The Student's Elements of Geology
by
Sir Charles Lyell
Part 7 out of 14
page 58, who cites Lander's Travels.) Besides, we know not, in such cases, how
far the fluviatile sediment and organic remains of the river and the land may be
carried out from the coast, and spread over the bed of the sea. I have shown,
when treating of the Mississippi, that a more ancient delta, including species
of shells such as now inhabit Louisiana, has been upraised, and made to occupy a
wide geographical area, while a newer delta is forming; and the possibility of
such movements and their effects must not be lost sight of when we speculate on
the origin of the Wealden. (See Chapter 6 and Second Visit to the United States
volume 2 chapter 34.)
It may be asked where the continent was placed, from the ruins of which the
Wealden strata were derived, and by the drainage of which a great river was fed.
If the Wealden was gradually going downward 1000 feet or more perpendicularly, a
large body of fresh-water would not continue to be poured into the sea at the
same point. The adjoining land, if it participated in the movement, could not
escape being submerged. But we may suppose such land to have been stationary, or
even undergoing contemporaneous slow upheaval. There may have been an ascending
movement in one region, and a descending one in a contiguous parallel zone of
country. But even if that were the case, it is clear that finally an extensive
depression took place in that part of Europe where the deep sea of the
Cretaceous period was afterwards brought in.
THICKNESS OF THE WEALDEN.
In the Weald area itself, between the North and South Downs, fresh-water beds to
the thickness of 1600 feet are known, the base not being reached. Probably the
thickness of the whole Wealden series, as seen in Swanage Bay, can not be
estimated as less than 2000 feet.
WEALDEN FLORA.
The flora of the Wealden is characterised by a great abundance of Coniferae,
Cycadeae, anD Ferns, and by the absence of leaves and fruits of Dicotyledonous
Angiosperms. The discovery in 1855, in the Hastings beds of the Isle of Wight,
of Gyrogonites, or spore-vessels of the Chara, was the first example of that
genus of plants, so common in the tertiary strata, being found in a Secondary or
Mesozoic rock.
CHAPTER XIX.
JURASSIC GROUP.-- PURBECK BEDS AND OOLITE.
The Purbeck Beds a Member of the Jurassic Group.
Subdivisions of that Group.
Physical Geography of the Oolite in England and France.
Upper Oolite.
Purbeck Beds.
New Genera of fossil Mammalia in the Middle Purbeck of Dorsetshire.
Dirt-bed or ancient Soil.
Fossils of the Purbeck Beds.
Portland Stone and Fossils.
Kimmeridge Clay.
Lithographic Stone of Solenhofen.
Archaeopteryx.
Middle Oolite.
Coral Rag.
Nerinaea Limestone.
Oxford Clay, Ammonites and Belemnites.
Kelloway Rock.
Lower, or Bath, Oolite.
Great Plants of the Oolite.
Oolite and Bradford Clay.
Stonesfield Slate.
Fossil Mammalia.
Fuller's Earth.
Inferior Oolite and Fossils.
Northamptonshire Slates.
Yorkshire Oolitic Coal-field.
Brora Coal.
Palaeontological Relations of the several Subdivisions of the Oolitic group.
CLASSIFICATION OF THE OOLITE.
Immediately below the Hastings Sands we find in Dorsetshire another remarkable
fresh-water formation, called THE PURBECK, because it was first studied in the
sea-cliffs of the peninsula of Purbeck in that county. These beds are for the
most part of fresh-water origin, but the organic remains of some few
intercalated beds are marine, and show that the Purbeck series has a closer
affinity to the Oolitic group, of which it may be considered as the newest or
uppermost member.
In England generally, and in the greater part of Europe, both the Wealden and
Purbeck beds are wanting, and the marine cretaceous group is followed
immediately, in the descending order, by another series called the Jurassic. In
this term, the formations commonly designated as "the Oolite and Lias" are
included, both being found in the Jura Mountains. The Oolite was so named
because in the countries where it was first examined the limestones belonging to
it had an Oolitic structure (see Chapter 3). These rocks occupy in England a
zone nearly thirty miles in average breadth, which extends across the island,
from Yorkshire in the north-east, to Dorsetshire in the south-west. Their
mineral characters are not uniform throughout this region; but the following are
the names of the principal subdivisions observed in the central and south-
eastern parts of England.
TABLE 19.1. OOLITE.
UPPER OOLITE:
a. Purbeck beds.
b. Portland stone and sand.
c. Kimmeridge clay.
MIDDLE OOLITE:
d. Coral rag.
e. Oxford clay, and Kelloway rock.
LOWER OOLITE:
f. Cornbrash and Forest marble.
g. Great Oolite and Stonesfield slate.
h. Fuller's earth.
i. Inferior Oolite.
The Upper Oolitic system of the Table 19.1 has usually the Kimmeridge clay for
its base; the Middle Oolitic system, the Oxford clay. The Lower system reposes
on the Lias, an argillo-calcareous formation, which some include in the Lower
Oolite, but which will be treated of separately in the next chapter. Many of
these subdivisions are distinguished by peculiar organic remains; and, though
varying in thickness, may be traced in certain directions for great distances,
especially if we compare the part of England to which the above-mentioned type
refers with the north-east of France and the Jura Mountains adjoining. In that
country, distant above 400 geographical miles, the analogy to the accepted
English type, notwithstanding the thinness or occasional absence of the clays,
is more perfect than in Yorkshire or Normandy.
PHYSICAL GEOGRAPHY.
The alternation, on a grand scale, of distinct formations of clay and limestone
has caused the oolitic and liassic series to give rise to some marked features
in the physical outline of parts of England and France. Wide valleys can usually
be traced throughout the long bands of country where the argillaceous strata
crop out; and between these valleys the limestones are observed, forming ranges
of hills or more elevated grounds. These ranges terminate abruptly on the side
on which the several clays rise up from beneath the calcareous strata.
(FIGURE 298. Section through Lias (left), Lower Oolite, Oxford Clay, Middle
Oolite, Kim. Clay. Upper Oolite. Gault, Chalk and London Clay (right).)
Figure 298 will give the reader an idea of the configuration of the surface now
alluded to, such as may be seen in passing from London to Cheltenham, or in
other parallel lines, from east to west, in the southern part of England. It has
been necessary, however, in this drawing, greatly to exaggerate the inclination
of the beds, and the height of the several formations, as compared to their
horizontal extent. It will be remarked, that the lines of steep slope, or
escarpment, face towards the west in the great calcareous eminences formed by
the chalk and the Upper, Middle, and Lower Oolites; and at the base of which we
have respectively the Gault, Kimmeridge clay, Oxford clay, and Lias. This last
forms, generally, a broad vale at the foot of the escarpment of inferior Oolite,
but where it acquires considerable thickness, and contains solid beds of
marlstone, it occupies the lower part of the escarpment.
The external outline of the country which the geologist observes in travelling
eastward from Paris to Metz, is precisely analogous, and is caused by a similar
succession of rocks intervening between the tertiary strata and the Lias; with
this difference, however, that the escarpments of Chalk, Upper, Middle, and
Lower Oolites face towards the east instead of the west. It is evident,
therefore, that the denuding causes (see Chapter 6) have acted similarly over an
area several hundred miles in diameter, removing the softer clays more
extensively than the limestones, and causing these last to form steep slopes or
escarpments wherever the harder calcareous rock was based upon a more yielding
and destructible formation.
UPPER OOLITE.
PURBECK BEDS.
These strata, which we class as the uppermost member of the Oolite, are of
limited geographical extent in Europe, as already stated, but they acquire
importance when we consider the succession of three distinct sets of fossil
remains which they contain. Such repeated changes in organic life must have
reference to the history of a vast lapse of ages. The Purbeck beds are finely
exposed to view in Durdlestone Bay, near Swanage, Dorsetshire, and at Lulworth
Cove and the neighbouring bays between Weymouth and Swanage. At Meup's Bay, in
particular, Professor E. Forbes examined minutely, in 1850, the organic remains
of this group, displayed in a continuous sea-cliff section, and it appears from
his researches that the Upper, Middle, and Lower Purbecks are each marked by
peculiar species of organic remains, these again being different, so far as a
comparison has yet been instituted, from the fossils of the overlying Hastings
Sands and Weald Clay.
UPPER PURBECK.
(FIGURE 299. Cyprides from the Upper Purbeck.
a. Cypris gibbosa, E. Forbes.
b. Cypris tuberculata, E. Forbes.
c. Cypris leguminella, E. Forbes.)
The highest of the three divisions is purely fresh-water, the strata, about
fifty feet in thickness, containing shells of the genera Paludina, Physa,
Limnaea, Planorbis, Valvata, Cyclas, and Unio, with Cyprides and fish. All the
species seem peculiar, and among these the Cyprides are very abundant and
characteristic (see Figure 299, a, b, c.)
The stone called "Purbeck Marble," formerly much used in ornamental architecture
in the old English cathedrals of the southern counties, is exclusively procured
from this division.
MIDDLE PURBECK.
Next in succession is the Middle Purbeck, about thirty feet thick, the uppermost
part of which consists of fresh-water limestone, with cyprides, turtles, and
fish, of different species from those in the preceding strata. Below the
limestone are brackish-water beds full of Cyrena, and traversed by bands
abounding in Corbula and Melania. These are based on a purely marine deposit,
with Pecten, Modiola, Avicula, and Thracia. Below this, again, come limestones
and shales, partly of brackish and partly of fresh-water origin, in which many
fish, especially species of Lepidotus and Microdon radiatus, are found, and a
crocodilian reptile named Macrorhynchus. Among the mollusks, a remarkable ribbed
Melania, of the section Chilina, occurs.
(FIGURE 300. Ostrea distorta, Sowerby. Cinder-bed. Middle Purbeck.)
(FIGURE 301. Hemicidaris Purbeckensis, E. Forbes. Middle Purbeck.)
(FIGURE 302. Cyprides from the Middle Purbecks.
a. Cypris striato-punctata, E. Forbes.
b. Cypris fasciculata, E. Forbes.
c. Cypris granulata, Sowerby.)
(FIGURE 303. Physa Bristovii, E. Forbes. Middle Purbeck.)
Immediately below is a great and conspicuous stratum, twelve feet thick, formed
of a vast accumulation of shells of Ostrea distorta (Figure 300), long familiar
to geologists under the local name of "Cinder-bed." In the uppermost part of
this bed Professor Forbes discovered the first echinoderm (Figure 301) as yet
known in the Purbeck series, a species of Hemicidaris, a genus characteristic of
the Oolitic period, and scarcely, if at all, distinguishable from a previously
known Oolitic fossil. It was accompanied by a species of Perna. Below the
Cinder-bed fresh-water strata are again seen, filled in many places with species
of Cypris (Figure 302, a, b, c), and with Valvata, Paludina, Planorbis, Limnaea,
Physa (Figure 303), and Cyclas, all different from any occurring higher in the
series. It will be seen that Cypris fasciculata (Figure 302, b) has tubercles at
the end only of each valve, a character by which it can be immediately
recognised. In fact, these minute crustaceans, almost as frequent in some of the
shales as plates of mica in a micaceous sandstone, enable geologists at once to
identify the Middle Purbeck in places far from the Dorsetshire cliffs, as, for
example, in the Vale of Wardour in Wiltshire. Thick beds of chert occur in the
Middle Purbeck filled with mollusca and cyprides of the genera already
enumerated, in a beautiful state of preservation, often converted into
chalcedony. Among these Professor Forbes met with gyrogonites (the spore-vessels
of Chara), plants never until 1851 discovered in rocks older than the Eocene.
About twenty feet below the "Cinder-bed" is a stratum two or three inches thick,
in which fossil mammalia presently to be mentioned occur, and beneath this a
thin band of greenish shales, with marine shells and impressions of leaves like
those of a large Zostera, forming the base of the Middle Purbeck.
FOSSIL MAMMALIA OF THE MIDDLE PURBECK.
In 1852, after alluding to the discovery of numerous insects and air-breathing
mollusca in the Purbeck strata, I remarked that, although no mammalia had then
been found, "it was too soon to infer their non-existence on mere negative
evidence." (Elements of Geology 4th edition.) Only two years after this remark
was in print, Mr. W.R. Brodie found in the Middle Purbeck, about twenty feet
below the "Cinder-bed" above alluded to, in Durdlestone Bay, portions of several
small jaws with teeth, which Professor Owen recognised as belonging to a small
mammifer of the insectivorous class, more closely allied in its dentition to the
Amphitherium (or Thylacotherium) than to any existing type.
Four years later (in 1856) the remains of several other species of warm-blooded
quadrupeds were exhumed by Mr. S.H. Beckles, F.R.S., from the same thin bed of
marl near the base of the Middle Purbeck. In this marly stratum many reptiles,
several insects, and some fresh-water shells of the genera Paludina, Planorbis,
and Cyclas, were found.
