendorf describes the occurrence in
Siberia of masses of ice, found at various depths from the surface after digging
through drift. Whenever the intercalation of snow and ice with drift, whether
stratified or unstratified, has taken place, the melting of the ice will cause
such a failure of support as may give rise to flexures, and sometimes to the
most complicated foldings. But in many cases the strata may have been bent and
deranged by the mechanical pressure of an advancing glacier, or by the sideway
thrust of huge islands of ice running aground against sandbanks; in which case,
the position of the beds forming the foundation of the banks may not be at all
disturbed by the shock.
There are indeed many signs in Scotland of the action of floating ice, as might
have been expected where proofs of submergence in the Glacial Period are not
wanting. Among these are the occurrence of large erratic blocks, frequently in
clusters at or near the tops of hills or ridges, places which may have formed
islets or shallows in the sea where floating ice would mostly ground and
discharge its cargo on melting. Glaciers or land-ice would, on the contrary,
chiefly discharge their cargoes at the bottom of valleys. Traces of an earlier
and independent glaciation have also been observed in some regions where the
striation, apparently produced by ice proceeding from the north-west, is not
explicable by the radiation of land-ice from a central mountainous region.
(Milne Home Transactions of the Royal Society Edinburgh volume 25 1868-9.)
GLACIATION OF WALES AND ENGLAND.
The mountains of North Wales were recognised, in 1842, by Dr. Buckland, as
having been an independent centre of the dispersion of erratics-- great
glaciers, long since extinct, having radiated from the Snowdonian heights in
Carnarvonshire, through seven principal valleys towards as many points of the
compass, carrying with them large stony fragments, and grooving the subjacent
rocks in as many directions.
Besides this evidence of land-glaciers, Mr. Trimmer had previously, in 1831,
detected the signs of a great submergence in Wales in the Post-pliocene period.
He had observed stratified drift, from which he obtained about a dozen species
of marine shells, near the summit of Moel Tryfaen, a hill 1400 feet high, on the
south side of the Menai Straits. I had an opportunity of examining in the summer
of 1863, together with the Reverend W.S. Symonds, a long and deep cutting made
through this drift by the Alexandra Mining Company in search of slates. At the
top of the hill above-mentioned we saw a stratified mass of incoherent sand and
gravel 35 feet thick, from which no less than 54 species of mollusca, besides
three characteristic arctic varieties-- in all 57 forms-- have been obtained by
Mr. Darbishire. They belong without exception to species still living in British
or more northern seas; eleven of them being exclusively arctic, four common to
the arctic and British seas, and a large proportion of the remainder having a
northward range, or, if found at all in the southern seas of Britain, being
comparatively less abundant. In the lowest beds of the drift were large heavy
boulders of far-transported rocks, glacially polished and scratched on more than
one side. Underneath the whole we saw the edges of vertical slates exposed to
view, which here, like the rocks in other parts of Wales, both at greater and
less elevations, exhibit beneath the drift unequivocal marks of prolonged
glaciation. The whole deposit has much the appearance of an accumulation in
shallow water or on a beach, and it probably acquired its thickness during the
gradual subsidence of the coast-- an hypothesis which would require us to
ascribe to it a high antiquity, since we must allow time, first for its sinking,
and then for its re-elevation.
The height reached by these fossil shells on Moel Tryfaen is no less than 1300
feet-- a most important fact when we consider how very few instances we have on
record beyond the limits of Wales, whether in Europe or North America, of marine
shells having been found in glacial drift at half the height above indicated. A
marine molluscous fauna, however, agreeing in character with that of Moel
Tryfaen, and comprising as many species, has been found in drift at Macclesfield
and other places in central England, sometimes reaching an elevation of 1200
feet.
Professor Ramsay estimated the probable amount of submergence during some part
of the glacial period at about 2300 feet; for he was unable to distinguish the
superficial sands and gravel which reached that high elevation from the drift
which, at Moel Tryfaen and at lower points, contains shells of living species.
The evidence of the marine origin of the highest drift is no doubt inconclusive
in the absence of shells, so great is the resemblance of the gravel and sand of
a sea beach and of a river's bed, when organic remains are wanting; but, on the
other hand, when we consider the general rarity of shells in drift which we know
to be of marine origin, we can not suppose that, in the shelly sands of Moel
Tryfaen, we have hit upon the exact uppermost limit of marine deposition, or, in
other words, a precise measure of the submergence of the land beneath the sea
since the glacial period.
We are gradually obtaining proofs of the larger part of England, north of a line
drawn from the mouth of the Thames to the Bristol Channel, having been under the
sea and traversed by floating ice since the commencement of the glacial epoch.
Among recent observations illustrative of this point, I may allude to the
discovery, by Mr. J.F. Bateman, near Blackpool, in Lancashire, fifty miles from
the sea, and at the height of 568 feet above its level, of till containing
rounded and angular stones and marine shells, such as Turritella communis,
Purpura lapillus, Cardium edule, and others, among which Trophon clathratum
(=Fusus Bamffius), though still surviving in North British seas, indicates a
cold climate.
ERRATICS NEAR CHICHESTER.
The most southern memorials of ice-action and of a Post-pliocene fauna in Great
Britain is on the coast of the county of Sussex, about 25 miles west of
Brighton, and 15 south of Chichester. A marine deposit exposed between high and
low tide occurs on both sides of the promontory called Selsea Bill, in which Mr.
Godwin-Austen found thirty-eight species of shells, and the number has since
been raised to seventy.
This assemblage is interesting because on the whole, while all the species are
recent, they have a somewhat more southern aspect than those of the present
British Channel. It is true that about forty of them range from British to high
northern latitudes; but several of them, as, for example, Lutraria rugosa and
Pecten polymorphous, which are abundant, are not known at present to range
farther north than the coast of Portugal, and seem to indicate a warmer
temperature than now prevails on the coast where we find them fossil. What
renders this curious is the fact that the sandy loam in which they occur is
overlaid by yellow clayey gravel with large erratic blocks which must have been
drifted into their present position by ice when the climate had become much
colder. These transported fragments of granite, syenite, and greenstone, as well
as of Devonian and Silurian rocks, may have come from the coast of Normandy and
Brittany, and are many of them of such large size that we must suppose them to
have been drifted into their present site by coast-ice. I measured one of
granite, at Pagham, 21 feet in circumference. In the gravel of this drift with
erratics are a few littoral shells of living species, indicating an ancient
coast-line.
GLACIAL FORMATIONS OF NORTH AMERICA.
In the western hemisphere, both in Canada and as far south as the 40th and even
38th parallel of latitude in the United States, we meet with a repetition of all
the peculiarities which distinguish the European boulder formation. Fragments of
rock have travelled for great distances, especially from north to south: the
surface of the subjacent rock is smoothed, striated, and fluted; unstratified
mud or TILL containing boulders is associated with strata of loam, sand, and
clay, usually devoid of fossils. Where shells are present, they are of species
still living in northern seas, and not a few of them identical with those
belonging to European drift, including most of those already given in Figures
107 to 112. The fauna also of the glacial epoch in North America is less rich in
species than that now inhabiting the adjacent sea, whether in the Gulf of St.
Lawrence, or off the shores of Maine, or in the Bay of Massachusetts.
The extension on the American continent of the range of erratics during the
Post-pliocene period to lower latitudes than they reached in Europe, agrees well
with the present southward deflection of the isothermal lines, or rather the
lines of equal winter temperature. It seems that formerly, as now, a more
extreme climate and a more abundant supply of ice prevailed on the western side
of the Atlantic. Another resemblance between the distribution of the drift
fossils in Europe and North America has yet to be pointed out. In Canada and the
United States, as in Europe, the marine shells are generally confined to very
moderate elevations above the sea (between 100 and 700 feet), while the erratic
blocks and the grooved and polished surfaces of rock extend to elevations of
several thousand feet.
