The Student's Elements of Geology
by
Sir Charles Lyell
Part 11 out of 14
with carbonate of lime, and with a zeolite resembling analcime, which has been
called cyclopite. The latter mineral has also been found in small fissures
traversing the altered marl, showing that the same cause which introduced the
minerals into the cavities of the lava, whether we suppose sublimation or
aqueous infiltration, conveyed it also into the open rents of the contiguous
sedimentary strata.
DIKES OF PALAGONIA.
(FIGURES 602 and 603. Ground-plan of dikes near Palagonia.)
(FIGURE 602. Ground-plan of dikes near Palagonia.
a. Lava.
b. Peperino, consisting of volcanic sand, mixed with fragments of lava and
limestone.)
(FIGURE 603. Ground-plan of dikes near Palagonia.
a. Lava.
b. Peperino, consisting of volcanic sand, mixed with fragments of lava and
limestone.))
Dikes of vesicular and amygdaloidal lava are also seen traversing marine tuff or
peperino, west of Palagonia, some of the pores of the lava being empty, while
others are filled with carbonate of lime. In such cases we may suppose the tuff
to have resulted from showers of volcanic sand and scoriae, together with
fragments of limestone, thrown out by a submarine explosion, similar to that
which gave rise to Graham Island in 1831. When the mass was, to a certain
degree, consolidated, it may have been rent open, so that the lava ascended
through fissures, the walls of which were perfectly even and parallel. In one
case, after the melted matter that filled the rent (Figure 602) had cooled down,
it must have been fractured and shifted horizontally by a lateral movement.
In Figure 603, the lava has more the appearance of a vein, which forced its way
through the peperino. It is highly probable that similar appearances would be
seen, if we could examine the floor of the sea in that part of the Mediterranean
where the waves have recently washed away the new volcanic island; for when a
superincumbent mass of ejected fragments has been removed by denudation, we may
expect to see sections of dikes traversing tuff, or, in other words, sections of
the channels of communication by which the subterranean lavas reached the
surface.
MADEIRA.
Although the more ancient portion of the volcanic eruptions by which the island
of Madeira and the neighbouring one of Porto Santo were built up occurred, as we
shall presently see, in the Upper Miocene Period, a still larger part of the
island is of Pliocene date. That the latest outbreaks belonged to the Newer
Pliocene Period, I infer from the close affinity to the present flora of Madeira
of the fossil plants preserved in a leaf-bed in the north-eastern part of the
island. These fossils, associated with some lignite in the ravine of the river
San Jorge, can none of them be proved to be of extinct species, but their
antiquity may be inferred from the following considerations: Firstly-- The leaf-
bed, discovered by Mr. Hartung and myself in 1853, at the height of 1000 feet
above the level of the sea, crops out at the base of a cliff formed by the
erosion of a gorge cut through alternating layers of basalt and scoriae, the
product of a vast succession of eruptions of unknown date, piled up to a
thickness of 1000 feet, and which were all poured out after the plants, of which
about twenty species have been recognised, flourished in Madeira. These lavas
are inclined at an angle of about 15 degrees to the north, and came down from
the great central region of eruption. Their accumulation implies a long period
of intermittent volcanic action, subsequently to which the ravine of San Jorge
was hollowed out. Secondly-- Some few of the plants, though perhaps all of
living species, are supposed to be of genera not now existing in the island.
They have been described by Sir Charles Bunbury and Professor Heer, and the
former first pointed out that many of the leaves are of the laurel type, and
analogous to those now flourishing in the modern forests of Madeira. He also
recognised among them the leaves of Woodwardia radicans, and Davallia
Canariensis, ferns now abundant in Madeira. Thirdly-- the great age of this
leaf-bed of San Jorge, which was perhaps originally formed in the crater of some
ancient volcanic cone afterwards buried under lava, is proved by its belonging
to a part of the eastern extremity of Madeira, which, after the close of the
igneous eruptions, became covered in the adjoining district of Canical with
blown sand in which a vast number of land-shells were buried. These fossil
shells belonged to no less than 36 species, among which are many now extremely
rare in the island, and others, about five per cent, extinct or unknown in any
part of the world. Several of these of the genus Helix are conspicuous from the
peculiarity of their forms, others from their large dimensions. The geographical
configuration of the country shows that this shell-bed is considerably more
modern than the leaf-bed; it must therefore be referred to the Newer Pliocene,
according to the definition of this period given in Chapter 9.
OLDER PLIOCENE PERIOD.-- ITALY.
In Tuscany, as at Radicofani, Viterbo, and Aquapendente, and in the Campagna di
Roma, submarine volcanic tuffs are interstratified with the Older Pliocene
strata of the Sub-apennine hills in such a manner as to leave no doubt that they
were the products of eruptions which occurred when the shelly marls and sands of
the Sub-appenine hills were in the course of deposition. This opinion I
expressed after my visit to Italy in 1828 (See 1st edition of Principles of
Geology volume 3 chapters 8 and 14 1833 and former editions of this work chapter
31.), and it has recently (1850) been confirmed by the argument adduced by Sir
R. Murchison in favour of the submarine origin of the tertiary volcanic rocks of
Italy. (Quarterly Geological Journal volume 6 page 281.) These rocks are well-
known to rest conformably on the Sub-apennine marls, even as far south as Monte
Mario, in the suburbs of Rome. On the exact age of the deposits of Monte Mario
new light has recently been thrown by a careful study of their marine fossil
shells, undertaken by MM. Rayneval, Van den Hecke, and Ponzi. They have compared
no less than 160 species with the shells of the Coralline Crag of Suffolk, so
well described by Mr. Searles Wood; and the specific agreement between the
British and Italian fossils is so great, if we make due allowance for
geographical distance and the difference of latitude, that we can have little
hesitation in referring both to the same period, or to the Older Pliocene of
this work. It is highly probable that, between the oldest trachytes of Tuscany
and the newest rocks in the neighbourhood of Naples, a series of volcanic
products might be detected of every age from the Older Pliocene to the
historical epoch.