Mr. Beckles had determined thoroughly to explore the thin layer of calcareous
mud from which in the suburbs of Swanage the bones of the Spalacotherium had
already been obtained, and in three weeks he brought to light from an area forty
feet long and ten wide, and from a layer the average thickness of which was only
five inches, portions of the skeletons of six new species of mammalia, as
interpreted by Dr. Falconer, who first examined them. Before these interesting
inquiries were brought to a close, the joint labours of Professor Owen and Dr.
Falconer had made it clear that twelve or more species of mammalia characterised
this portion of the Middle Purbeck, most of them insectivorous or predaceous,
varying in size from that of a mole to that of the common polecat, Mustela
putorius. While the majority had the character of insectivorous marsupials, Dr.
Falconer selected one as differing widely from the rest, and pointed out that in
certain characters it was allied to the living Kangaroo-rat, or Hypsiprymnus,
ten species of which now inhabit the prairies and scrub-jungle of Australia,
feeding on plants, and gnawing scratched-up roots. A striking peculiarity of
their dentition, one in which they differ from all other quadrupeds, consists in
their having a single large pre-molar, the enamel of which is furrowed with
vertical grooves, usually seven in number.
(FIGURE 304. Pre-molar of the recent Australian Hypsiprymnus Gaimardi, showing 7
grooves, at right angles to the length of the jaw, magnified 3 1/2 diameters.)
(FIGURE 305. Third and largest pre-molar (lower jaw) of Plagiaulax Becklesii,
magnified 5 1/2 diameters, showing 7 diagonal grooves.)
(FIGURE 306. Plagiaulex Becklesii, Falconer. Middle Purbeck. Right ramus of
lower jaw, magnified two diameters.
a. Incisor.
b, c. Line of vertical fracture behind the pre-molars.
pm. Three pre-molars, the third and last (much larger than the other two taken
together) being divided by a crack.
m. Sockets of two missing molars.)
The largest pre-molar (see Figure 305) in the fossil genus exhibits in like
manner seven parallel grooves, producing by their termination a similar serrated
edge in the crown; but their direction is diagonal-- a distinction, says Dr.
Falconer, which is "trivial, not typical." As these oblique furrows form so
marked a character of the majority of the teeth, Dr. Falconer gave to the fossil
the generic name of Plagiaulax. The shape and relative size of the incisor, a,
Figure 306, exhibit a no less striking similarity to Hypsiprymnus. Nevertheless,
the more sudden upward curve of this incisor, as well as other characters of the
jaw, indicate a great deviation in the form of Plagiaulax from that of the
living kangaroo-rats.
There are two fossil specimens of lower jaws of this genus evidently referable
to two distinct species extremely unequal in size and otherwise distinguishable.
The Plagiaulax Becklesii (Figure 306) was about as big as the English squirrel
or the flying phalanger of Australia (Petaurus Australis, Waterhouse). The
smaller fossil, having only half the linear dimensions of the other, was
probably only one-twelfth of its bulk. It is of peculiar geological interest,
because, as shown by Dr. Falconer, its two back molars bear a decided
resemblance to those of the Triassic Microlestes (Figure 389 Chapter 19), the
most ancient of known mammalia, of which an account will be given in Chapter 21.
Up to 1857 all the mammalian remains discovered in secondary rocks had consisted
solely of single branches of the lower jaw, but in that year Mr. Beckles
obtained the upper portion of a skull, and on the same slab the lower jaw of
another quadruped with eight molars, a large canine, and a broad and thick
incisor. It has been named Triconodon from its bicuspid teeth, and is supposed
to have been a small insectivorous marsupial, about the size of a hedgehog.
Other jaws have since been found indicating a larger species of the same genus.
Professor Owen has proposed the name of Galestes for the largest of the mammalia
discovered in 1858 in Purbeck, equalling the polecat (Mustela putorius) in size.
It is supposed to have been predaceous and marsupial.
Between forty and fifty pieces or sides of lower jaws with teeth have been found
in oolitic strata in Purbeck; only five upper maxillaries, together with one
portion of a separate cranium, occur at Stonesfield, and it is remarkable that
with these there were no examples in Purbeck of an entire skeleton, nor of any
considerable number of bones in juxtaposition. In several portions of the matrix
there were detached bones, often much decomposed, and fragments of others
apparently mammalian; but if all of them were restored, they would scarcely
suffice to complete the five skeletons to which the five upper maxillaries above
alluded to belonged. As the average number of pieces in each mammalian skeleton
is about 250, there must be many thousands of missing bones; and when we
endeavour to account for their absence, we are almost tempted to indulge in
speculations like those once suggested to me by Dr. Buckland, when he tried to
solve the enigma in reference to Stonesfield; "The corpses," he said, "of
drowned animals, when they float in a river, distended by gases during
putrefaction, have often their lower jaw hanging loose, and sometimes it has
dropped off. The rest of the body may then be drifted elsewhere, and sometimes
may be swallowed entire by a predaceous reptile or fish, such as an ichthyosaur
or a shark."
As all the above-mentioned Purbeck marsupials, belonging to eight or nine genera
and to about fourteen species, insectivorous, predaceous, and herbivorous, have
been obtained from an area less than 500 square yards in extent, and from a
single stratum no more than a few inches thick, we may safely conclude that the
whole lived together in the same region, and in all likelihood they constituted
a mere fraction of the mammalia which inhabited the lands drained by one river
and its tributaries. They afford the first positive proof as yet obtained of the
co-existence of a varied fauna of the highest class of vertebrata with that
ample development of reptile life which marks all the periods from the Trias to
the Lower Cretaceous inclusive, and with a gymnospermous flora, or that state of
the vegetable kingdom when cycads and conifers predominated over all kinds of
plants, except the ferns, so far, at least, as our present imperfect knowledge
of fossil botany entitles us to speak.
TABLE 19.2. NUMBER AND DISTRIBUTION OF ALL THE KNOWN SPECIES OF FOSSIL MAMMALIA
FROM STRATA OLDER THAN THE PARIS GYPSUM, OR THAN THE BEMBRIDGE SERIES OF THE
ISLE OF WIGHT.
TERTIARY:
Headon Series and beds between the Paris Gypsum and the Gres de Beauchamp: 14:
10 English, 4 French.
Barton Clay and Sables de Beauchamp: 0.
Bagshot Beds, Calcaire Grossier, and Upper Soissonnais of Cuisse-Lamotte: 20: 16
French, 1 English, 3 United States (I allude to several Zeuglodons found in
Alabama, and referred by some zoologists to three species.)
London Clay, including the Kyson Sand: 7 English.
Plastic Clay and Lignite: 9: 7 French, 2 English.
Sables de Bracheux: 1 French.
Thanet Sands and Lower Landenian of Belgium: 0.
SECONDARY:
Maestricht Chalk: 0.
White Chalk: 0.
Chalk Marl: 0.
Chloritic Series (Upper Greensand): 0.
Gault: 0.
Neocomian (Lower Greensand): 0.
Wealden: 0.
Upper Purbeck Oolite : 0.
Middle Purbeck Oolite : 14 Swanage.
Lower Purbeck Oolite: 0.
Portland Oolite: 0.
Kimmeridge Clay: 0.
Coral Rag: 0.
Oxford Clay: 0.
Great Oolite: 4 Stonesfield.
Inferior Oolite: 0.
Lias: 0.
Upper Trias: 4 Wurtemberg, Somersetshire. N. Carolina.
Middle Trias: 0.
Lower Trias: 0.
PRIMARY.
Permian: 0.
Carboniferous : 0.
Devonian: 0.
Silurian: 0.
Cambrian: 0.
Laruentian: 0.
Table 19.2 will enable the reader to see at a glance how conspicuous a part,
numerically considered, the mammalian species of the Middle Purbeck now play
when compared with those of other formations more ancient than the Paris gypsum,
and, at the same time, it will help him to appreciate the enormous hiatus in the
history of fossil mammalia which at present occurs between the Eocene and
Purbeck periods, and between the latter and the Stonesfield Oolite, and between
this again and the Trias.
The Sables de Bracheux, enumerated in the Tertiary division of the table,
supposed by Mr. Prestwich to be somewhat newer than the Thanet Sands, and by M.
Hebert to be of about that age, have yielded at La Fere the Arctocyon
(Palaeocyon) primaevus, the oldest known tertiary mammal.
It is worthy of notice, that in the Hastings Sands there are certain layers of
clay and sandstone in which numerous footprints of quadrupeds have been found by
Mr. Beckles, and traced by him in the same set of rocks through Sussex and the
Isle of Wight. They appear to belong to three or four species of reptiles, and
no one of them to any warm-blooded quadruped. They ought, therefore, to serve as
a warning to us, when we fail in like manner to detect mammalian footprints in
older rocks (such as the New Red Sandstone), to refrain from inferring that
quadrupeds, other than reptilian, did not exist or pre-exist.
But the most instructive lesson read to us by the Purbeck strata consists in
this: They are all, with the exception of a few intercalated brackish and marine
layers, of fresh-water origin; they are 160 feet in thickness, have been well
searched by skillful collectors, and by the late Edward Forbes in particular,
who studied them for months consecutively. They have been numbered, and the
contents of each stratum recorded separately, by the officers of the Geological
Survey of Great Britain. They have been divided into three distinct groups by
Forbes, each characterised by the same genera of pulmoniferous mollusca and
cyprides, these genera being represented in each group by different species;
they have yielded insects of many orders, and the fruits of several plants; and
lastly, they contain "dirt-beds," or old terrestrial surfaces and vegetable
soils at different levels, in some of which erect trunks and stumps of cycads
and conifers, with their roots still attached to them, are preserved. Yet when
the geologist inquires if any land-animals of a higher grade than reptiles lived
during any one of these three periods, the rocks are all silent, save one thin
layer a few inches in thickness; and this single page of the earth's history has
suddenly revealed to us in a few weeks the memorials of so many species of
fossil mammalia, that they already outnumber those of many a subdivision of the
tertiary series, and far surpass those of all the other secondary rocks put
together!
LOWER PURBECK.
(FIGURE 307. Cyprides from the Lower Purbeck.
a. Cypris Purbeckensis, Forbes.
b. Same magnified.
c. Cypris punctata, Forbes.
d, e. Two views magnified of the same.)
Beneath the thin marine band mentioned above as the base of the Middle Purbeck,
some purely fresh-water marls occur, containing species of Cypris (Figure 307 a,
c), Valvata, and Limnaea, different from those of the Middle Purbeck. This is
the beginning of the inferior division, which is about 80 feet thick. Below the
marls are seen, at Meup's Bay, more than thirty feet of brackish-water strata,
abounding in a species of Serpula, allied to, if not identical with, Serpula
coacervites, found in beds of the same age in Hanover. There are also shells of
the genus Rissoa (of the subgenus Hydrobia), and a little Cardium of the
subgenus Protocardium, in these marine beds, together with Cypris. Some of the
cypris-bearing shales are strangely contorted and broken up, at the west end of
the Isle of Purbeck. The great dirt-bed or vegetable soil containing the roots
and stools of Cycadeae, which I shall presently describe, underlies these marls,
and rests upon the lowest fresh-water limestone, a rock about eight feet thick,
containing Cyclas, Valvata, and Limnaea, of the same species as those of the
uppermost part of the Lower Purbeck, or above the dirt-bed. The fresh-water
limestone in its turn rests upon the top beds of the Portland stone, which,
although it contains purely marine remains, often consists of a rock
undistinguishable in mineral character from the Lowest Purbeck limestone.
DIRT-BED OR ANCIENT SURFACE-SOIL.
(FIGURE 308. Mantellia nidiformis, Brongniart. The upper part shows the woody
stem, the lower part the bases of the leaves.)
The most remarkable of all the varied succession of beds enumerated in the above
list is that called by the quarrymen "the dirt," or "black dirt," which was
evidently an ancient vegetable soil. It is from 12 to 18 inches thick, is of a
dark brown or black colour, and contains a large proportion of earthy lignite.
Through it are dispersed rounded and sub-angular fragments of stone, from 3 to 9
inches in diameter, in such numbers that it almost deserves the name of gravel.
I also saw in 1866, in Portland, a smaller dirt-bed six feet below the principal
one, six inches thick, consisting of brown earth with upright Cycads of the same
species, Mantellia nidiformis, as those found in the upper bed, but no
Coniferae. The weight of the incumbent strata squeezing down the compressible
dirt-bed has caused the Cycads to assume that form which has led the quarrymen
to call them "petrified birds' nests," which suggested to Brongniart the
specific name of nidiformis. I am indebted to Mr. Carruthers for Figure 308 of
one of these Purbeck specimens, in which the original cylindrical figure has
been less distorted than usual by pressure.
Many silicified trunks of coniferous trees, and the remains of plants allied to
Zamia and Cycas, are buried in this dirt-bed, and must have become fossil on the
spots where they grew. The stumps of the trees stand erect for a height of from
one to three feet, and even in one instance to six feet, with their roots
attached to the soil at about the same distances from one another as the trees
in a modern forest. The carbonaceous matter is most abundant immediately around
the stumps, and round the remains of fossil Cycadeae.