I have already mentioned that in Europe several quadrupeds of living, as well as
extinct, species were common to pre-glacial and post-glacial times. In like
manner there is reason to suppose that in North America much of the ancient
mammalian fauna, together with nearly all the invertebrata, lived through the
ages of intense cold. That in the United States the Mastodon giganteus was very
abundant after the drift period, is evident from the fact that entire skeletons
of this animal are met with in bogs and lacustrine deposits occupying hollows in
the glacial drift. They sometimes occur in the bottom even of small ponds
recently drained by the agriculturist for the sake of the shell-marl. In 1845 no
less than six skeletons of the same species of Mastodon were found in Warren
county, New Jersey, six feet below the surface, by a farmer who was digging out
the rich mud from a small pond which he had drained. Five of these skeletons
were lying together, and a large part of the bones crumbled to pieces as soon as
they were exposed to the air.
It would be rash, however, to infer from such data that these quadrupeds were
mired in MODERN times, unless we use that term strictly in a geological sense. I
have shown that there is a fluviatile deposit in the valley of the Niagara,
containing shells of the genera Melania, Lymnea, Planorbis, Velvata, Cyclaz,
Unio, Helix, etc., all of recent species, from which the bones of the great
Mastodon have been taken in a very perfect state. Yet the whole excavation of
the ravine, for many miles below the Falls, has been slowly effected since that
fluviatile deposit was thrown down. Other extinct animals accompany the Mastodon
giganteus in the post-glacial deposits of the United States, and this, taken
with the fact that so few of the mollusca, even of the commencement of the cold
period, differ from species now living is important, as refuting the hypothesis,
for which some have contended, that the intensity of the glacial cold
annihilated all the species in temperate and arctic latitudes.
CONNECTION OF THE PREDOMINANCE OF LAKES WITH GLACIAL ACTION.
It was first pointed out by Professor Ramsay in 1862, that lakes are exceedingly
numerous in those countries where erratics, striated blocks, and other signs of
ice-action abound; and that they are comparatively rare in tropical and sub-
tropical regions. Generally in countries where the winter cold is intense, such
as Canada, Scandinavia, and Finland, even the plains and lowlands are thickly
strewn with innumerable ponds and small lakes, together with some others of a
larger size; while in more temperate regions, such as Great Britain, Central and
Southern Europe, the United States, and New Zealand, lake districts occur in all
such mountainous tracts as can be proved to have been glaciated in times
comparatively modern or since the geographical configuration of the surface bore
a considerable resemblance to that now prevailing. In the same countries, beyond
the glaciated regions, lakes abruptly cease, and in warmer and tropical
countries are either entirely absent, or consist, as in equatorial Africa, of
large sheets of water unaccompanied so far as we yet know by numerous smaller
ponds and tarns.
The southern limits of the lake districts of the Northern Hemisphere are found
at about 40 degrees N. latitude on the American continent, and about 50 degrees
in Europe, or where the Alps intervene four degrees farther south. A large
proportion of the smaller lakes are dammed up by barriers of unstratified drift,
having the exact character of the moraines of glaciers, and are termed by
geologists "morainic," but some of them are true rock-basins, and would hold
water even if all the loose drift now resting on their margins were removed.
In a paper read before the Geological Society of London in 1862, Professor
Ramsay maintained that the first formation of most existing lakes took place
during the glacial epoch, and was due, not to elevation or subsidence, but to
actual erosion of their basins by glaciers. M. Mortillet in the same year
advanced the theory that after the Alpine lake-basins had been filled up with
loose fluviatile deposits, they were re-excavated by the great glaciers which
passed down the valleys at the time of the greatest cold, a doctrine which would
attribute to moving ice almost as great a capacity of erosion as that which
assumed that the original basins were scooped out of solid rock by glaciers. It
is impossible to deny that the mere geographical distribution of lakes points to
the intimate connection of their origin with the abundance of ice during a
former excess of cold, but how far the erosive action of moving ice has been the
sole or even the principal cause of lake-basins, is a question still open to
discussion.
The lakes of Switzerland and the north of Italy are some of them twenty and
thirty miles in length, and so deep that their bottoms are in some cases from
1000 to 2000 feet beneath the level of the sea. It is admitted on all hands that
they were once filled with ice, and as the existing glaciers polish and grind
down, as before stated, the surface of the rocks, we are prepared to find that
every lake-basin in countries once covered by ice should bear the marks of
superficial glaciation, and also that the ice during its advance and retreat
should have left behind it much transported matter as well as some evidence of
its having enlarged the pre-existing cavity. But much more than this is demanded
by the advocates of glacial erosion. They suggest that as the old extinct
glaciers were several thousand feet thick, they were able in some places
gradually to scoop out of the solid rock cavities twenty or thirty miles in
length, and as in the case of Lago Maggiore from a thousand to two thousand six
hundred feet below the previous level of the river-channel, and also that the
ice had the power to remove from the cavity formed by its grinding action all
the materials of the missing rocks. A constant supply, it is argued, of fine mud
issues from the termination of every glacier in the stream which is produced by
the melting of the ice, and this result of friction is exhibited both during
winter and summer, affording evidence of the continual deepening and widening of
the valleys through which glaciers pass. As the fine mud is carried away by a
river from the deep pool which is formed from the base of every cataract, so it
seems to be imagined that lake-basins may be gradually emptied of the mud formed
by abrasion during the glacial period.
I am by no means disposed to object to this theory on the ground of the
insufficiency of the time during which the extreme cold endured, but we must
carefully consider whether that same time is not so vast as to make it probable
that other forces, besides the motion of glaciers, must have cooperated in
converting some parts of the ancient valley courses into lake-basins. They who
have formed the most exalted conceptions of the erosive energy of moving ice do
not deny that during the period termed "Glacial" there have been movements of
the earth's crust sufficient to produce oscillations of level in Europe
amounting to 1000 feet or more in both directions. M. Charpentier, indeed,
attributed some of the principal changes of climate in Switzerland, during the
glacial period, to a depression of the central Alps to the extent of 3000 feet,
and Swiss geologists have long been accustomed to attribute their lake basins,
in part, to those convulsions by which the shape and course of the valleys may
have been modified. Our experience, in the lifetime of the present generation,
of the changes of level witnessed in New Zealand during great earthquakes is
entirely opposed to the notion that the movements, whether upward or downward,
are uniform in amount or direction throughout areas of indefinite extent. On the
contrary, the land has been permanently raised in one region several feet or
yards, and the rise has been found gradually to die out, so as to be
imperceptible at a distance of twenty miles, and in some areas is even exchanged
for a simultaneous downward movement of several feet.
But, it is asked, if such inequality of movement can have contributed towards
the production of lake basins, does it not leave unexplained the comparative
rarity of lakes in tropical and subtropical countries. In reply to this question
it may be observed that in our endeavour to estimate the effects of subterranean
movements in modifying the superficial geography of a country we must remember
that each convulsion effects a very slight change. If it interferes with the
drainage, whether by raising the lower or sinking the higher portion of a
hydrographical basin, the upheaval or depression will only amount to a few feet
at a time, and there may be an interval of years or centuries before any further
movement takes place in the same region. In the mean time an incipient lake if
produced may be filled up with sediment, and the recently-formed barrier will
then be cut through by the river, whereas in a country where glacial conditions
prevail no such obliteration of the temporary lake-basin would take place; for
however deep it became by repeated sinking of the upper or rising of the lower
extremity, being always filled with ice it might remain, throughout the greater
part of its extent, free from sediment or drift until the ice melted at the
close of the glacial period.
One of the most serious objections to the exclusive origin by ice-erosion of
wide and deep lake-basins arises from their capricious distribution, as for
example in Piedmont, both to the eastward and westward of Turin, where great
lakes are wanting (Antiquity of Man page 313.), although some of the largest
extinct glaciers descending from Mont Blanc and Monte Rosa came down from the
Alps, leaving their gigantic moraines in the low country. Here, therefore, we
might have expected to find lakes of the first magnitude rivalling the
contiguous Lago Maggiore in importance.
A still more striking illustration of the same absence of lakes where large
glaciers abound is afforded by the Caucasus, a chain more than 300 miles long,
and the loftiest peaks of which attain heights from 16,000 to 18,000 feet. This
greatest altitude is reached by Elbruz, a mountain in latitude 43 degrees north
three degrees south of Mont Blanc, but on the other hand 3000 feet higher. The
present Caucasian glaciers are equal or superior in dimensions to those of
Switzerland, and like them give rise occasionally to temporary lakes by
obstructing the course of rivers, and causing great floods when the icy barriers
give way. Mr. Freshfield, a careful observer, writing in 1869, says: "A total
absence of lakes on both sides of the chains is the most marked feature. Not
only are there no great subalpine sheets of water, like Como or Geneva, but
mountain tarns, such as the Dauben See on the Gemmi, or the Klonthal See near
Glarus, are equally wanting." (Travels in Central Caucasus 1869 page 452.) The
same author states on the authority of the eminent Swiss geologist, Mons. E.
Favre, who also explored the Caucasus in 1868, that moraines of great height and
huge erratics of granite and other rocks "justify the assertion that the present
glaciers of the Caucasus, like those of the Alps, are only the shadows of their
former selves."
It seems safe to assume that the chain of lakes, of which the Albert Nyanza
forms one in equatorial Africa, was due to causes other than glacial. Yet if we
could imagine a glacial period to visit that region filling the lakes with ice
and scoring the rocks which form their sides and bottoms, we should be unable to
decide how much the capacity of the basins had been enlarged and the surface
modified by glacial erosion. The same may be true of the Lago Maggiore and Lake
Superior, although the present basins of both of them afford abundant
superficial markings due to ice-action.
But to whatever combination of causes we attribute the great Alpine lakes one
thing is clear, namely, that they are, geologically speaking, of modern origin.
Every one must admit that the upper valley of the Rhone has been chiefly caused
by fluviatile denudation, and it is obvious that the quantity of matter removed
from that valley previous to the glacial period would have been amply sufficient
to fill up with sediment the basin of the Lake of Geneva, supposing it to have
been in existence, even if its capacity had been many times greater than it is
now. (See Principles volume 1 page 420 10th edition 1867.)
On the whole, it appears to me, in accordance with the views of Professor
Ramsay, M. Mortillet, Mr. Geikie, and others, that the abrading action of ice
has formed some mountain tarns and many morainic lakes; but when it is a
question of the origin of larger and deeper lakes, like those of Switzerland or
the north of Italy, or inland fresh-water seas, like those of Canada, it will
probably be found that ice has played a subordinate part in comparison with
those movements by which changes of level in the earth's crust are gradually
brought about.
TERTIARY OR CAINOZOIC PERIOD.
CHAPTER XIII.
PLIOCENE PERIOD.
Glacial Formations of Pliocene Age.
Bridlington Beds.
Glacial Drifts of Ireland.
Drift of Norfolk Cliffs.
Cromer Forest-bed.
Aldeby and Chillesford Beds.
Norwich Crag.
Older Pliocene Strata.
Red Crag of Suffolk.
Coprolitic Bed of Red Crag.
White or Coralline Crag.
Relative Age, Origin, and Climate of the Crag Deposits.
Antwerp Crag.
Newer Pliocene Strata of Sicily.
Newer Pliocene Strata of the Upper Val d'Arno.
Older Pliocene of Italy.
Subapennine Strata.
Older Pliocene Flora of Italy.
It will be seen in the description given in the last chapter of the Post-
pliocene formations of the British Isles that they comprise a large proportion
of those commonly termed glacial, characterised by shells which, although
referable to living species, usually indicate a colder climate than that now
belonging to the latitudes where they occur fossil. But in parts of England,
more especially in Yorkshire, Norfolk, and Suffolk, there are superficial
formations of clay with glaciated boulders, and of sand and pebbles, containing
occasional, though rare, patches of shells, in which the marine fauna begins to
depart from that now inhabiting the neighbouring sea, and comprises some species
of mollusca not yet known as living, as well as extinct varieties of others,
entitling us to class them as Newer Pliocene, although belonging to the close of
that period and chronologically on the verge of the later or Post-pliocene
epoch.
BRIDLINGTON DRIFT.
To this era belongs the well-known locality of Bridlington, near the mouth of
the Humber, in Yorkshire, where about seventy species or well-marked varieties
of shells have been found on the coast, near the sea-level, in a bed of sand
several feet thick resting on glacial clay with much chalk debris, and covered
by a deposit of purple clay with glaciated boulders. More than a third of the
species in this drift are now inhabitants of arctic regions, none of them
extending southward to the British seas; which is the more remarkable as
Bridlington is situated in latitude 54 degrees north. Fifteen species are
British and Arctic, a very few belong to those species which range south of our
British seas. Five species or well-marked varieties are not known living,
namely, the variety of Astarte borealis (called A. Withami); A. mutabilis; the
sinistral form of Tritonium carinatum, Cardita analis, and Tellina obliqua,
Figure 120. Mr. Searles Wood also inclines to consider Nucula Cobboldiae, Figure
119, now absent from the European seas and the Atlantic, as specifically
distinct from a closely-allied shell now living in the seas surrounding
Vancouver's Island, which some conchologists regard as a variety. Tellina
obliqua also approaches very near to a shell now living in Japan.
GLACIAL DRIFT OF IRELAND.
Marine drift containing the last-mentioned Nucula and other glacial shells
reaches a height of from 1000 to 1200 feet in the county of Wexford, south of
Dublin. More than eighty species have already been obtained from this formation,
of which two, Conovulus pyramidalis and Nassa monensis, are not known as living;
while Turritella incrassata and Cypraea lucida no longer inhabit the British
seas, but occur in the Mediterranean. The great elevation of these shells, and
the still greater height to which the surface of the rocks in the mountainous
regions of Ireland have been smoothed and striated by ice-action, has led
geologists to the opinion that that island, like the greater part of England and
Scotland, after having been united with the continent of Europe, from whence it
received the plants and animals now inhabiting it, was in great part submerged.
The conversion of this and other parts of Great Britain into an archipelago was
followed by a re-elevation of land and a second continental period. After all
these changes the final separation of Ireland from Great Britain took place, and
this event has been supposed to have preceded the opening of the straits of
Dover. (See Antiquity of Man chapter 14.)
DRIFT OF NORFOLK CLIFFS.
(FIGURE 116. Tellina balthica (T. solidula).)
There are deposits of boulder clay and till in the Norfolk cliffs principally
made up of the waste of white chalk and flints which, in the opinion of Mr.
Searles Wood, jun., and others, are older than the Bridlington drift, and
contain a larger proportion of shells common to the Norwich and Red Crag,
including a certain number of extinct forms, but also abounding in Tellina
balthica (T. solidula, Figure 116), which is found fossil at Bridlington, and
living in our British seas, but wanting in all the formations, even the newest,
afterwards to be described as Crag. As the greater part of these drifts are
barren of organic remains, their classification is at present a matter of great
uncertainty.