(FIGURE 309. Section in Isle of Portland, Dorset. (Buckland and De la
Beche.)showing layers (from top to bottom): Fresh-water calcareous slate: Dirt-
bed and ancient forest: Lowest fresh-water beds of the Lower Purbeck: and
Portland stone, marine.)
Besides the upright stumps above mentioned, the dirt-bed contains the stems of
silicified trees laid prostrate. These are partly sunk into the black earth, and
partly enveloped by a calcareous slate which covers the dirt-bed. The fragments
of the prostrate trees are rarely more than three or four feet in length; but by
joining many of them together, trunks have been restored, having a length from
the root to the branches of from 20 to 23 feet, the stems being undivided for 17
or 20 feet, and then forked. The diameter of these near the root is about one
foot; but I measured one myself, in 1866, which was 3 1/2 feet in diameter, said
by the quarrymen to be unusually large. Root-shaped cavities were observed by
Professor Henslow to descend from the bottom of the dirt-bed into the subjacent
fresh-water stone, which, though now solid, must have been in a soft and
penetrable state when the trees grew. The thin layers of calcareous slate
(Figure 309) were evidently deposited tranquilly, and would have been horizontal
but for the protrusion of the stumps of the trees, around the top of each of
which they form hemispherical concretions.
(FIGURE 310. Section of cliff east of Lulworth Cove. (Buckland and De la Beche.)
showing layers (from top to bottom): Fresh-water calcareous slate: Dirt-bed,
with stools of trees: Fresh-water: Portland stone, marine.)
The dirt-bed is by no means confined to the island of Portland, where it has
been most carefully studied, but is seen in the same relative position in the
cliffs east of Lulworth Cove, in Dorsetshire, where, as the strata have been
disturbed, and are now inclined at an angle of 45 degrees, the stumps of the
trees are also inclined at the same angle in an opposite direction-- a beautiful
illustration of a change in the position of beds originally horizontal (see
Figure 310).
From the facts above described we may infer, first, that those beds of the Upper
Oolite, called "the Portland," which are full of marine shells, were overspread
with fluviatile mud, which became dry land, and covered by a forest, throughout
a portion of the space now occupied by the south of England, the climate being
such as to permit the growth of the Zamia and Cycas. Secondly. This land at
length sank down and was submerged with its forests beneath a body of fresh-
water, from which sediment was thrown down enveloping fluviatile shells.
Thirdly. The regular and uniform preservation of this thin bed of black earth
over a distance of many miles, shows that the change from dry land to the state
of a fresh-water lake or estuary, was not accompanied by any violent denudation,
or rush of water, since the loose black earth, together with the trees which lay
prostrate on its surface, must inevitably have been swept away had any such
violent catastrophe taken place.
The forest of the dirt-bed, as before hinted, was not everywhere the first
vegetation which grew in this region. Besides the lower bed containing upright
Cycadeae, before mentioned, another has sometimes been found above it, which
implies oscillations in the level of the same ground, and its alternate
occupation by land and water more than once.
SUBDIVISIONS OF THE PURBECK.
It will be observed that the division of the Purbecks into upper, middle, and
lower, was made by Professor Forbes strictly on the principle of the entire
distinctness of the species of organic remains which they include. The lines of
demarkation are not lines of disturbance, nor indicated by any striking physical
characters or mineral changes. The features which attract the eye in the
Purbecks, such as the dirt-beds, the dislocated strata at Lulworth, and the
Cinder-bed, do not indicate any breaks in the distribution of organised beings.
"The causes which led to a complete change of life three times during the
deposition of the fresh-water and brackish strata must," says this naturalist,
"be sought for, not simply in either a rapid or a sudden change of their area
into land or sea, but in the great lapse of time which intervened between the
epochs of deposition at certain periods during their formation."
Each dirt-bed may, no doubt, be the memorial of many thousand years or
centuries, because we find that two or three feet of vegetable soil is the only
monument which many a tropical forest has left of its existence ever since the
ground on which it now stands was first covered with its shade. Yet, even if we
imagine the fossil soils of the Lower Purbeck to represent as many ages, we need
not be surprised to find that they do not constitute lines of separation between
strata characterised by different zoological types. The preservation of a layer
of vegetable soil, when in the act of being submerged, must be regarded as a
rare exception to a general rule. It is of so perishable a nature, that it must
usually be carried away by the denuding waves or currents of the sea, or by a
river; and many Purbeck dirt-beds were probably formed in succession and
annihilated, besides those few which now remain.
The plants of the Purbeck beds, so far as our knowledge extends at present,
consist chiefly of Ferns, Coniferae, and Cycadeae (Figure 308), without any
angiosperms; the whole more allied to the Oolitic than to the Cretaceous
vegetation. The same affinity is indicated by the vertebrate and invertebrate
animals. Mr. Brodie has found the remains of beetles and several insects of the
homopterous and trichopterous orders, some of which now live on plants, while
others are of such forms as hover over the surface of our present rivers.
PORTLAND OOLITE AND SAND (b, TABLE 19.1).
(FIGURE 311. Cerithium Portlandicum (=Terebra) Sowerby.
a. Cast of shell known as "Portland screw."
b. The shell itself. )
(FIGURE 312. Isastraea oblonga, M. Edw. and J. Haime. As seen on a polished
slab of chert from the Portland Sand, Tisbury.)
(FIGURE 313. Trigonia gibbosa. 1/2 natural size. Portland Stone, Tisbury.
a. The hinge.)
(FIGURE 314. Cardium dissimile. 1/4 natural size. Portland Stone.)
(FIGURE 315. Ostrea expansa. Portland Sand.)
The Portland Oolite has already been mentioned as forming in Dorsetshire the
foundation on which the fresh-water limestone of the Lower Purbeck reposes (see
above). It supplies the well-known building-stone of which St. Paul's and so
many of the principal edifices of London are constructed. About fifty species of
mollusca occur in this formation, among which are some ammonites of large size.
The cast of a spiral univalve called by the quarrymen the "Portland screw" (a,
Figure 311), is common; the shell of the same (b) being rarely met with. Also
Trigonia gibbosa (Figure 313) and Cardium dissimile (Figure 314). This upper
member rests on a dense bed of sand, called the Portland Sand, containing
similar marine fossils, below which is the Kimmeridge Clay. In England these
Upper Oolite formations are almost wholly confined to the southern counties. But
some fragments of them occur beneath the Neocomian or Speeton Clay on the coast
of Yorkshire, containing many more fossils common to the Portlandian of the
Continent than does the same formation in Dorsetshire. Corals are rare in this
formation, although one species is found plentifully at Tisbury, Wiltshire, in
the Portland Sand, converted into flint and chert, the original calcareous
matter being replaced by silex (Figure 312).
KIMMERIDGE CLAY.
The Kimmeridge Clay consists, in great part, of a bituminous shale, sometimes
forming an impure coal, several hundred feet in thickness. In some places in
Wiltshire it much resembles peat; and the bituminous matter may have been, in
part at least, derived from the decomposition of vegetables. But as impressions
of plants are rare in these shales, which contain ammonites, oysters, and other
marine shells, with skeletons of fish and saurians, the bitumen may perhaps be
of animal origin. Some of the saurians (Pliosaurus) in Dorsetshire are among the
most gigantic of their kind.
(FIGURE 316. Cardium striatulum. Kimmeridge Clay, Hartwell.)
(FIGURE 317. Ostrea deltoidea. Kimmeridge Clay, 1/4 natural size.)
(FIGURE 318. Gryphaea (Exogyra) virgula. Kimmeridge Clay.)
(FIGURE 319. Trigonellites latus, Park, Kimmeridge Clay.)
Among the fossils, amounting to nearly 100 species, may be mentioned Cardium
striatulum (Figure 316) and Ostrea deltoidea (Figure 317), the latter found in
the Kimmeridge Clay throughout England and the north of France, and also in
Scotland, near Brora. The Gryphaea virgula (Figure 318), also met with in the
Kimmeridge Clay near Oxford, is so abundant in the Upper Oolite of parts of
France as to have caused the deposit to be termed "marnes a gryphees virgules."
Near Clermont, in Argonne, a few leagues from St. Menehould, where these
indurated marls crop out from beneath the Gault, I have seen them, on
decomposing, leave the surface of every ploughed field literally strewed over
with this fossil oyster. The Trigonellites latus (Aptychus of some
authors)(Figure 319) is also widely dispersed through this clay. The real nature
of the shell, of which there are many species in oolitic rocks, is still a
matter of conjecture. Some are of opinion that the two plates have been the
gizzard of a cephalopod; others, that it may have formed a bivalve operculum of
the same.
SOLENHOFEN STONE.
(FIGURE 320. Skeleton of Pterodactylus crassirostris. Oolite of Pappenheim, near
Solenhofen.
a. This bone, consisting of four joints, is part of the fifth or outermost digit
elongated, as in bats, for the support of a wing.)
The celebrated lithographic stone of Solenhofen in Bavaria, appears to be of
intermediate age between the Kimmeridge clay and the Coral Rag, presently to be
described. It affords a remarkable example of the variety of fossils which may
be preserved under favourable circumstances, and what delicate impressions of
the tender parts of certain animals and plants may be retained where the
sediment is of extreme fineness. Although the number of testacea in this slate
is small, and the plants few, and those all marine, count Munster had determined
no less than 237 species of fossils when I saw his collection in 1833; and among
them no less than seven SPECIES of flying reptiles or pterodactyls (see Figure
320), six saurians, three tortoises, sixty species of fish, forty-six of
crustacea, and twenty-six of insects. These insects, among which is a libellula,
or dragon-fly, must have been blown out to sea, probably from the same land to
which the pterodactyls, and other contemporaneous air-breathers, resorted.
(FIGURE 321. Tail and feather of Archaeopteryx, from Solenhofen, and tail of
living bird for comparison.
A. Caudal vertebrae of Archaeopteryx macrura, Owen; with impression of tail-
feathers; one-fifth natural size.
B. Two caudal vertebrae of same; natural size.
C. Single feather, found in 1861 at Solenhofen, by Von Meyer, and called
Archaeopteryx lithographica; natural size.
D. Tail of recent vulture (Gyps Bengalensis) showing attachment of tail-feathers
in living birds; one-quarter natural size.
E. Profile of caudal vertebrae of same; one-third natural size.
e, e. Direction of tail-feathers when seen in profile.
f. Ploughshare bone or broad terminal joint (seen also in f, D.))
In the same slate of Solenhofen a fine example was met with in 1862 of the
skeleton of a bird almost entire, and retaining even its feathers so perfect
that the vanes as well as the shaft are preserved. The head was at first
supposed to be wanting, but Mr. Evans detected on the slab what seems to be the
impression of the cranium and beak, much resembling in size and shape that of
the jay or woodcock. This valuable specimen is now in the British Museum, and
has been called by Professor Owen Archaeopteryx macrura. Although anatomists
agree that it is a true bird, yet they also find that in the length of the bones
of the tail, and some other minor points of its anatomy, it approaches more
nearly to reptiles than any known living bird. In the living representatives of
the class Aves, the tail-feathers are attached to a coccygian bone, consisting
of several vertebrae united together, whereas in the Archaeopteryx the tail is
composed of twenty vertebrae, each of which supports a pair of quill-feathers.
The first five only of the vertebrae, as seen in A, have transverse processes,
the fifteen remaining ones become gradually longer and more tapering. The
feathers diverge outward from them at an angle of 45 degrees.
Professor Huxley in his late memoirs on the order of reptiles called
Dinosaurians, which are largely represented in all the formations, from the
Neocomian to the Trias inclusive, has shown that they present in their structure
many remarkable affinities to birds. But a reptile about two feet long, called
Compsognathus, lately found in the Stonesfield slate, makes a much greater
approximation to the class Aves than any Dinosaur, and therefore forms a closer
link between the classes Aves and Reptilia than does the Archaeopteryx.
It appears doubtful whether any species of British fossil, whether of the
vertebrate or invertebrate class, is common to the Oolite and Chalk. But there
is no similar break or discordance as we proceed downward, and pass from one to
another of the several leading members of the Jurassic group, the Upper, Middle,
and Lower Oolite, and the Lias, there being often a considerable proportion of
the mollusca, sometimes as much as a fourth, common to such divisions as the
Upper and Middle Oolite.
MIDDLE OOLITE.
CORAL RAG.
(FIGURE 322. Thecosmilia annularis, Milne Edwards and J. Haime. Coral Rag,
Steeple Ashton.)
(FIGURE 323. Thamnastraea. Coral Rag. Steeple Ashton.)
(FIGURE 324. Ostrea gregaria, Coral Rag, Steeple Ashton.)
One of the limestones of the Middle Oolite has been called the "Coral Rag,"
because it consists, in part, of continuous beds of petrified corals, most of
them retaining the position in which they grew at the bottom of the sea. In
their forms they more frequently resemble the reef-building polyparia of the
Pacific than do the corals of any other member of the Oolite. They belong
chiefly to the genera Thecosmilia (Figure 322), Protoseris, and Thamnastraea,
and sometimes form masses of coral fifteen feet thick. In Figure 323 of a
Thamnastraea from this formation, it will be seen that the cup-shaped cavities
are deepest on the right-hand side, and that they grow more and more shallow,
until those on the left side are nearly filled up. The last-mentioned stars are
supposed to represent a perfected condition, and the others an immature state.