They can nowhere be so advantageously studied as on the coast between
Happisburgh and Cromer. Here we may see vertical cliffs, sometimes 300 feet and
more in height, exposed for a distance of fifty miles, at the base of which the
chalk with flints crops out in nearly horizontal strata. Beds of gravel and sand
repose on this undisturbed chalk. They are often strangely contorted, and
envelop huge masses or erratics of chalk with layers of vertical flint. I
measured one of these fragments in 1839 at Sherringham, and found it to be
eighty feet in its longest diameter. It has been since entirely removed by the
waves of the sea. In the floor of the chalk beneath it the layers of flint were
horizontal. Such erratics have evidently been moved bodily from their original
site, probably by the same glacial action which has polished and striated some
of the accompanying granitic and other boulders, occasionally six feet in
diameter, which are imbedded in the drift.
CROMER FOREST-BED.
Intervening between these glacial formations and the subjacent chalk lies what
has been called the Cromer Forest-bed. This buried forest has been traced from
Cromer to near Kessingland, a distance of more than forty miles, being exposed
at certain seasons between high and low water mark. It is the remains of an old
land and estuarine deposit, containing the submerged stumps of trees standing
erect with their roots in the ancient soil. Associated with the stumps and
overlying them, are lignite beds with fresh-water shells of recent species, and
laminated clay without fossils. Through the lignite and forest-bed are scattered
cones of the Scotch and spruce firs with the seeds of recent plants, and the
bones of at least twenty species of terrestrial mammalia. Among these are two
species of elephant, E. meridionalis, Nesti, and E. antiquus, the former found
in the Newer Pliocene beds of the Val d'Arno, near Florence. In the same bed
occur Hippopotamus major, Rhinoceros etruscus, both of them also Val d'Arno
species, many species of deer considered by Mr. Boyd Dawkins to be
characteristic of warmer countries, and also a horse, beaver, and field-mouse.
Half of these mammalia are extinct, and the rest still survive in Europe. The
vegetation taken alone does not imply a temperature higher than that now
prevailing in the British Isles. There must have been a subsidence of the forest
to the amount of 400 or 500 feet, and a re-elevation of the same to an equal
extent in order to allow the ancient surface of the chalk or covering of soil,
on which the forest grew, to be first covered with several hundred feet of
drift, and then upheaved so that the trees should reach their present level.
Although the relative antiquity of the forest-bed to the overlying glacial till
is clear, there is some difference of opinion as to its relation to the crag
presently to be described.
CHILLESFORD AND ALDEBY BEDS.
(FIGURE 117. Natica helicoides, Johnson.)
It is in the counties of Norfolk, Suffolk, and Essex, that we obtain our most
valuable information respecting the British Pliocene strata, whether newer or
older. They have obtained in those counties the provincial name of "Crag,"
applied particularly to masses of shelly sand which have long been used in
agriculture to fertilise soils deficient in calcareous matter. At Chillesford,
between Woodbridge and Aldborough in Suffolk, and Aldeby, near Beccles, in the
same county, there occur stratified deposits, apparently older than any of the
preceding drifts of Yorkshire, Norfolk, and Suffolk. They are composed at
Chillesford of yellow sands and clays, with much mica, forming horizontal beds
about twenty feet thick. Messrs. Prestwich and Searles Wood, senior, who first
described these beds, point out that the shells indicate on the whole a colder
climate than the Red Crag; two-thirds of them being characteristic of high
latitudes. Among these are Cardium Groenlandicum, Leda limatula, Tritonium
carinatum, and Scalaria Groenlandica. In the upper part of the laminated clays a
skeleton of a whale was found associated with casts of the characteristic
shells, Nucula Cobboldiae and Tellina obliqua, already referred to as no longer
inhabiting our seas, and as being extinct varieties if not species. The same
shells occur in a perfect state in the lower part of the formation. Natica
helicoides (Figure 117) is an example of a species formerly known only as
fossil, but which has now been found living in our seas.
At Aldeby, where beds occur decidedly similar in mineral character as well as
fossil remains, Messrs. Crowfoot and Dowson have now obtained sixty-six species
of mollusca, comprising the Chillesford species and some others. Of these about
nine-tenths are recent. They are in a perfect state, clearly indicating a cold
climate; as two-thirds of them are now met with in arctic regions. As a rule,
the lamellibranchiate molluscs have both valves united, and many of them, such
as Mya arenaria, stand with the siphonal end upward, as when in a living state.
Tellina balthica, before mentioned (Figure 116) as so characteristic of the
glacial beds, including the drift of Bridlington, has not yet been found in
deposits of Chillesford and Aldeby age, whether at Sudbourn, East Bavent,
Horstead, Coltishall, Burgh, or in the highest beds overlying the Norwich Crag
proper at Bramerton and Thorpe.
NORWICH OR FLUVIO-MARINE CRAG.
(FIGURE 118. Mastodon arvernensis, third milk molar, left side, upper jaw;
grinding surface, natural size. Norwich Crag, Postwick, also found in Red Crag,
see below.)
The beds above alluded to ought, perhaps, to be regarded as beds of passage
between the glacial formations and those called from a provincial name "Crag,"
the newest member of which has been commonly called the "Norwich Crag." It is
chiefly seen in the neighbourhood of Norwich, and consists of beds of incoherent
sand, loam, and gravel, which are exposed to view on both banks of the Yare, as
at Bramerton and Thorpe. As they contain a mixture of marine, land, and fresh-
water shells, with bones of fish and mammalia, it is clear that these beds have
been accumulated at the bottom of a sea near the mouth of a river. They form
patches rarely exceeding twenty feet in thickness, resting on white chalk. At
their junction with the chalk there invariably intervenes a bed called the
"Stone-bed," composed of unrolled chalk-flints, commonly of large size, mingled
with the remains of a land fauna comprising Mastodon arvernensis, Elephas
meridionalis, and an extinct species of deer. The mastodon, which is a species
characteristic of the Pliocene strata of Italy and France, is the most abundant
fossil, and one not found in the Cromer forest before mentioned. When these
flints, probably long exposed in the atmosphere, became submerged, they were
covered with barnacles, and the surface of the chalk became perforated by the
Pholas crispata, each fossil shell still remaining at the bottom of its
cylindrical cavity, now filled up with loose sand from the incumbent crag. This
species of Pholas still exists, and drills the rocks between high and low water
on the British coast. The name of "Fluvio-marine" has often been given to this
formation, as no less than twenty species of land and fresh-water shells have
been found in it. They are all of living species; at least only one univalve,
Paludina lenta, has any, and that a very doubtful, claim to be regarded as
extinct.
(FIGURE 119. Nucula Cobboldiae.)
(FIGURE 120. Tellina obliqua.)
Of the marine shells, 124 in number, about 18 per cent are extinct, according to
the latest estimate given me by Mr. Searles Wood; but, for reasons presently to
be mentioned, this percentage must be only regarded as provisional. It must also
be borne in mind that the proportion of recent shells would be augmented if the
uppermost beds at Bramerton, near Norwich, which belong to the most modern or
Chillesford division of the Crag, had been included, as they were formerly, by
Mr. Woodward and myself, in the Norwich series. Arctic shells, which formed so
large a proportion in the Chillesford and Aldeby beds, are more rare in the
Norwich Crag, though many northern species-- such as Rhynchonella psittacea,
Scalaria Groenlandica, Astarte borealis, Panopaea Norvegia, and others-- still
occur. The Nucula Cobboldiae and Tellina obliqua, Figures 119 and 120, before
mentioned, are frequent in these beds, as are also Littorina littorea, Cardium
edule, and Turritella communis, of our seas, proving the littoral origin of the
beds.
OLDER PLIOCENE STRATA.
RED CRAG.
(FIGURE 121. Section through (left) sea, Red Crag, London Clay and Chalk
(right).)
Among the English Pliocene beds the next in antiquity is the Red Crag, which
often rests immediately on the London Clay, as in the county of Essex,
illustrated in Figure 121.
It is chiefly in the county of Suffolk that it is found, rarely exceeding twenty
feet in thickness, and sometimes overlying another Pliocene deposit, the
Coralline Crag, to be mentioned in the sequel. It has yielded-- exclusive of 25
species regarded by Mr. Wood as derivative-- 256 species of mollusca, of which
65, or 25 per cent, are extinct. Thus, apart from its order of superposition,
its greater antiquity than the Norwich and glacial beds, already described, is
proved by the greater departure from the fauna of our seas. It may also be
observed that in most of the deposits of this Red Crag, the northern forms of
the Norwich Crag, and of such glacial formations as Bridlington, are less
numerous, while those having a more southern aspect begin to make their
appearance. Both the quartzose sand, of which it chiefly consists, and the
included shells, are most commonly distinguished by a deep ferruginous or
ochreous colour, whence its name. The shells are often rolled, sometimes
comminuted, and the beds have much the appearance of having been shifting sand-
banks, like those now forming on the Dogger-bank, in the sea, sixty miles east
of the coast of Northumberland. Cross stratification is almost always present,
the planes of the strata being sometimes directed towards one point of the
compass, sometimes to the opposite, in beds immediately overlying. That such a
structure is not deceptive or due to any subsequent concretionary rearrangement
of particles, or to mere bands of colour produced by the iron, is proved by each
bed being made up of flat pieces of shell which lie parallel to the planes of
the smaller strata.
(FIGURE 122. Purpura tetragona, Sowerby; natural size.)
(FIGURE 123. Voluta Lamberti, Sowerby. Variety characteristic of Suffolk Crag.
Pliocene.)
(FIGURE 124. Voluta Lamberti, young individual, Cor. and Red Crag.)
It has long been suspected that the different patches of Red Crag are not all of
the same age, although their chronological relation can not be decided by
superposition. Separate masses are characterised by shells specifically distinct
or greatly varying in relative abundance, in a manner implying that the deposits
containing them were separated by intervals of time. At Butley, Tunstall,
Sudbourn, and in the Red Crag of Chillesford, the mollusca appear to assume
their most modern aspect when the climate was colder than when the earliest
deposits of the same period were formed. At Butley, Nucula Cobboldiae, so common
in the Norwich and certain glacial beds, is found, and Purpura tetragona (Figure
122) is very abundant. On the other hand, at Walton-on-the-Naze, in Essex, we
seem to have an exhibition of the oldest phase of the Red Crag; and a warmer
climate seems indicated, not only by the absence of many northern forms, but
also by the abundance of some now living in the British seas and the
Mediterranean. Voluta Lamberti (see Figures 123 and 124), an extinct form, which
seems to have flourished chiefly in the antecedent Coralline Crag period, is
still represented here by individuals of every age.
(FIGURE 125. Trophon antiquum, Muller. (Fusus contrarius) half natural size.)
The reversed whelk (Figure 125) is common at Walton, where the dextral form of
that shell is unknown. Here also we find most frequently specimens of
lamellibranchiate molluscs, with both the valves united, showing that they
belonged to this sea of the Upper Crag, and were not washed in from an older
bed, such as the Coralline, in which case the ligament would not have held
together the valves in strata so often showing signs of the boisterous action of
the waves. No less than forty species of lamellibranchiate molluscs, with double
valves, have been collected by Mr. Bell from the various localities of the Red
Crag.
At and near the base of the Red Crag is a loose bed of brown nodules, first
noticed by Professor Henslow as containing a large percentage of earthy
phosphates. This bed of coprolites (as it is called, because they were
originally supposed to be the faeces of animals) does not always occur at one
level, but is generally in largest quantity at the junction of the Crag and the
underlying formation. In thickness it usually varies from six to eighteen
inches, and in some rare cases amounts to many feet. It has been much used in
agriculture for manure, as not only the nodules, but many of the separate bones
associated with them, are largely impregnated with phosphate of lime, of which
there is sometimes as much as sixty per cent. They are not unfrequently covered
with barnacles, showing that they were not formed as concretions in the stratum
where they now lie buried, but had been previously consolidated. The phosphatic
nodules often collect fossil crabs and fishes from the London Clay, together
with the teeth of gigantic sharks. In the same bed have been found many ear-
bones of whales, and the teeth of Mastodon arvernensis, Rhinoceros
Schleiermacheri, Tapirus priscus, and Hipparion (a quadruped of the horse
family), and antlers of a stag, Cervus anoceros. Organic remains also of the
older chalk and Lias are met with, showing how great was the denudation of
previous formations during the Pliocene period. As the older White Crag,
presently to be mentioned, contains similar phosphatic nodules near its base,
those of the Red Crag may be partly derived from this source.
WHITE OR CORALLINE CRAG.
The lower or Coralline Crag is of very limited extent, ranging over an area
about twenty miles in length, and three or four in breadth, between the rivers
Stour and Alde, in Suffolk. It is generally calcareous and marly-- often a mass
of comminuted shells, and the remains of bryozoa (or polyzoa), passing
occasionally into a soft building-stone. (Ehrenberg proposed in 1831 the term
Bryozoum, or "Moss-animal," for the molluscous or ascidian form of polyp,
characterised by having two openings to the digestive sack, as in Eschara,
Flustra, Retepora, and other zoophytes popularly included in the corals, but now
classed by naturalists as mollusca. The term Polyzoum, synonymous with Bryozoum,
was, it seems, proposed in 1830, or the year before, by Mr. J.O. Thompson.) At
Sudbourn and Gedgrave, near Orford, this building-stone has been largely
quarried. At some places in the neighbourhood the softer mass is divided by thin
flags of hard limestone, and bryozoa placed in the upright position in which
they grew. From the abundance of these coralloid mollusca the lowest or White
Crag obtained its popular name, but true corals, as now defined, or zoantharia,
are very rare in this formation.
The Coralline Crag rarely, if ever, attains a thickness of thirty feet in any
one section. Mr. Prestwich imagines that if the beds found at different
localities were united in the probable order of their succession, they might
exceed eighty feet in thickness, but Mr. Searles Wood does not believe in the
possibility of establishing such a chronological succession by aid of the
organic remains, and questions whether proof could be obtained of more than
forty feet. I was unable to come to any satisfactory opinion on the subject,
although at Orford, especially at Gedgrave, in the neighbourhood of that place,
I saw many sections in pits, where this crag is cut through. These pits are so
unconnected, and of such limited extent, that no continuous section of any
length can be obtained, so that speculations as to the thickness of the whole
deposit must be very vague. At the base of the formation at Sutton a bed of
phosphatic nodules, very similar to that before alluded to in the Red Crag, with
remains of mammalia, has been met with.
(FIGURE 126. Section near Woodbridge, in Suffolk.
Through Sutton (left), Shottisham Creek, Ramsholt (right) and R. Deben.
a. Red Crag.
b. Coralline Crag.
c. London Clay.)