These coralline strata extend through the calcareous hills of the north-west of
Berkshire, and north of Wilts, and again recur in Yorkshire, near Scarborough.
The Ostrea gregarea (Figure 324) is very characteristic of the formation in
England and on the Continent.
(FIGURE 325. Nerinaea Goodhallii, Fitton. Coral Rag, Weymouth. 1/4 natural
size.)
One of the limestones of the Jura, referred to the age of the English Coral Rag,
has been called "Nerinaean limestone" (Calcaire a Nerinees) by M. Thirria;
Nerinaea being an extinct genus of univalve shells (Figure 325) much resembling
the Cerithium in external form. Figure 325 shows the curious and continuous
ridges on the columnella and whorls.
OXFORD CLAY.
(FIGURE 326. Belemnites hastatus. Oxford Clay.)
(FIGURE 327. Ammonites Jason, Reinecke. (Syn. A. Elizabethae, Pratt. Oxford
Clay, Christian Malford, Wiltshire.)
(FIGURE 328. Belemnites Puzosianus, d'Orbigny. B. Owenii, Pierce. Oxford Clay,
Christian Malford, Wiltshire.
a. Section of the shell projecting from the phragmacone.
b-c. External covering to the ink-bag and phragmacone.
c, d. Osselet, or that portion commonly called the belemnite.
e. Conical chambered body called the phragmacone.
f. Position of ink-bag beneath the shelly covering.)
The coralline limestone, or "Coral Rag," above described, and the accompanying
sandy beds, called "calcareous grits," of the Middle Oolite, rest on a thick bed
of clay, called the "Oxford Clay," sometimes not less than 600 feet thick. In
this there are no corals, but great abundance of cephalopoda, of the genera
Ammonite and Belemnite (Figures 326 and 327). In some of the finely laminated
clays ammonites are very perfect, although somewhat compressed, and are
frequently found with the lateral lobe extended on each side of the opening of
the mouth into a horn-like projection (Figure 327). These were discovered in the
cuttings of the Great Western Railway, near Chippenham, in 1841, and have been
described by Mr. Pratt (Annals of Natural History November 1841).
Similar elongated processes have been also observed to extend from the shells of
some Belemnites discovered by Dr. Mantell in the same clay (see Figure 328),
who, by the aid of this and other specimens, has been able to throw much light
on the structure of singular extinct forms of cuttle-fish. (See Philosophical
Transactions 1850 page 363; also Huxley Memoirs of Geological Survey 1864;
Phillips Palaeontological Society.)
KELLOWAY ROCK.
The arenaceous limestone which passes under this name is generally grouped as a
member of the Oxford clay, in which it forms, in the south-west of England,
lenticular masses, 8 or 10 feet thick, containing at Kelloway, in Wiltshire,
numerous casts of ammonites and other shells. But in Yorkshire this calcareo-
arenaceous formation thickens to about 30 feet, and constitutes the lower part
of the Middle Oolite, extending inland from Scarborough in a southerly
direction. The number of mollusca which it contains is, according to Mr.
Etheridge, 143, of which only 34, or 23 1/2 per cent, are common to the Oxford
clay proper. Of the 52 Cephalopoda, 15 (namely 13 species of ammonite, the
Ancyloceras Calloviense and one Belemnite) are common to the Oxford Clay, giving
a proportion of nearly 30 per cent.
LOWER OOLITE.
CORNBRASH AND FOREST MARBLE.
The upper division of this series, which is more extensive than the preceding or
Middle Oolite, is called in England the Cornbrash, as being a brashy, easily
broken rock, good for corn land. It consists of clays and calcareous sandstones,
which pass downward into the Forest Marble, an argillaceous limestone, abounding
in marine fossils. In some places, as at Bradford, this limestone is replaced by
a mass of clay. The sandstones of the Forest Marble of Wiltshire are often
ripple-marked and filled with fragments of broken shells and pieces of drift-
wood, having evidently been formed on a coast. Rippled slabs of fissile oolite
are used for roofing, and have been traced over a broad band of country from
Bradford in Wilts, to Tetbury in Gloucestershire. These calcareous tile-stones
are separated from each other by thin seams of clay, which have been deposited
upon them, and have taken their form, preserving the undulating ridges and
furrows of the sand in such complete integrity, that the impressions of small
footsteps, apparently of crustaceans, which walked over the soft wet sands, are
still visible. In the same stone the claws of crabs, fragments of echini, and
other signs of a neighbouring beach, are observed. (P. Scrope Proceedings of the
Geological Society March 1831.)
GREAT (OR BATH) OOLITE.
(FIGURE 329. Eunomia radiata, Lamouroux. (Calamophyllia, Milne Edwards.)
a. Section transverse to the tubes.
b. Vertical section, showing the radiation of the tubes.
c. Portion of interior of tubes magnified, showing striated surface.)
Although the name of Coral Rag has been appropriated, as we have seen, to a
member of the Middle Oolite before described, some portions of the Lower Oolite
are equally entitled in many places to be called coralline limestones. Thus the
Great Oolite near Bath contains various corals, among which the Eunomia radiata
(Figure 329) is very conspicuous, single individuals forming masses several feet
in diameter; and having probably required, like the large existing brain-coral
(Meandrina) of the tropics, many centuries before their growth was completed.
(FIGURE 330. Apiocrinites rotundus, or Pear Encrinite; Miller. Fossil at
Bradford, Wilts.
a. Stem of Apiocrinites, and one of the articulations, natural size.
b. Section at Bradford of Great Oolite and overlying clay, containing the fossil
encrinites. (See text.)
c. Three perfect individuals of Apiocrinites, represented as they grew on the
surface of the Great Oolite.
d. Body of the Apiocrinites rotundus. Half natural size.)
(FIGURE 331. Apiocrinus.
a. Single plate of body of Apiocrinus, overgrown with serpulae and bryozoa.
Natural size. Bradford Clay.
b. Portion of the same magnified, showing the bryozoan Diastopora diluviana
covering one of the serpulae.)
Different species of crinoids, or stone-lilies, are also common in the same
rocks with corals; and, like them, must have enjoyed a firm bottom, where their
base of attachment remained undisturbed for years (c, Figure 330). Such fossils,
therefore, are almost confined to the limestones; but an exception occurs at
Bradford, near Bath, where they are enveloped in clay sometimes 60 feet thick.
In this case, however, it appears that the solid upper surface of the "Great
Oolite" had supported, for a time, a thick submarine forest of these beautiful
zoophytes, until the clear and still water was invaded by a current charged with
mud, which threw down the stone-lilies, and broke most of their stems short off
near the point of attachment. The stumps still remain in their original
position; but the numerous articulations, once composing the stem, arms, and
body of the encrinite, were scattered at random through the argillaceous deposit
in which some now lie prostrate. These appearances are represented in the
section b, Figure 330, where the darker strata represent the Bradford clay,
which is however a formation of such local development that in many places it
can not easily be separated from the clays of the overlying "forest-marble" and
underlying "fuller's earth." The upper surface of the calcareous stone below is
completely incrusted over with a continuous pavement, formed by the stony roots
or attachments of the Crinoidea; and besides this evidence of the length of time
they had lived on the spot, we find great numbers of single joints, or circular
plates of the stem and body of the encrinite, covered over with serpulae. Now
these serpulae could only have begun to grow after the death of some of the
stone-lilies, parts of whose skeletons had been strewed over the floor of the
ocean before the irruption of argillaceous mud. In some instances we find that,
after the parasitic serpulae were full grown, they had become incrusted over
with a bryozoan, called Diastopora diluviana (see b, Figure 331); and many
generations of these molluscoids had succeeded each other in the pure water
before they became fossil.
We may, therefore, perceive distinctly that, as the pines and cycadeous plants
of the ancient "dirt-bed," or fossil forest, of the Lower Purbeck were killed by
submergence under fresh water, and soon buried beneath muddy sediment, so an
invasion of argillaceous matter put a sudden stop to the growth of the Bradford
Encrinites, and led to their preservation in marine strata.
Such differences in the fossils as distinguish the calcareous and argillaceous
deposits from each other, would be described by naturalists as arising out of a
difference in the STATIONS of species; but besides these, there are variations
in the fossils of the higher, middle, and lower part of the oolitic series,
which must be ascribed to that great law of change in organic life by which
distinct assemblages of species have been adapted, at successive geological
periods, to the varying conditions of the habitable surface. In a single
district it is difficult to decide how far the limitation of species to certain
minor formations has been due to the local influence of STATIONS, or how far it
has been caused by time or the law of variation above alluded to. But we
recognise the reality of the last-mentioned influence, when we contrast the
whole oolitic series of England with that of parts of the Jura, Alps, and other
distant regions, where, although there is scarcely any lithological resemblance,
yet some of the same fossils remain peculiar in each country to the Upper,
Middle, and Lower Oolite formations respectively. Mr. Thurmann has shown how
remarkably this fact holds true in the Bernese Jura, although the argillaceous
divisions, so conspicuous in England, are feebly represented there, and some
entirely wanting.
(FIGURE 332. Terebratula digona, Sowerby. Natural size. Bradford Clay.)
(FIGURE 333. Purpuroidea nodulata. One-fourth natural size. Great Oolite,
Minchinhampton.)
(FIGURE 334. Cylindrites acutus. Sowb. Syn. Actaeon acutus. Great Oolite,
Minchinhampton.)
(FIGURE 335. Patella rugosa, Sowerby. Great Oolite.)
(FIGURE 336. Nerita costulata, Desh. Great Oolite.)
(FIGURE 337. Rimula (Emarginula) clathrata, Sowerby. Great Oolite.)
The calcareous portion of the Great Oolite consists of several shelly
limestones, one of which, called the Bath Oolite, is much celebrated as a
building-stone. In parts of Gloucestershire, especially near Minchinhampton, the
Great Oolite, says Mr. Lycett, "must have been deposited in a shallow sea, where
strong currents prevailed, for there are frequent changes in the mineral
character of the deposit, and some beds exhibit false stratification. In others,
heaps of broken shells are mingled with pebbles of rocks foreign to the
neighbourhood, and with fragments of abraded madrepores, dicotyledonous wood,
and crabs' claws. The shelly strata, also, have occasionally suffered
denudation, and the removed portions have been replaced by clay." In such
shallow-water beds shells of the genera Patella, Nerita, Rimula, Cylindrites are
common (see Figures 334 to 337); while cephalopods are rare, and instead of
ammonites and belemnites, numerous genera of carnivorous trachelipods appear.
Out of 224 species of univalves obtained from the Minchinhampton beds, Mr.
Lycett found no less than 50 to be carnivorous. They belong principally to the
genera Buccinum, Pleurotoma, Rostellaria, Murex, Purpuroidea (Figure 333), and
Fusus, and exhibit a proportion of zoophagous species not very different from
that which obtains in seas of the Recent period. These zoological results are
curious and unexpected, since it was imagined that we might look in vain for the
carnivorous trachelipods in rocks of such high antiquity as the Great Oolite,
and it was a received doctrine that they did not begin to appear in considerable
numbers till the Eocene period, when those two great families of cephalopoda,
the ammonites and belemnites, and a great number of other representatives of the
same class of chambered shells, had become extinct.
STONESFIELD SLATE: MAMMALIA.
(FIGURE 338. Elytron of Buprestis? Stonesfield.)
The slate of Stonesfield has been shown by Mr. Lonsdale to lie at the base of
the Great Oolite. (Proceedings of the Geological Society volume 1 page 414.) It
is a slightly oolitic shelly limestone, forming large lenticular masses imbedded
in sand only six feet thick, but very rich in organic remains. It contains some
pebbles of a rock very similar to itself, and which may be portions of the
deposit, broken up on a shore at low water or during storms, and redeposited.
The remains of belemnites, trigoniae, and other marine shells, with fragments of
wood, are common, and impressions of ferns, cycadeae, and other plants. Several
insects, also, and, among the rest, the elytra or wing-covers of beetles, are
perfectly preserved (see Figure 338), some of them approaching nearly to the
genus Buprestis. The remains, also, of many genera of reptiles, such as
Plesiosaur, Crocodile, and Pterodactyl, have been discovered in the same
limestone.
But the remarkable fossils for which the Stonesfield slate is most celebrated
are those referred to the mammiferous class. The student should be reminded that
in all the rocks described in the preceding chapters as older than the Eocene,
no bones of any land-quadruped, or of any cetacean, had been discovered until
the Spalacotherium of the Purbeck beds came to light in 1854. Yet we have seen
that terrestrial plants were not wanting in the Upper Cretaceous formation (see
Chapter 17), and that in the Wealden there was evidence of fresh-water sediment
on a large scale, containing various plants, and even ancient vegetable soils.
We had also in the same Wealden many land-reptiles and winged insects, which
render the absence of terrestrial quadrupeds the more striking. The want,
however, of any bones of whales, seals, dolphins, and other aquatic mammalia,
whether in the chalk or in the upper or middle oolite, is certainly still more
remarkable.
These observations are made to prepare the reader to appreciate more justly the
interest felt by every geologist in the discovery in the Stonesfield slate of no
less than ten specimens of lower jaws of mammiferous quadrupeds, belonging to
four different species and to three distinct genera, for which the names of
Amphitherium, Phascolotherium, and Stereognathus have been adopted.
(FIGURE 339. Tupaia tana. Right ramus of lower jaw. Natural size. A recent
insectivorous placental mammal, from Sumatra.)
(Figures 340 and 341. Part of lower jaw of Tupaia tana. Twice natural size.
(FIGURE 340. End view seen from behind, showing the very slight inflection of
the angle at c.)
(FIGURE 341. Side view of same.))
(Figures 342 and 343. Part of lower jaw of Didelphys Azarae; recent, Brazil.
Natural size.
(FIGURE 342. End view seen from behind, showing the inflection of the angle of
the jaw, c, d.)
(FIGURE 343. Side view of same.))
(FIGURE 344. Amphitherium Prevostii, Cuvier sp. Stonesfield Slate. Syn.
Thylacotherium Prevostii, Valenc.
a. Coronoid process.
b. Condyle.
c. Angle of jaw.
d. Double-fanged molars.)
(FIGURE 345. Amphitheriumm Broderipii, Owen. Natural size. Stonesfield Slate.)
It is now generally admitted that these or really the remains of mammalia
(although it was at first suggested that they might be reptiles), and the only
question open to controversy is limited to this point, whether the fossil
mammalia found in the Lower Oolite of Oxfordshire ought to be referred to the
marsupial quadrupeds, or to the ordinary placental series. Cuvier had long ago
pointed out a peculiarity in the form of the angular process (c, Figures 342 and
343) of the lower jaw, as a character of the genus Didelphys; and Professor Owen
has since confirmed the doctrine of its generality in the entire marsupial
series. In all these pouched quadrupeds this process is turned inward, as at c,
d, Figure 342, in the Brazilian opossum, whereas in the placental series, as at
c, Figures 340 and 341, there is an almost entire absence of such inflection.
The Tupaia Tana of Sumatra has been selected by Mr. Waterhouse for this
illustration, because the jaws of that small insectivorous quadruped bear a
great resemblance to those of the Stonesfield Amphitherium. By clearing away the
matrix from the specimen of Amphitherium Prevostii here represented (Figure
344), Professor Owen ascertained that the angular process (c) bent inward in a
slighter degree than in any of the known marsupialia; in short, the inflection
does not exceed that of the mole or hedgehog. This fact made him doubt whether
the Amphitherium might not be an insectivorous placental, although it offered
some points of approximation in its osteology to the marsupials, especially to
the Myrmecobius, a small insectivorous quadruped of Australia, which has nine
molars on each side of the lower jaw, besides a canine and three incisors. (A
figure of this recent Myrmecobius will be found in my Principles of Geology
chapter 9.) Another species of Amphitherium has been found at Stonesfield
(Figure 345), which differs from the former (Figure 344) principally in being
larger.
(FIGURE 346. Phascolotherium Bucklandi, Broderip, sp.
a. Natural size.
b. Molar of same, magnified.)
The second mammiferous genus discovered in the same slates was named originally
by Mr. Broderip Didelphys Bucklandi (see Figure 346), and has since been called
Phascolotherium by Owen. It manifests a much stronger likeness to the marsupials
in the general form of the jaw, and in the extent and position of its inflected
angle, while the agreement with the living genus Didelphys in the number of the
pre-molar and molar teeth is complete. (Owen's British Fossil Mammals page 62.)
In 1854 the remains of another mammifer, small in size, but larger than any of
those previously known, was brought to light. The generic name of Stereognathus
was given to it, and, as is usually the case in these old rocks (see above), it
consisted of part of a lower jaw, in which were implanted three double-fanged
teeth, differing in structure from those of all other known recent or extinct
mammals.
PLANTS OF THE OOLITE.
(FIGURE 347. Portion of a fossil fruit of Podocarya Bucklandi, Ung., magnified.
(Buckland's Bridgewater Treatise Plate 63.) Inferior Oolite, Charmouth, Dorset.)
(FIGURE 348. Cone of fossil Araucaria sphaerocarpa, Carruthers. Inferior Oolite.
Bruton, Somersetshire. One-third diameter of original. In the collection of the
British Museum.)
The Araucarian pines, which are now abundant in Australia and its islands,
together with marsupial quadrupeds, are found in like manner to have accompanied
the marsupials in Europe during the Oolitic period (see Figure 348). In the same
rock endogens of the most perfect structure are met with, as, for example,
fruits allied to the Pandanus, such as the Kaidacarpum ooliticum of Carruthers
in the Great Oolite, and the Podocarya of Buckland (see Figure 347) in the
Inferior Oolite.
FULLER'S EARTH.
(FIGURE 349. Ostrea acuminata. Fuller's Earth.)
Between the Great and Inferior Oolite near Bath, an argillaceous deposit, called
"the fuller's earth," occurs; but it is wanting in the north of England. It
abounds in the small oyster represented in Figure 349. The number of mollusca
known in this deposit is about seventy; namely, fifty Lamellibranchiate
Bivalves, ten Brachiopods, three Gasteropods, and seven or eight Cephalopods.
INFERIOR OOLITE.
This formation consists of a calcareous freestone, usually of small thickness,
but attaining in some places, as in the typical area of Cheltenham and the
Western Cotswolds, a thickness of 250 feet. It sometimes rests upon yellow
sands, formerly classed as the sands of the Inferior Oolite, but now regarded as
a member of the Upper Lias. These sands repose upon the Upper Lias clays in the
south and west of England. The Collyweston slate, formerly classed with the
Great Oolite, and supposed to represent in Northamptonshire the Stonesfield
slate, is now found to belong to the Inferior Oolite, both by community of
species and position in the series. The Collyweston beds, on the whole, assume a
much more marine character than the Stonesfield slate. Nevertheless, one of the
fossil plants Aroides Stutterdi, Carruthers, remarkable, like the Pandanaceous
species before mentioned (Figure 347) as a representative of the
monocotyledonous class, is common to the Stonesfield beds in Oxfordshire.
(FIGURE 350. Hemitelites Brownii, Goepp. Syn. Phlebopteris contigua, Lind. and
Hutt. Lower carbonaceous strata, Inferior Oolite shales. Gristhorpe, Yorkshire.)
The Inferior Oolite of Yorkshire consists largely of shales and sandstones,
which assume much the aspect of a true coal-field, thin seams of coal having
actually been worked in them for more than a century. A rich harvest of fossil
ferns has been obtained from them, as at Gristhorpe, near Scarborough (Figure
350). They contain also Cycadeae, of which family a magnificent specimen has
been described by Mr. Williamson under the name Zamia gigas, and a fossil called
Equisetum Columnare (see Figure 397), which maintains an upright position in
sandstone strata over a wide area. Shells of Estheria and Unio, collected by Mr.
Bean from these Yorkshire coal-bearing beds, point to the estuary or fluviatile
origin of the deposit.
At Brora, in Sutherlandshire, a coal formation, probably coeval with the above,
or at least belonging to some of the lower divisions of the Oolitic period, has
been mined extensively for a century or more. It affords the thickest stratum of
pure vegetable matter hitherto detected in any secondary rock in England. One
seam of coal of good quality has been worked three and a half feet thick, and
there are several feet more of pyritous coal resting upon it.
(FIGURE 351. Terebratula fimbria, Sowerby. Inferior Oolite marl. Cotswold
Hills.)
(FIGURE 352. Rhynchonella spinosa, Schloth. Inferior Oolite.)
(FIGURE 353. Pholadomya fidicula, Sowerby. One-third natural size. Inferior
Oolite.)
(FIGURE 354. Pleurotomaria granulata, Sowerby. Ferruginous Oolite, Normandy.
Inferior Oolite, England.)
(FIGURE 355. Pleurotomaria ornata, Sowerby Sp. Inferior Oolite.)
(FIGURE 356. Collyrites (Dysaster) ringens, Agassiz. Inferior Oolite,
Somersetshire.)
(FIGURE 357. Ammonites Humphresianus, Sowerby. Inferior Oolite.)
(FIGURE 358. Ammonites Braikenridgii, Sowerby. Oolite, Scarborough. Inferior
Oolite, Dundry; Calvados; etc.)
(FIGURE 359. Ostrea Marshii. One-half natural size. Middle and Lower Oolite.)
Among the characteristic shells of the Inferior Oolite, I may instance
Terebratula fimbria (Figure 351), Rhynchonella spinosa (Figure 352), and
Pholadomya fidicula (Figure 353). The extinct genus Pleurotomaria is also a form
very common in this division as well as in the Oolitic system generally. It
resembles the Trochus in form, but is marked by a deep cleft (a, Figures 354,
355) on one side of the mouth. The Collyrites (Dysaster) ringens (Figure 356) is
an Echinoderm common to the Inferior Oolite of England and France, as are the
two Ammonites (Figures 357, 358).
PALAEONTOLOGICAL RELATIONS OF THE OOLITIC STRATA.
Observations have already been made on the distinctness of the organic remains
of the Oolitic and Cretaceous strata, and the proportion of species common to
the different members of the Oolite. Between the Lower Oolite and the Lias there
is a somewhat greater break, for out of 256 mollusca of the Upper Lias, thirty-
seven species only pass up into the Inferior Oolite.
In illustration of shells having a great vertical range, it may be stated that
in England some few species pass up from the Lower to the Upper Oolite, as, for
example, Rhynchonella obsoleta, Lithodomus inclusus, Pholadomya ovalis, and
Trigonia costata.
(FIGURE 360. Ammonites macrocephalus, Schloth. One-third natural size. Great
Oolite and Oxford Clay.)
Of all the Jurassic Ammonites of Great Britain, A. macrocephalus (Figure 360),
which is common to the Great Oolite and Oxford Clay, has the widest range.
We have every reason to conclude that the gaps which occur, both between the
larger and smaller sections of the English Oolites, imply intervals of time,
elsewhere represented by fossiliferous strata, although no deposit may have
taken place in the British area. This conclusion is warranted by the partial
extent of many of the minor and some of the larger divisions even in England.
CHAPTER XX.
JURASSIC GROUP-- CONTINUED.-- LIAS.
Mineral Character of Lias.
Numerous successive Zones in the Lias, marked by distinct Fossils, without
Unconformity in the Stratification, or Change in the Mineral Character of the
Deposits.
Gryphite Limestone.
Shells of the Lias.
Fish of the Lias.
Reptiles of the Lias.
Ichthyosaur and Plesiosaur.
Marine Reptile of the Galapagos Islands.
Sudden Destruction and Burial of Fossil Animals in Lias.
Fluvio-marine Beds in Gloucestershire, and Insect Limestone.
Fossil Plants.
The origin of the Oolite and Lias, and of alternating Calcareous and
Argillaceous Formations.
LIAS.
The English provincial name of Lias has been very generally adopted for a
formation of argillaceous limestone, marl, and clay, which forms the base of the
Oolite, and is classed by many geologists as part of that group. The peculiar
aspect which is most characteristic of the Lias in England, France, and Germany,
is an alternation of thin beds of blue or grey limestone, having a surface which
becomes light-brown when weathered, these beds being separated by dark-coloured,
narrow argillaceous partings, so that the quarries of this rock, at a distance,
assume a striped and ribbon-like appearance.
The Lias has been divided in England into three groups, the Upper, Middle, and
Lower. The Upper Lias consists first of sands, which were formerly regarded as
the base of the Oolite, but which, according to Dr. Wright, are by their fossils
more properly referable to the Lias; secondly, of clay shale and thin beds of
limestone. The Middle Lias, or marl-stone series, has been divided into three
zones; and the Lower Lias, according to the labours of Quenstedt, Oppel,
Strickland, Wright, and others, into seven zones, each marked by its own group
of fossils. This Lower Lias averages from 600 to 900 feet in thickness.
From Devon and Dorsetshire to Yorkshire all these divisions, observes Professor
Ramsay, are constant; and from top to bottom we can not assert that anywhere
there is actual unconformity between any two subdivisions, whether of the larger
or smaller kind.
In the whole of the English Lias there are at present known about 937 species of
mollusca, and of these 267 are Cephalopods, of which class more than two-thirds
are Ammonites, the Nautilus and Belemnite also abounding. The whole series has
been divided by zones characterised by particular Ammonites; for while other
families of shells pass from one division to another in numbers varying from
about 20 to 50 per cent, these cephalopods are almost always limited to single
zones, as Quenstedt and Oppel have shown for Germany, and Dr. Wright and others
for England.