Whenever the Red and Coralline Crag occur in the same district, the Red Crag
lies uppermost; and in some cases, as in the section represented in Figure 126,
which I had an opportunity of seeing exposed to view in 1839, it is clear that
the older deposit, or Coralline Crag, b, had suffered denudation, before the
newer formation, a, was thrown down upon it. At D there was not only seen a
distinct cliff, eight or ten feet high, of Coralline Crag, running in a
direction N.E. and S.W., against which the Red Crag abuts with its horizontal
layers, but this cliff occasionally overhangs. The rock composing it is drilled
everywhere by Pholades, the holes which they perforated having been afterwards
filled with sand, and covered over when the newer beds were thrown down. The
older formation is shown by its fossils to have accumulated in a deeper sea, and
contains none of those littoral forms such as the limpet, Patella, found in the
Red Crag. So great an amount of denudation could scarcely take place, in such
incoherent materials, without some of the fossils of the inferior beds becoming
mixed up with the overlying crag, so that considerable difficulty must be
occasionally experienced by the palaeontologist in deciding which species belong
severally to each group.
(FIGURE 127. Fascicularia aurantium, Milne Edwards. Family, Tubuliporidae, of
same author. Bryozoan of extinct genus, from the inferior or Coralline Crag,
Suffolk.
a. Exterior.
b. Vertical section of interior.
c. Portion of exterior magnified.
d. Portion of interior magnified, showing that it is made up of long, thin,
straight tubes, united in conical bundles.)
(FIGURE 128. Astarte omalii, laj.; species common to Upper and lower crag.)
Mr. Searles Wood estimates the total number of marine testaceous mollusca of the
Coralline Crag at 350, of which 110 are not known as living, being in the
proportion of thirty-one per cent extinct. No less than 130 species of bryozoa
have been found in the Coralline Crag, and some belong to genera unknown in the
living creation, and of a very peculiar structure; as, for example, that
represented in Figure 127, which is one of several species having a globular
form. Among the testacea the genus Astarte (see Figure 128) is largely
represented, no less than fourteen species being known, and many of these being
rich in individuals. There is an absence of genera peculiar to hot climates,
such as Conus, Oliva, Fasciolaria, Crassatella, and others. The absence also of
large cowries (Cyprea), those found belonging exclusively to the section Trivia,
is remarkable. The large volute, called Voluta Lamberti (Figure 123), may seem
an exception; but it differs in form from the volutes of the torrid zone, and,
like the living Voluta Magellanica, must have been fitted for an extra-tropical
climate.
(FIGURE 129. Lingula Dumortieri, Nyst; Suffolk and Antwerp Crag.)
(FIGURE 130. Pyrula reticulata, Lam.; Coralline Crag, Ramsholt.)
(FIGURE 131. Temnechinus excavatus, Forbes; Temnopleurus excavatus, Wood;
Coralline Crag, Ramsholt.)
The occurrence of a species of Lingula at Sutton (see Figure 129) is worthy of
remark, as these Brachiopoda seem now confined to more equatorial latitudes; and
the same may be said still more decidedly of a species of Pyrula, supposed by
Mr. Wood to be identical with P. reticulata (Figure 130), now living in the
Indian Ocean. A genus also of echinoderms, called by Professor Forbes
Temnechinus (Figure 131), occurs in the Red and Coralline Crag of Suffolk, and
until lately was unknown in a living state, but it has been brought to light as
an existing form by the deep-sea dredgings, both of the United States survey,
off Florida, at a depth of from 180 to 480 feet, and more recently (1869), in
the British seas, during the explorations of the "Porcupine."
CLIMATE OF THE CRAG DEPOSITS.
One of the most interesting conclusions deduced from a careful comparison of the
shells of the British Pliocene strata and the fauna of our present seas has been
pointed out by Professor E. Forbes. It appears that, during the Glacial period,
a period intermediate, as we have seen, between that of the Crag and our own
time, many shells, previously established in the temperate zone, retreated
southward to avoid an uncongenial climate, and they have been found fossil in
the Newer Pliocene strata of Sicily, Southern Italy, and the Grecian
Archipelago, where they may have enjoyed, during the era of floating icebergs, a
climate resembling that now prevailing in higher European latitudes. (E. Forbes
Mem. Geological Survey of Great Britain volume 1 page 386.) The Professor gave a
list of fifty shells which inhabited the British seas while the Coralline and
Red Crag were forming, and which, though now living in our seas, were wanting,
as far as was then known, in the glacial deposits. Some few of these species
have subsequently been found in the glacial drift, but the general conclusion of
Forbes remains unshaken.
The transport of blocks by ice, when the Red Crag was being deposited, appears
to me evident from the large size of some huge, irregular, quite unrounded chalk
flints, retaining their white coating, and 2 feet long by 18 inches broad, in
beds worked for phosphatic nodules at Foxhall, four miles south-east of Ipswich.
These must have been tranquilly drifted to the spot by floating ice. Mr.
Prestwich also mentions the occurrence of a large block of porphyry in the base
of the Coralline Crag at Sutton, which would imply that the ice-action had begun
in our seas even in this older period. The cold seems to have gone on increasing
from the time of the Coralline to that of the Norwich Crag, and became more and
more severe, not perhaps without some oscillations of temperature, until it
reached its maximum in what has been called the Glacial period, or at the close
of the Newer Pliocene, and in the Post-pliocene periods.
RELATION OF THE FAUNA OF THE CRAG TO THAT OF THE RECENT SEAS.
By far the greater number of the recent marine species occurring in the several
Crag formations are still inhabitants of the British seas; but even these differ
considerably in their relative abundance, some of the commonest of the Crag
shells being now extremely scarce-- as, for example, Buccinum Dalei-- while
others, rarely met with in a fossil state, are now very common, as Murex
erinaceus and Cardium echinatum. Some of the species also, the identity of which
with the living would not be disputed by any conchologist, are nevertheless
distinguishable as varieties, whether by slight deviations in form or a
difference in average dimensions. Since Mr. Searles Wood first described the
marine testacea of the Crags, the additions made to that fossil fauna have not
been considerable, whereas we have made in the same period immense progress in
our knowledge of the living testacea of the British and arctic seas, and of the
Mediterranean. By this means the naturalist has been enabled to identify with
existing species many forms previously supposed to be extinct.
In the forthcoming supplement to the invaluable monograph communicated by Mr.
Wood to the Palaeontographical Society, in which he has completed his figures
and descriptions of the British crag shells of every age, list will be found of
all the fossil shells, of which a summary is given in the table below.
TABLE OF NUMBER OF KNOWN SPECIES OF MARINE TESTACEA IN THE CRAG.
COLUMN 1: KNOWN SPECIES.
COLUMN 2: TOTAL NUMBER OF KNOWN SPECIES.
COLUMN 3: NUMBER OF SPECIES NOT KNOWN AS LIVING.
CHILLESFORD AND ALDEBY BEDS:
Bivalves: 61 : 4.
Univalves: 33 : 5.
Brachiopods: 0 : 0.
PERCENTAGE OF SHELLS NOT KNOWN AS LIVING : 9.5.
NORWICH OR FLUVIO-MARINE CRAG:
Bivalves: 61 : 10.
Univalves: 64 : 12.
Brachiopods: 1 : 0.
PERCENTAGE OF SHELLS NOT KNOWN AS LIVING : 17.5.
RED CRAG (Exclusive of many derivative shells):
Bivalves: 128 : 31.
Univalves: 127 : 33.
Brachiopods: 1 : 1.
PERCENTAGE OF SHELLS NOT KNOWN AS LIVING : 25.0.
CORALLINE CRAG:
Bivalves: 161 : 47.