As no actual unconformity is known from the top of the Upper to the bottom of
the Lower Lias, and as there is a marked uniformity in the mineral character of
almost all the strata, it is somewhat difficult to account even for such partial
breaks as have been alluded to in the succession of species, if we reject the
hypothesis that the old species were in each case destroyed at the close of the
deposition of the rocks containing them, and replaced by the creation of new
forms when the succeeding formation began. I agree with Professor Ramsay in not
accepting this hypothesis. No doubt some of the old species occasionally died
out, and left no representatives in Europe or elsewhere; others were locally
exterminated in the struggle for life by species which invaded their ancient
domain, or by varieties better fitted for a new state of things. Pauses also of
vast duration may have occurred in the deposition of strata, allowing time for
the modification of organic life throughout the globe, slowly brought about by
variation accompanied by extinction of the original forms.
FOSSILS OF THE LIAS.
(FIGURE 361. Plagiostoma (Lima) giganteum, Sowerby. Inferior Oolite and Lias.)
(FIGURE 362. Gryphaea incurva, Sowerby. (G. arcuata, Lam.) Lias.)
(FIGURE 363. Avicula inaequivalvis, Sowerby. Lower Lias.)
(FIGURE 364. Avicula cygnipes, Phil. Lower Lias, Gloucestershire and Yorkshire.
a. Lower valve.
b. Upper valve.)
(FIGURE 365. Hippopodium ponderosum, Sowerby. 1/4 diameter. Lias, Cheltenham)
(FIGURE 366. Spiriferina (Spirifera) Walcotti, Sowerby. Lower Lias.)
(FIGURE 367. Leptaena Moorei, Davidson. Upper Lias, Ilminster.)
The name of Gryphite limestone has sometimes been applied to the Lias, in
consequence of the great number of shells which it contains of a species of
oyster, or Gryphaea (Figure 362). A large heavy shell called Hippopodium (Figure
365), allied to Cypricardia, is also characteristic of the upper part of the
Lower Lias. In this formation occur also the Aviculas, Figures 363 and 364. The
Lias formation is also remarkable for being the newest of the secondary rocks in
which brachiopoda of the genera Spirifer and Leptaena (Figures 366, 367) occur,
although the former is slightly modified in structure so as to constitute the
subgenus Spiriferina, Davidson, and the Leptaena has dwindled to a shell smaller
in size than a pea. No less than eight or nine species of Spiriferina are
enumerated by Mr. Davidson as belonging to the Lias. Palliobranchiate mollusca
predominate greatly in strata older than the Trias; but, so far as we yet know,
they did not survive the Liassic epoch.
(FIGURE 368. Ammonites Bucklandi, Sowerby. Ammonites bisulcatus, Brug. One-
eighth diameter of original.
a. Side view.
b. Front view, showing mouth and bisulcated keel. Characteristic of the lower
part of the Lias of England and the Continent.)
(FIGURE 369. Ammonites planorbis, Sowerby. One-half diameter of original. From
the base of the Lower Lias of England and the Continent.)
(FIGURE 370. Nautilus truncatus, Sowerby. Lias.)
(FIGURE 371. Ammonites bifrons, Brug. Ammonites Walcotti, Sowerby. Upper Lias
shales.)
(FIGURE 372. Ammonites margaritatus, Montf. Syn. Ammonites Stokesi, Sowerby.
Middle Lias.)
Allusion has already been made to numerous zones in the Lias having each their
peculiar Ammonites. Two of these occur near the base of the Lower Lias, having a
united thickness, varying from 40 to 80 feet. The upper of these is
characterised by Ammonites Bucklandi, and the lower by Ammonites planorbis (see
Figures 368, 369). (Quarterly Journal volume 16 page 376.) Sometimes, however,
there is a third intermediate zone, that of Ammonites angulatus, which is the
equivalent of the zone called the infra-lias on the Continent, the species of
which are for the most part common to the superior group marked by Ammonites
Bucklandi.
(FIGURE 373. Extracrinus (Pentacrinus) Briareus. Miller. 1/2 natural size.
(Body, arms, and part of stem.) Lower Lias, Lyme Regis.)
(FIGURE 374. Palaeocoma (Ophioderma) tenuibrachiata. E. Forbes. Middle Lias,
Seatown, Dorset.)
Among the Crinoids or Stone-lilies of the Lias, the Pentacrinites are
conspicuous. (See Figure 373.) Of Palaeocoma (Ophioderma) Egertoni (Figure 374),
referable to the Ophiuridae of Muller, perfect specimens have been met with in
the Middle Lias beds of Dorset and Yorkshire.
The Extracrinus Briareus (removed by Major Austin from Pentacrinus on account of
generic differences) occurs in tangled masses, forming thin beds of considerable
extent, in the Lower Lias of Dorset, Gloucestershire, and Yorkshire. The remains
are often highly charged with pyrites. This Crinoid, with its innumerable
tentacular arms, appears to have been frequently attached to the driftwood of
the liassic sea, in the same manner as Barnacles float about on wood at the
present day. There is another species of Extracrinus and several of Pentacrinus
in the Lias; and the latter genus is found in nearly all the formations from the
Lias to the London Clay inclusive. It is represented in the present seas by the
delicate and rare Pentacrinus caput-medusae of the Antilles, which, with
Comatula, is one of the few surviving members of the ancient family of the
Crinoids, represented by so many extinct genera in the older formations.
FISHES OF THE LIAS.
(FIGURE 375. Scales of Lepidotus gigas. Agass.
a. Two of the scales detached.)
(FIGURE 376. Aechmodus Leachii and Dapedius monilifer.
a. Aechmodus. Restored outline.
b. Scales of Aechmodus Leachii.
c. Scales of Dapedius monilifer.)
(FIGURE 377. Acrodus nobilis, Agassiz (tooth); commonly called "fossil leech."
Lias, Lyme Regis, and Germany.)
The fossil fish, of which there are no less than 117 species known as British,
resemble generically those of the Oolite, but differ, according to M. Agassiz,
from those of the Cretaceous period. Among them is a species of Lepidotus (L.
gigas, Agassiz), Figure 375, which is found in the Lias of England, France, and
Germany. (Agassiz Poissons Fossiles volume 2 tab. 28, 29.) This genus was before
mentioned (Chapter 18) as occurring in the Wealden, and is supposed to have
frequented both rivers and sea-coasts. Another genus of Ganoids (or fish with
hard, shining, and enamelled scales), called Aechmodus (Figure 376), is almost
exclusively Liassic. The teeth of a species of Acrodus, also, are very abundant
in the Lias (Figure 377).
(FIGURE 378. Hybodus reticulatus, Agassiz. Lias, Lyme Regis.
a. Part of fin, commonly called Ichthyodorulite.
b. Tooth.)
(FIGURE 379. Chimaera monstrosa. (Agassiz Poissons Fossiles volume 3 tab. C
Figure 1.)
a. Spine forming anterior part of the dorsal fin.)
SBut the remains of fish which have excited more attention than any others are
those large bony spines called ichthyodorulites (a, Figure 378), which were once
supposed by some naturalists to be jaws, and by others weapons, resembling those
of the living Balistes and Silurus; but which M. Agassiz has shown to be neither
the one nor the other. The spines, in the genera last mentioned, articulate with
the backbone, whereas there are no signs of any such articulation in the
ichthyodorulites. These last appear to have been bony spines which formed the
anterior part of the dorsal fin, like that of the living genera Cestracion and
Chimaera (see a, Figure 379). In both of these genera, the posterior concave
face is armed with small spines, as in that of the fossil Hybodus (Figure 378),
a placoid fish of the shark family found fossil at Lyme Regis. Such spines are
simply imbedded in the flesh, and attached to strong muscles. "They serve," says
Dr. Buckland, "as in the Chimaera (Figure 379), to raise and depress the fin,
their action resembling that of a movable mast, raising and lowering backward
the sail of a barge." (Bridgewater Treatise page 290.)
REPTILES OF THE LIAS.
(FIGURE 380. Skeleton of Ichthyosaurus communis, restored by Conybeare and
Cuvier.
a. Costal vertebrae.)
(FIGURE 381. Skeleton of Plesiosaurus dolichodeirus, restored by Reverend W.D.
Conybeare.
a. Cervical vertebra.)
It is not, however, the fossil fish which form the most striking feature in the
organic remains of the Lias; but the Enaliosaurian reptiles, which are
extraordinary for their number, size, and structure. Among the most singular of
these are several species of Ichthyosaurus and Plesiosaurus (Figures 380, 381).
The genus Ichthyosaurus, or fish-lizard, is not confined to this formation, but
has been found in strata as high as the White Chalk of England, and as low as
the Trias of Germany, a formation which immediately succeeds the Lias in the
descending order. It is evident from their fish-like vertebrae, their paddles,
resembling those of a porpoise or whale, the length of their tail, and other
parts of their structure, that the Ichthyosaurs were aquatic. Their jaws and
teeth show that they were carnivorous; and the half-digested remains of fishes
and reptiles, found within their skeletons, indicate the precise nature of their
food.
Mr. Conybeare was enabled, in 1824, after examining many skeletons nearly
perfect, to give an ideal restoration of the osteology of this genus, and of
that of the Plesiosaurus (Geological Society Transactions Second Series volume 1
page 49.). (See Figures 380, 381.) The latter animal had an extremely long neck
and small head, with teeth like those of the crocodile, and paddles analogous to
those of the Ichthyosaurus, but larger. It is supposed to have lived in shallow
seas and estuaries, and to have breathed air like the Ichthyosaur and our modern
cetacea. (Conybeare and De la Beche, Geological Transactions First Series volume
5 page 559; and Buckland Bridgewater Treatise page 203.) Some of the reptiles
above mentioned were of formidable dimensions. One specimen of Ichthyosaurus
platydon, from the Lias at Lyme, now in the British Museum, must have belonged
to an animal more than 24 feet in length; and there are species of Plesiosaurus
which measure from 18 to 20 feet in length. The form of the Ichthyosaurus may
have fitted it to cut through the waves like the porpoise; as it was furnished
besides its paddles with a tail-fin so constructed as to be a powerful organ of
motion; but it is supposed that the Plesiosaurus, at least the long-necked
species (Figure 381), was better suited to fish in shallow creeks and bays
defended from heavy breakers.
It is now very generally agreed that these extinct saurians must have inhabited
the sea; and it was urged that as there are now chelonians, like the tortoise,
living in fresh water, and others, as the turtle, frequenting the ocean, so
there may have been formerly some saurians proper to salt, others to fresh
water. The common crocodile of the Ganges is well-known to frequent equally that
river and the brackish and salt water near its mouth; and crocodiles are said in
like manner to be abundant both in the rivers of the Isla de Pinos (Isle of
Pines), south of Cuba, and in the open sea round the coast. In 1835 a curious
lizard (Amblyrhynchus cristatus) was discovered by Mr. Darwin in the Galapagos
Islands. (See Darwin Naturalist's Voyage page 385 Murray.) It was found to be
exclusively marine, swimming easily by means of its flattened tail, and
subsisting chiefly on seaweed. One of them was sunk from the ship by a heavy
weight, and on being drawn up after an hour was quite unharmed.
The families of Dinosauria, crocodiles, and Pterosauria or winged reptiles, are
also represented in the Lias.
SUDDEN DESTRUCTION OF SAURIANS.
It has been remarked, and truly, that many of the fish and saurians, found
fossil in the Lias, must have met with sudden death and immediate burial; and
that the destructive operation, whatever may have been its nature, was often
repeated.
"Sometimes," says Dr. Buckland, "scarcely a single bone or scale has been
removed from the place it occupied during life; which could not have happened
had the uncovered bodies of these saurians been left, even for a few hours,
exposed to putrefaction, and to the attacks of fishes and other smaller animals
at the bottom of the sea." (Bridgewater Treatise page 115.) Not only are the
skeletons of the Ichthyosaurs entire, but sometimes the contents of their
stomachs still remain between their ribs, as before remarked, so that we can
discover the particular species of fish on which they lived, and the form of
their excrements. Not unfrequently there are layers of these coprolites, at
different depths in the Lias, at a distance from any entire skeletons of the
marine lizards from which they were derived; "as if," says Sir H. De la Beche,
"the muddy bottom of the sea received small sudden accessions of matter from
time to time, covering up the coprolites and other exuviae which had accumulated
during the intervals." (Geological Researches page 334.) It is further stated
that, at Lyme Regis, those surfaces only of the coprolites which lay uppermost
at the bottom of the sea have suffered partial decay, from the action of water
before they were covered and protected by the muddy sediment that has afterwards
permanently enveloped them.
Numerous specimens of the Calamary or pen-and-ink fish, (Geoteuthis bollensis)
have also been met with in the Lias at Lyme, with the ink-bags still distended,
containing the ink in a dried state, chiefly composed of carbon, and but
slightly impregnated with carbonate of lime. These Cephalopoda, therefore, must,
like the saurians, have been soon buried in sediment; for, if long exposed after
death, the membrane containing the ink would have decayed. (Buckland Bridgewater
Treatise page 307.)
As we know that river-fish are sometimes stifled, even in their own element, by
muddy water during floods, it can not be doubted that the periodical discharge
of large bodies of turbid fresh water in the sea may be still more fatal to
marine tribes. In the "Principles of Geology" I have shown that large quantities
of mud and drowned animals have been swept down into the sea by rivers during
earthquakes, as in Java in 1699; and that indescribable multitudes of dead
fishes have been seen floating on the sea after a discharge of noxious vapours
during similar convulsions. But in the intervals between such catastrophes,
strata may have accumulated slowly in the sea of the Lias, some being formed
chiefly of one description of shell, such as ammonites, others of gryphites.
FRESH-WATER DEPOSITS.-- INSECT BEDS.
(FIGURE 382. Wing of a neuropterous insect, from the Lower Lias,
Gloucestershire. (Reverend P.B. Brodie.))
From the above remarks the reader will infer that the Lias is for the most part
a marine deposit. Some members, however, of the series have an estuarine
character, and must have been formed within the influence of rivers. At the base
of the Upper and Lower Lias respectively, insect-beds appear to be almost
everywhere present throughout the Midland and South-western districts of
England. These beds are crowded with the remains of insects, small fish, and
crustaceans, with occasional marine shells. One band in Gloucestershire, rarely
exceeding a foot in thickness, has been named the "insect limestone." It passes
upward, says the Reverend P.B. Brodie, into a shale containing Cypris and
Estheria, and is full of the wing-cases of several genera of Coleoptera, with
some nearly entire beetles, of which the eyes are preserved. (A History of
Fossil Insects etc 1846 London.) The nervures of the wings of neuropterous
insects (Figure 382) are beautifully perfect in this bed. Ferns, with Cycads and
leaves of monocotyledonous plants, and some apparently brackish and fresh-water
shells, accompany the insects in several places, while in others marine shells
predominate, the fossils varying apparently as we examine the bed nearer or
farther from the ancient land, or the source whence the fresh water was derived.
After studying 300 specimens of these insects from the Lias, Mr. Westwood
declares that they comprise both wood-eating and herb-devouring beetles, of the
Linnean genera Elater, Carabus, etc., besides grasshoppers (Gryllus), and
detached wings of dragon-flies and may-flies, or insects referable to the
Linnean genera Libellula, Ephemera, Hemerobius, and Panorpa, in all belonging to
no less than twenty-four families. The size of the species is usually small, and
such as taken alone would imply a temperate climate; but many of the associated
organic remains of other classes must lead to a different conclusion.
FOSSIL PLANTS.
Among the vegetable remains of the Lias, several species of Zamia have been
found at Lyme Regis, and the remains of coniferous plants at Whitby. M. Ad.
Brongniart enumerates forty-seven liassic acrogens, most of them ferns; and
fifty gymnosperms, of which thirty-nine are cycads, and eleven conifers. Among
the cycads the predominance of Zamites, and among the ferns the numerous genera
with leaves having reticulated veins (as in Figure 349), are mentioned as
botanical characteristics of this era. (Tableau des Veg. Foss. 1849 page 105.)
The absence as yet from the Lias and Oolite of all signs of dicotyledonous
angiosperms is worthy of notice. The leaves of such plants are frequent in
tertiary strata, and occur in the Cretaceous, though less plentifully (see
Chapter 17). The angiosperms seem, therefore, to have been at the least
comparatively rare in these older secondary periods, when more space was
occupied by the Cycads and Conifers.
ORIGIN OF THE OOLITE AND LIAS.
The entire group of Oolite and Lias consists of repeated alternations of clay,
sandstone, and limestone, following each other in the same order. Thus the clays
of the Lias are followed by the sands now considered (see Chapter 20) as
belonging to the same formation, though formerly referred to the Inferior
Oolite, and these sands again by the shelly and coralline limestone called the
Great or Bath Oolite. So, in the Middle Oolite, the Oxford Clay is followed by
calcareous grit and coral rag; lastly, in the Upper Oolite, the Kimmeridge Clay
is followed by the Portland Sand and limestone (see Figure 298). (Conybeare and
Philips's Outlines etc. page 166.) The clay beds, however, as Sir H. de la Beche
remarks, can be followed over larger areas than the sand or sandstones.
(Geological Researches page 337.) It should also be remembered that while the
Oolite system becomes arenaceous and resembles a coal-field in Yorkshire, it
assumes in the Alps an almost purely calcareous form, the sands and clays being
omitted; and even in the intervening tracts it is more complicated and variable
than appears in ordinary descriptions. Nevertheless, some of the clays and
intervening limestones do retain, in reality, a pretty uniform character for
distances of from 400 to 600 miles from east to west and north to south.
In order to account for such a succession of events, we may imagine, first, the
bed of the ocean to be the receptacle for ages of fine argillaceous sediment,
brought by oceanic currents, which may have communicated with rivers, or with
part of the sea near a wasting coast. This mud ceases, at length, to be conveyed
to the same region, either because the land which had previously suffered
denudation is depressed and submerged, or because the current is deflected in
another direction by the altered shape of the bed of the ocean and neighbouring
dry land. By such changes the water becomes once more clear and fit for the
growth of stony zoophytes. Calcareous sand is then formed from comminuted shell
and coral, or, in some cases, arenaceous matter replaces the clay; because it
commonly happens that the finer sediment, being first drifted farthest from
coasts, is subsequently overspread by coarse sand, after the sea has grown
shallower, or when the land, increasing in extent, whether by upheaval or by
sediment filling up parts of the sea, has approached nearer to the spots first
occupied by fine mud.
The increased thickness of the limestones in those regions, as in the Alps and
Jura, where the clays are comparatively thin, arises from the calcareous matter
having been derived from species of corals and other organic beings which live
in clear water, far from land, to the growth of which the influx of mud would be
unfavourable. Portions therefore of these clays and limestones have probably
been formed contemporaneously to a greater extent than we can generally prove,
for the distinctness of the species of organic beings would be caused by the
difference of conditions between the more littoral and the more pelagic areas
and the different depths and nature of the sea-bottom. Independently of those
ascending and descending movements which have given rise to the superposition of
the limestones and clays, and by which the position of land and sea are made in
the course of ages to vary, the geologist has the difficult task of allowing for
the contemporaneous thinning out in one direction and thickening in another, of
the successive organic and inorganic deposits of the same era.
CHAPTER XXI.
TRIAS, OR NEW RED SANDSTONE GROUP.
Beds of Passage between the Lias and Trias, Rhaetic Beds.
Triassic Mammifer.
Triple Division of the Trias.
Keuper, or Upper Trias of England.
Reptiles of the Upper Trias.
Foot-prints in the Bunter formation in England.
Dolomitic Conglomerate of Bristol.
Origin of Red Sandstone and Rock-salt.
Precipitation of Salt from inland Lakes and Lagoons.
Trias of Germany.
Keuper.
St. Cassian and Hallstadt Beds.
Peculiarity of their Fauna.
Muschelkalk and its Fossils.
Trias of the United States.
Fossil Foot-prints of Birds and Reptiles in the Valley of the Connecticut.
Triassic Mammifer of North Carolina.
Triassic Coal-field of Richmond, Virginia.
Low Grade of early Mammals favourable to the Theory of Progressive Development.
BEDS OF PASSAGE BETWEEN THE LIAS AND TRIAS-- RHAETIC BEDS.
We have mentioned in the last chapter that the base of the Lower Lias is
characterised, both in England and Germany, by beds containing distinct species
of Ammonites, the lowest subdivision having been called the zone of Ammonites
planorbis. Below this zone, on the boundary line between the Lias and the strata
of which we are about to treat, called "Trias," certain cream-coloured
limestones devoid of fossils are usually found. These white beds were called by
William Smith the White Lias, and they have been shown by Mr. Charles Moore to
belong to a formation similar to one in the Rhaetian Alps of Bavaria, to which
Mr. Gumbel has applied the name of Rhaetic. They have also long been known as
the Koessen beds in Germany, and may be regarded as beds of passage between the
Lias and Trias. They are named the Penarth beds by the Government surveyors of
Great Britain, from Penarth, near Cardiff, in Glamorganshire, where they
sometimes attain a thickness of fifty feet.
(FIGURE 383. Cardium rhaeticum, Merrian. Natural size. Rhaetic Beds.)
(FIGURE 384. Pecten Valoniensis. Dfr. 1/2 natural size. Portrush, Ireland, etc.
Rhaetic Beds.)
(FIGURE 385. Avicula contorta. Portlock. Portrush, Ireland, etc. Natural size.
Rhaetic Beds.)
The principal member of this group has been called by Dr. Wright the Avicula
contorta bed, as this shell is very abundant, and has a wide range in Europe.
(Dr. Wright, on Lias and Bone Bed, Quarterly Geological Journal 1860 volume 16.)
General Portlock first described the formation as it occurs at Portrush, in
Antrim, where the Avicula contorta is accompanied by Pecten Valoniensis, as in
Germany.
The best known member of the group, a thin band or bone-breccia, is conspicuous
among the black shales in the neighbourhood of Axmouth in Devonshire, and in the
cliffs of Westbury-on-Severn, as well as at Aust and other places on the borders
of the Bristol Channel. It abounds in the remains of saurians and fish, and was
formerly classed as the lowest bed of the Lias; but Sir P. Egerton first pointed
out, in 1841, that it should be referred to the Upper New Red Sandstone, because
it contained an assemblage of fossil fish which are either peculiar to this
stratum, or belong to species well-known in the Muschelkalk of Germany. These
fish belong to the genera Acrodus, Hybodus, Gyrolepis, and Saurichthys.
(FIGURE 386. Hybodus plicatilis, Agassiz. Teeth. Bone-bed, Aust and Axmouth.)
(FIGURE 387. Saurichthys apicalis, Agassiz. Tooth; natural size and magnified.
Axmouth.)
(FIGURE 388. Gyrolepis tenuistriatus, Agassiz. Scale; natural size and
magnified. Axmouth.)
Among those common to the English bone-bed and the Muschelkalk of Germany are
Hybodus plicatilis (Figure 386), Saurychthys apicalis (Figure 387), Gyrolepis
tenuistriatus (Figure 388), and G. Albertii. Remains of saurians, Plesiosaurus
among others, have also been found in the bone-bed, and plates of an Encrinus.
It may be questioned whether some of those fossils which have the most Triassic
character may not have been derived from the destruction of older strata, since
in bone-beds, in general, many of the organic remains are undoubtedly
derivative.
TRIASSIC MAMMIFER.
(FIGURE 389. Microlestes antiquus, Plieninger. Molar tooth, magnified. Upper
Trias. Diegerloch, near Stuttgart, Wurtemberg.
a. View of inner side?
b. Same, outer side?
c. Same in profile.
d. Crown of same.)
In North-western Germany, as in England, there occurs beneath the Lias a
remarkable bone breccia. It is filled with shells and with the remains of fishes
and reptiles, almost all the genera of which, and some even of the species,
agree with those of the subjacent Trias. This breccia has accordingly been
considered by Professor Quenstedt, and other German geologists of high
authority, as the newest or uppermost part of the Trias. Professor Plieninger
found in it, in 1847, the molar tooth of a small Triassic mammifer, called by
him Microlestes antiquus. He inferred its true nature from its double fangs, and
from the form and number of the protuberances or cusps on the flat crown; and
considering it as predaceous, probably insectivorous, he called it Microlestes
from micros, little, and lestes, a beast of prey. Soon afterwards he found a
second tooth, also at the same locality, Diegerloch, about two miles to the
south-east of Stuttgart.
No anatomist had been able to give any feasible conjecture as to the affinities
of this minute quadruped until Dr. Falconer, in 1857, recognised an unmistakable
resemblance between its teeth and the two back molars of his new genus
Plagiaulax (Figure 306), from the Purbeck strata. This would lead us to the
conclusion that Microlestes was marsupial and plant-eating.
In Wurtemberg there are two bone-beds, namely, that containing the Microlestes,
which has just been described, which constitutes, as we have seen, the uppermost
member of the Trias, and another of still greater extent, and still more rich in
the remains of fish and reptiles, which is of older date, intervening between
the Keuper and Muschelkalk.
The genera Saurichthys, Hybodus, and Gyrolepis are found in both these breccias,
and one of the species, Saurichthys Mongeoti, is common to both bone-beds, as is
also a remarkable reptile called Nothosaurus mirabilis. The saurian called
Belodon by H. von Meyer, of the Thecodont family, is another Triassic form,
associated at Diegerloch with Microlestes.
TRIAS OF ENGLAND.
Between the Lias and the Coal (or Carboniferous group) there is interposed, in
the midland and western counties of England, a great series of red loams,
shales, and sandstones, to which the name of the "New Red Sandstone formation"
was first given, to distinguish it from other shales and sandstones called the
"Old Red," often identical in mineral character, which lie immediately beneath
the coal. The name of "Red Marl" has been incorrectly applied to the red clays
of this formation, as before explained (Chapter 2), for they are remarkably free
from calcareous matter. The absence, indeed, of carbonate of lime, as well as
the scarcity of organic remains, together with the bright red colour of most of
the rocks of this group, causes a strong contrast between it and the Jurassic
formations before described.