Univalves: 184 : 60
Brachiopods: 5 : 3
PERCENTAGE OF SHELLS NOT KNOWN AS LIVING : 31.5
To begin with the uppermost or Chillesford beds, it will be seen that about 9
per cent only are extinct, or not known as living, whereas in the Norwich, which
succeeds in the descending order, seventeen in a hundred are extinct. Formerly,
when the Norwich or Fluvio-marine Crag was spoken of, both these formations were
included under the same head, for both at Bramerton and Thorpe, the chief
localities where the Norwich Crag was studied, an overlying deposit occurs
referable to the Chillesford age. If now the two were fused together as of old,
their shells would, according to Mr. Wood, yield a percentage of fifteen in a
hundred of species extinct or not known as living.
To come next to the Red Crag, the reader will observe that a percentage of 25 is
given of shells unknown as living, and this increases to 31 in the antecedent
Coralline Crag. But the gap between these two stages of our Pliocene deposits is
really wider than these numbers would indicate, for several reasons. In the
first place, the Coralline Crag is more strictly the product of a single period,
the Red Crag, as we have seen, consisting of separate and independent patches,
slightly varying in age, of which the newest is probably not much anterior to
the Norwich Crag. Secondly, there was a great change of conditions, both as to
the depth of the sea and climate, between the periods of the Coralline and Red
Crag, causing the fauna in each to differ far more widely than would appear from
the above numerical results.
The value of the analysis given in the above table of the shells of the Red and
Coralline Crags is in no small degree enhanced by the fact that they were all
either collected by Mr. Wood himself, or obtained by him direct from their
discoverers, so that he was enabled in each case to test their authenticity, and
as far as possible to avoid those errors which arise from confounding together
shells belonging to the sea of a newer deposit, and those washed into it from a
formation of older date. The danger of this confusion may be conceived when we
remember that the number of species rejected from the Red Crag as derivative by
Mr. Wood is no less than 25. Some geologists have held that on the same grounds
it is necessary to exclude as spurious some of the species found in the Norwich
Crag proper; but Mr. Wood does not entertain this view, believing that the
spurious shells which have sometimes found their way into the lists of this crag
have been introduced by want of care from strata of Red Crag.
There can be no doubt, on the other hand, that conchologists have occasionally
rejected from the Red and Norwich Crags, as derivative, shells which really
belonged to the seas of those periods, because they were extinct or unknown as
living, which in their eyes afforded sufficient ground for suspecting them to be
intruders. The derivative origin of a species may sometimes be indicated by the
extreme scarcity of the individuals, their colour, and worn condition; whereas
an opposite conclusion may be arrived at by the integrity of the shells,
especially when they are of delicate and tender structure, or their abundance,
and, in the case of the lamellibranchiata, by their being held together by the
ligament, which often happens when the shells have been so broken that little
more than the hinges of the two valves are preserved. As to the univalves, I
have seen from a pit of Red Crag, near Woodbridge, a large individual of the
extinct Voluta Lamberti, seven inches in length, of which the lip, then perfect,
had in former stages of its growth been frequently broken, and as often
repaired. It had evidently lived in the sea of the Red Crag, where it had been
exposed to rough usage, and sustained injuries like those which the reversed
whelk, Trophon antiquum, so characteristic of the same formation, often
exhibits. Additional proofs, however, have lately been obtained by Mr. Searles
Wood that this shell had not died out in the era of the Red Crag by the
discovery of the same fossil near Southwold, in beds of the later Norwich Crag.
ANTWERP CRAG.
Strata of the same age as the Red and Coralline Crag of Suffolk have been long
known in the country round Antwerp, and on the banks of the Scheldt, below that
city; and the lowest division, or Black Crag, there found, is shown by the
shells to be somewhat more ancient than any of our British series, and probably
forms the first links of a downward passage from the strata of the Pliocene to
those of the Upper Miocene period.
NEWER PLIOCENE STRATA OF SICILY.
(FIGURE 132. Murex vaginatus, Phil.)
At several points north of Catania, on the eastern sea-coast of Sicily-- as at
Aci-Castello, for example, Trezza, and Nizzeti-- marine strata, associated with
volcanic tuffs and basaltic lavas, are seen, which belong to a period when the
first igneous eruptions of Mount Etna were taking place in a shallow bay of the
Mediterranean. They contain numerous fossil shells, and out of 142 species that
have been collected all but eleven are identical with species now living. Some
few of these eleven shells may possibly still linger in the depths of the
Mediterranean, like Murex vaginatus, see Figure 132. The last-mentioned shell
had already become rare when the associated marine and volcanic strata above
alluded to were formed. On the whole, the modern character of the testaceous
fauna under consideration is expressed not only by the small proportion of
extinct species, but by the relative number of individuals by which most of the
other species are represented, for the proportion agrees with that observed in
the present fauna of the Mediterranean. The rarity of individuals in the extinct
species is such as to imply that they were already on the point of dying out,
having flourished chiefly in the earlier Pliocene times, when the Subapennine
strata were in progress.
Yet since the accumulation of these Newer Pliocene sands and clays, the whole
cone of Etna, 11,000 feet in height and about 90 miles in circumference at its
base, has been slowly built up; an operation requiring many tens of thousands of
years for its accomplishment, and to estimate the magnitude of which it is
necessary to study in detail the internal structure of the mountain, and to see
the proofs of its double axis, or the evidence of the lavas of the present great
centre of eruption having gradually overwhelmed and enveloped a more ancient
cone, situated 3 1/2 miles to the east of the present one. (See a Memoir on the
Lavas and Mode of Origin of Mount Etna by the Author in Philosophical
Transactions 1858.)
It appears that while Etna was increasing in bulk by a series of eruptions, its
whole mass, comprising the foundations of subaqueous origin above alluded to,
was undergoing a slow upheaval, by which those marine strata were raised to the
height of 1200 feet above the sea, as seen at Catera, and perhaps to greater
heights, for we can not trace their extension westward, owing to the dense and
continuous covering of modern lava under which they are buried. During the
gradual rise of these Newer Pliocene formations (consisting of clays, sands, and
basalts) other strata of Post-pliocene date, marine as well as fluviatile,
accumulated round the base of the mountain, and these, in their turn, partook of
the upward movement, so that several inland cliffs and terraces at low levels,
due partly to the action of the sea and partly to the river Simeto, originated
in succession. Fossil remains of the elephant, and other extinct quadrupeds,
have been found in these Post-Pliocene strata, associated with recent shells.
There is probably no part of Europe where the Newer Pliocene formations enter so
largely into the structure of the earth's crust, or rise to such heights above
the level of the sea, as Sicily. They cover nearly half the island, and near its
centre, at Castrogiovanni, reach an elevation of 3000 feet. They consist
principally of two divisions, the upper calcareous and the lower argillaceous,
both of which may be seen at Syracuse, Girgenti, and Castrogiovanni. According
to Philippi, to whom we are indebted for the best account of the tertiary shells
of this island, thirty-five species out of one hundred and twenty-four obtained
from the beds in central Sicily are extinct.
A geologist, accustomed to see nearly all the Newer Pliocene formations in the
north of Europe occupying low grounds and very incoherent in texture, is
naturally surprised to behold formations of the same age so solid and stony, of
such thickness, and attaining so great an elevation above the level of the sea.
The upper or calcareous member of this group in Sicily consists in some places
of a yellowish-white stone, like the Calcaire Grossier of Paris; in others, of a
rock nearly as compact as marble. Its aggregate thickness amounts sometimes to
700 or 800 feet. It usually occurs in regular horizontal beds, and is
occasionally intersected by deep valleys, such as those of Sortino and
Pentalica, in which are numerous caverns. The fossils are in every stage of
preservation, from shells retaining portions of their animal matter and colour
to others which are mere casts. The limestone passes downward into a sandstone
and conglomerate, below which is clay and blue marl, from which perfect shells
and corals may be disengaged. The clay sometimes alternates with yellow sand.