The group in question is more fully developed in Germany than in England or
France. It has been called the Trias by German writers, or the Triple Group,
because it is separable into three distinct formations, called the "Keuper," the
"Muschelkalk," and the "Bunter-sandstein." Of these the middle division, or the
Muschelkalk, is wholly wanting in England, and the uppermost (Keuper) and lowest
(Bunter) members of the series are not rich in fossils.
UPPER TRIAS OR KEUPER.
In certain grey indurated marls below the bone-bed Mr. Boyd Dawkins has found at
Watchet, on the coast of Somersetshire, a molar tooth of Microlestes, enabling
him to refer to the Trias strata formerly supposed to be Liassic. Mr. Charles
Moore had previously discovered many teeth of mammalia of the same family near
Frome, in Somersetshire, in the contents of a vertical fissure traversing a mass
of carboniferous limestone. The top of this fissure must have communicated with
the bed of the Triassic sea, and probably at a point not far from the ancient
shore on which the small marsupials of that era abounded.
This upper division of the Trias called the Keuper is of great thickness in the
central counties of England, attaining, according to Mr. Hull's estimate, no
less than 3450 feet in Cheshire, and it covers a large extent of country between
Lancashire and Devonshire.
(FIGURE 390. Estheria minuta, Bronn.)
In Worcestershire and Warwickshire in sandstone belonging to the uppermost part
of the Keuper the bivalve crustacean Estheria minuta occurs. The member of the
English "New Red" containing this shell, in those parts of England, is,
according to Sir Roderick Murchison and Mr. Strickland, 600 feet thick, and
consists chiefly of red marl or slate, with a band of sandstone.
Ichthyodorulites, or spines of Hybodus, teeth of fishes, and footprints of
reptiles were observed by the same geologists in these strata.
(FIGURE 391. Hyperodapedon Gordoni. Left palate, maxillary. (Showing the two
rows of palatal teeth on opposite sides of the jaw.)
a. Under surface.
b. Exterior right side.)
In the Upper Trias or Keuper the remains of two saurians of the order Lacertilia
have been found. The one called Rhynchosaurus occurred at Grinsell near
Shrewsbury, and is characterised by having a small bird-like skull and jaws
without teeth. The other Hyperodapedon (Figure 391) was first noticed in 1858,
near Elgin, in strata now recognised as Upper Triassic, and afterwards in beds
of about the same age in the neighbourhood of Warwick. Remains of the same genus
have been found both in Central India and Southern Africa in rocks believed to
be of Triassic age. The Hyperodapedon has been shown by Professor Huxley to be a
terrestrial reptile having numerous palatal teeth, and closely allied to the
living Sphenodon of New Zealand.
The recent discoveries of a living saurian in New Zealand so closely allied to
this supposed extinct division of the Lacertilia seems to afford an illustration
of a principle pointed out by Mr. Darwin of the survival in insulated tracts,
after many changes in physical geography, of orders of which the congeners have
become extinct on continents where they have been exposed to the severer
competition of a larger progressive fauna.
(FIGURE 392. Tooth of Labyrinthodon; natural size. Warwick sandstone.)
(FIGURE 393. Transverse section of upper part of tooth of Labyrinthodon Jaegeri,
Owen (Mastodonsaurus Jaegeri, Meyer); natural size, and a segment magnified.
a. Pulp cavity, from which the processes of pulp and dentine radiate.)
Teeth of Labyrinthodon (Figure 392) found in the Keuper in Warwickshire were
examined microscopically by Professor Owen, and compared with other teeth from
the German Keuper. He found after careful investigation that neither of them
could be referred to true saurians, although they had been named Mastodonsaurus
and Phytosaurus by Jager. It appeared that they were of the Batrachian order,
and of gigantic dimensions in comparison with any representatives of that order
now living. Both the Continental and English fossil teeth exhibited a most
complicated texture, differing from that previously observed in any reptile,
whether recent or extinct, but most nearly analogous to the Ichthyosaurus. A
section of one of these teeth exhibits a series of irregular folds, resembling
the labyrinthic windings of the surface of the brain; and from this character
Professor Owen has proposed the name Labyrinthodon for the new genus. Figure 393
of part of one is given from his "Odontography," plate 64, a. The entire length
of this tooth is supposed to have been about three inches and a half, and the
breadth at the base one inch and a half.
ROCK-SALT.
In Cheshire and Lancashire there are red clays containing gypsum and salt of the
age of the Trias which are between 1000 and 1500 feet thick. In some places
lenticular masses of pure rock-salt nearly 100 feet thick are interpolated
between the argillaceous beds. At the base of the formation beneath the rock-
salt occur the Lower Sandstones and Marl, called provincially in Cheshire
"water-stones," which are largely quarried for building. They are often ripple-
marked, and are impressed with numerous footprints of reptiles.
The basement beds of the Keuper rest with a slight unconformability upon an
eroded surface of the "Bunter" next to be described.
LOWER TRIAS OR BUNTER.
(FIGURE 394. Single footstep of Cheirotherium. Bunter-sandstein, Saxony, one-
eighth of natural size.)
(FIGURE 395. Line of footsteps on slab of sandstone. Hildburghausen, in Saxony.)
The lower division or English representative of the "Bunter" attains a thickness
of 1500 feet in the counties last mentioned, according to Professor Ramsay.
Besides red and green shales and red sandstones, it comprises much soft white
quartzose sandstone, in which the trunks of silicified trees have been met with
at Allesley Hill, near Coventry. Several of them were a foot and a half in
diameter, and some yards in length, decidedly of coniferous wood, and showing
rings of annual growth. (Buckland Proceedings of the Geological Society volume 2
page 439 and Murchison and Strickland Geological Transactions Second Series
volume 5 page 347.) Impressions, also, of the footsteps of animals have been
detected in Lancashire and Cheshire in this formation. Some of the most
remarkable occur a few miles from Liverpool, in the whitish quartzose sandstone
of Storton Hill, on the west side of the Mersey. They bear a close resemblance
to tracks first observed in this member of the Upper New Red Sandstone, at the
village of Hesseberg, near Hildburghausen, in Saxony. For many years these
footprints have been referred to a large unknown quadruped, provisionally named
Cheirotherium by Professor Kaup, because the marks both of the fore and hind
feet resembled impressions made by a human hand. (See Figure 394.) The foot-
marks at Hesseberg are partly concave, and partly in relief, the former, or the
depressions, are seen upon the upper surface of the sandstone slabs, but those
in relief are only upon the lower surfaces, being, in fact, natural casts,
formed in the subjacent footprints as in moulds. The larger impressions, which
seem to be those of the hind foot, are generally eight inches in length, and
five in width, and one was twelve inches long. Near each large footstep, and at
a regular distance (about an inch and a half) before it, a smaller print of a
fore foot, four inches long and three inches wide, occurs. The footsteps follow
each other in pairs, each pair in the same line, at intervals of fourteen inches
from pair to pair. The large as well as the small steps show the great toes
alternately on the right and left side; each step makes the print of five toes,
the first, or great toe, being bent inward like a thumb. Though the fore and
hind foot differ so much in size, they are nearly similar in form.
As neither in Germany nor in England had any bones or teeth been met with in the
same identical strata as the footsteps, anatomists indulged, for several years,
in various conjectures respecting the mysterious animals from which they might
have been derived. Professor Kaup suggested that the unknown quadruped might
have been allied to the Marsupialia; for in the kangaroo the first toe of the
fore foot is in a similar manner set obliquely to the others, like a thumb, and
the disproportion between the fore and hind feet is also very great. But M. Link
conceived that some of the four species of animals of which the tracks had been
found in Saxony might have been gigantic Batrachians, and when it was afterwards
inferred that the Labyrinthodon was an air-breathing reptile, it was conjectured
by Professor Owen that it might be one and the same as the Cheirotherium.
DOLOMITIC CONGLOMERATE OF BRISTOL.
(FIGURE 396. Tooth of Thecodontosaurus; three times magnified. Riley and
Stutchbury. Dolomitic conglomerate. Redland, near Bristol.)
Near Bristol, in Somersetshire, and in other counties bordering the Severn, the
lowest strata belonging to the Triassic series consist of a conglomerate or
breccia resting unconformably upon the Old Red Sandstone, and on different
members of the Carboniferous rocks, such as the Coal Measures, Millstone Grit,
and Mountain Limestone. This mode of superposition will be understood by
reference to the section below Dundry Hill (Figure 85), where No. 4 is the
dolomitic conglomerate. Such breccias may have been partly the result of the
subaerial waste of an old land-surface which gradually sank down and suffered
littoral denudation in proportion as it became submerged. The pebbles and
fragments of older rocks which constitute the conglomerate are cemented together
by a red or yellow base of dolomite, and in some places the encrinites and other
fossils derived from the Mountain Limestone are so detached from the parent
rocks that they have the deceptive appearance of belonging to a fauna
contemporaneous with the dolomitic beds in which they occur. The imbedded
fragments are both rounded and angular, some consisting of sandstone from the
coal-measures, being of vast size, and weighing nearly a ton. Fractured bones
and teeth of saurians which are truly of contemporaneous origin are dispersed
through some parts of the breccia, and two of these reptiles called Thecodont
saurians, named from the manner in which the teeth were implanted in the
jawbone, obtained great celebrity because the patches of red conglomerate in
which they were found, near Bristol, were originally supposed to be of Permian
or Palaeozoic age, and therefore the only representatives in England of
vertebrate animals of so high a grade in rocks of such antiquity. The teeth of
these saurians are conical, compressed, and with finely serrated edges (see
Figure 396); they are referred by Professor Huxley to the Dinosaurian order.
ORIGIN OF RED SANDSTONE AND ROCK-SALT.
In various parts of the world, red and mottled clays and sandstones, of several
distinct geological epochs, are found associated with salt, gypsum, and
magnesian limestone, or with one or all of these substances. There is,
therefore, in all likelihood, a general cause for such a coincidence.
Nevertheless, we must not forget that there are dense masses of red and
variegated sandstones and clays, thousands of feet in thickness, and of vast
horizontal extent, wholly devoid of saliferous or gypseous matter. There are
also deposits of gypsum and of common salt, as in the blue-clay formation of
Sicily, without any accompanying red sandstone or red clay.
These red deposits may be accounted for by the decomposition of gneiss and mica
schist, which in the eastern Grampians of Scotland has produced a mass of
detritus of precisely the same colour as the Old Red Sandstone.
It is a general fact, and one not yet accounted for, that scarcely any fossil
remains are ever preserved in stratified rocks in which this oxide of iron
abounds; and when we find fossils in the New or Old Red Sandstone in England, it
is in the grey, and usually calcareous beds, that they occur. The saline or
gypseous interstratified beds may have been produced by submarine gaseous
emanations, or hot mineral springs, which often continue to flow in the same
spots for ages. Beds of rock-salt are, however, more generally attributed to the
evaporation of lakes or lagoons communicating at intervals with the ocean. In
Cheshire two beds of salt occur of the extraordinary thickness of 90 or even 100
feet, and extending over an area supposed to be 150 miles in diameter. The
adjacent beds present ripple-marked sandstones and footprints of animals at so
many levels as to imply that the whole area underwent a slow and gradual
depression during the formation of the red sandstone.
Major Harris, in his "Highlands of Ethiopia," describes a salt lake, called the
Bahr Assal, near the Abyssinian frontier, which once formed the prolongation of
the Gulf of Tadjara, but was afterwards cut off from the gulf by a broad bar of
lava or of land upraised by an earthquake. "Fed by no rivers, and exposed in a
burning climate to the unmitigated rays of the sun, it has shrunk into an
elliptical basin, seven miles in its transverse axis, half filled with smooth
water of the deepest caerulean hue, and half with a solid sheet of glittering
snow-white salt, the offspring of evaporation." "If," says Mr. Hugh Miller, "we
suppose, instead of a barrier of lava, that sand-bars were raised by the surf on
a flat arenaceous coast during a slow and equable sinking of the surface, the
waters of the outer gulf might occasionally topple over the bar, and supply
fresh brine when the first stock had been exhausted by evaporation."
The Runn of Cutch, as I have shown elsewhere (Principles of Geology chapter
27.), is a low region near the delta of the Indus, equal in extent to about a
quarter of Ireland, which is neither land nor sea, being dry during part of
every year, and covered by salt water during the monsoons. Here and there its
surface is incrusted over with a layer of salt caused by the evaporation of sea-
water. A subsiding movement has been witnessed in this country during
earthquakes, so that a great thickness of pure salt might result from a
continuation of such sinking.
TRIAS OF GERMANY.
In Germany, as before hinted, chapter 21, the Trias first received its name as a
Triple Group, consisting of two sandstones with an intermediate marine
calcareous formation, which last is wanting in England.
NOMENCLATURE OF TRIAS.
COLUMN 1: GERMAN.
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