South of the plain of Catania is a region in which the tertiary beds are
intermixed with volcanic matter, which has been for the most part the product of
submarine eruptions. It appears that, while the clay, sand, and yellow limestone
before mentioned were in course of deposition at the bottom of the sea,
volcanoes burst out beneath the waters, like that of Graham Island, in 1831, and
these explosions recurred again and again at distant intervals of time. Volcanic
ashes and sand were showered down and spread by the waves and currents so as to
form strata of tuff, which are found intercalated between beds of limestone and
clay containing marine shells, the thickness of the whole mass exceeding 2000
feet. The fissures through which the lava rose may be seen in many places,
forming what are called DIKES.
(FIGURE 133. Pecten jacobaeus; half natural size.)
No shell is more conspicuous in these Sicilian strata than the great scallop,
Pecten jacobaeus (Figure 133), now so common in the neighbouring seas. The more
we reflect on the preponderating number of this and other recent shells, the
more we are surprised at the great thickness, solidity, and height above the sea
of the rocky masses in which they are entombed, and the vast amount of
geographical change which has taken place since their origin. It must be
remembered that, before they began to emerge, the uppermost strata of the whole
must have been deposited under water. In order, therefore, to form a just
conception of their antiquity, we must first examine singly the innumerable
minute parts of which the whole is made up, the successive beds of shells,
corals, volcanic ashes, conglomerates, and sheets of lava; and we must
afterwards contemplate the time required for the gradual upheaval of the rocks,
and the excavation of the valleys. The historical period seems scarcely to form
an appreciable unit in this computation, for we find ancient Greek temples, like
those of Girgenti (Agrigentum), built of the modern limestone of which we are
speaking, and resting on a hill composed of the same; the site having remained
to all appearances unaltered since the Greeks first colonised the island.
It follows, from the modern geological date of these rocks, that the fauna and
flora of a large part of Sicily are of higher antiquity than the country itself.
The greater part of the island has been raised above the sea since the epoch of
existing species, and the animals and plants now inhabiting it must have
migrated from adjacent countries, with whose productions the species are now
identical. The average duration of species would seem to be so great that they
are destined to outlive many important changes in the configuration of the
earth's surface, and hence the necessity for those innumerable contrivances by
which they are enabled to extend their range to new lands as they are formed,
and to escape from those which sink beneath the sea.
NEWER PLIOCENE STRATA OF THE UPPER VAL D'ARNO.
When we ascend the Arno for about ten miles above Florence, we arrive at a deep
narrow valley called the Upper Val d'Arno, which appears once to have been a
lake, at a time when the valley below Florence was an arm of the sea. The
horizontal lacustrine strata of this upper basin are twelve miles long and two
broad. The depression which they fill has been excavated out of Eocene and
Cretaceous rocks, which form everywhere the sides of the valley in highly
inclined stratification. The thickness of the more modern and unconformable beds
is about 750 feet, of which the upper 200 feet consist of Newer Pliocene strata,
while the lower are Older Pliocene. The newer series are made up of sands and a
conglomerate called "sansino." Among the imbedded fossil mammalia are Mastodon
arvernensis, Elephas meridionalis, Rhinoceros etruscus, Hippopotamus major, and
remains of the genera bear, hyaena, and felis, nearly all of which occur in the
Cromer forest-bed (see Chapter 13).
In the same upper strata are found, according to M. Gaudin, the leaves and cones
of Glyptostrobus europaeus, a plant closely allied to G. heterophyllus, now
inhabiting the north of China and Japan. This conifer had a wide range in time,
having been traced back to the Lower Miocene strata of Switzerland, and being
common at Oeningen in the Upper Miocene, as we shall see in the sequel (Chapter
14.)
OLDER PLIOCENE OF ITALY.-- SUBAPENNINE STRATA.
The Apennines, it is well-known, are composed chiefly of Secondary or Mesozoic
rocks, forming a chain which branches off from the Ligurian Alps and passes down
the middle of the Italian peninsula. At the foot of these mountains, on the side
both of the Adriatic and the Mediterranean, are found a series of tertiary
strata, which form, for the most part, a line of low hills occupying the space
between the older chain and the sea. Brocchi was the first Italian geologist who
described this newer group in detail, giving it the name of the Subapennine.
Though chiefly composed of Older Pliocene strata, it belongs, nevertheless, in
part, both to older and newer members of the tertiary series. The strata, for
example, of the Superga, near Turin, are Miocene; those of Asti and Parma Older
Pliocene, as is the blue marl of Sienna; while the shells of the incumbent
yellow sand of the same territory approach more nearly to the recent fauna of
the Mediterranean, and may be Newer Pliocene.
We have seen that most of the fossil shells of the Older Pliocene strata of
Suffolk which are of recent species are identical with testacea now living in
British seas, yet some of them belong to Mediterranean species, and a few even
of the genera are those of warmer climates. We might therefore expect, in
studying the fossils of corresponding age in countries bordering the
Mediterranean, to find among them some species and genera of warmer latitudes.
Accordingly, in the marls belonging to this period at Asti, Parma, Sienna, and
parts of the Tuscan and Roman territories, we observe the genera Conus, Cypraea,
Strombus, Pyrula, Mitra, Fasciolaria, Sigaretus, Delphinula, Ancillaria, Oliva,
Terebellum, Terebra, Perna, Plicatula, and Corbis, some characteristic of
tropical seas, others represented by species more numerous or of larger size
than those now proper to the Mediterranean.
OLDER PLIOCENE FLORA OF ITALY.
(FIGURE 134. Oreodaphne Heerii.
Leaf half natural size. (Feuilles fossiles de la Toscane.))
I have already alluded to the Newer Pliocene deposits of the Upper Val d'Arno
above Florence, and stated that below those sands and conglomerates, containing
the remains of the Elephas meridionalis and other associated quadrupeds, lie an
older horizontal and conformable series of beds, which may be classed as Older
Pliocene. They consist of blue clays with some subordinate layers of lignite,
and exhibit a richer flora than the overlying Newer Pliocene beds, and one
receding farther from the existing vegetation of Europe. They also comprise more
species common to the antecedent Miocene period. Among the genera of flowering
plants, M. Gaudin enumerates pine, oak, evergreen oak, plum, plane, alder, elm,
fig, laurel, maple, walnut, birch, buckthorn, hickory, sumach, sarsaparilla,
sassafras, cinnamon, Glyptostrobus, Taxodium, Sequoia, Persea, Oreodaphne
(Figure 134), Cassia, and Psoralea, and some others. This assemblage of plants
indicates a warm climate, but not so subtropical an one as that of the Upper
Miocene period, which will presently be considered.
(FIGURE 135. Liquidambar europaeum, var. trilobatum, A. Br. (sometimes four-
lobed, and more commonly five-lobed).
a. Leaf, half natural size.
b. Part of same, natural size.
c. Fruit, natural size.
d. Seed, natural size. Oeningen.)
M. Gaudin, jointly with the Marquis Strozzi, has thrown much light on the botany
of beds of the same age in another part of Tuscany, at a place called Montajone,
between the rivers Elsa and Evola, where, among other plants, is found the
Oreodaphne Heerii, Gaud. (See Figure 134), which is probably only a variety of
Oreodaphne foetens, or the laurel called the Til in Madeira, where, as in the
Canaries, it constitutes a large portion of the native woods, but can not now
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