PLIOCENE VOLCANOES OF THE EIFEL.
Some of the most perfect cones and craters in Europe, not even excepting those
of the district round Vesuvius, may be seen on the left or west bank of the
Rhine, near Bonn and Andernach. They exhibit characters distinct from any which
I have observed elsewhere, owing to the large part which the escape of aqueous
vapour has played in the eruptions and the small quantities of lava emitted. The
fundamental rocks of the district are grey and red sandstones and shales, with
some associated limestones, replete with fossils of the Devonian or Old Red
Sandstone group. The volcanoes broke out in the midst of these inclined strata,
and when the present systems of hills and valleys had already been formed. The
eruptions occurred sometimes at the bottom of deep valleys, sometimes on the
summit of hills, and frequently on intervening platforms. In travelling through
this district we often come upon them most unexpectedly, and may find ourselves
on the very edge of a crater before we had been led to suspect that we were
approaching the site of any igneous outburst. Thus, for example, on arriving at
the village of Gemund, immediately south of Daun, we leave the stream, which
flows at the bottom of a deep valley in which strata of sandstone and shale crop
out. We then climb a steep hill, on the surface of which we see the edges of the
same strata dipping inward towards the mountain. When we have ascended to a
considerable height, we see fragments of scoriae sparingly scattered over the
surface; until at length, on reaching the summit, we find ourselves suddenly on
the edge of a tarn, or deep circular lake-basin called the Gemunder Maar. In it
we recognise the ordinary form of a crater, for which we have been prepared by
the occurrence of scoriae scattered over the surface of the soil. But on
examining the walls of the crater we find precipices of sandstone and shale
which exhibit no signs of the action of heat; and we look in vain for those beds
of lava and scoriae, dipping outward on every side, which we have been
accustomed to consider as characteristic of volcanic vents. As we proceed,
however, to the opposite side of the lake, we find a considerable quantity of
scoriae and some lava, and see the whole surface of the soil sparkling with
volcanic sand, and strewed with ejected fragments of half-fused shale, which
preserves its laminated texture in the interior, while it has a vitrified or
scoriform coating.
Other crater lakes of circular or oval form, and hollowed out of similar ancient
strata, occur in the Upper Eifel, where copious aeriform discharges have taken
place, throwing out vast heaps of pulverized shale into the air. I know of no
other extinct volcanoes where gaseous explosions of such magnitude have been
attended by the emission of so small a quantity of lava. Yet I looked in vain in
the Eifel for any appearances which could lend support to the hypothesis that
the sudden rushing out of such enormous volumes of gas had ever lifted up the
stratified rocks immediately around the vent so as to form conical masses,
having their strata dipping outward on all sides from a central axis, as is
assumed in the theory of elevation craters, alluded to in the last chapter.
I have already given (Figure 590) an example in the Eifel of a small stream of
lava which issued from one of the craters of that district at Bertrich-Baden. It
shows that when some of these volcanoes were in action the valleys had already
been eroded to their present depth.
TRASS.
The tufaceous alluvium called trass, which has covered large areas in the Eifel,
and choked up some valleys now partially re-excavated, is unstratified. Its base
consists almost entirely of pumice, in which are included fragments of basalt
and other lavas, pieces of burnt shale, slate, and sandstone, and numerous
trunks and branches of trees. If, as is probable, this trass was formed during
the period of volcanic eruptions, it may have originated in the manner of the
moya of the Andes.
We may easily conceive that a similar mass might now be produced, if a copious
evolution of gases should occur in one of the lake-basins. If a breach should be
made in the side of the cone, the flood would sweep away great heaps of ejected
fragments of shale and sandstone, which would be borne down into the adjoining
valleys. Forests might be torn up by such a flood, and thus the occurrence of
the numerous trunks of trees dispersed irregularly through the trass can be
explained. The manner in which this trass conforms to the shape of the present
valleys implies its comparatively modern origin, probably not dating farther
back than the Pliocene Period.
CHAPTER XXX.
AGE OF VOLCANIC ROCKS
CONTINUED.
Volcanic Rocks of the Upper Miocene Period.
Madeira.
Grand Canary.
Azores.
Lower Miocene Volcanic Rocks.
Isle of Mull.
Staffa and Antrim.
The Eifel.
Upper and Lower Miocene Volcanic Rocks of Auvergne.
Hill of Gergovia.
Eocene Volcanic Rocks of Monte Bolca.
Trap of Cretaceous Period.
Oolitic Period.
Triassic Period.
Permian Period.
Carboniferous Period.
Erect Trees buried in Volcanic Ash in the Island of Arran.
Old Red Sandstone Period.
Silurian Period.
Cambrian Period.
Laurentian Volcanic Rocks.
VOLCANIC ROCKS OF THE UPPER MIOCENE PERIOD.
MADEIRA.
The greater part of the volcanic eruptions of Madeira, as we have already seen
(Chapter 29), belong to the Pliocene Period, but the most ancient of them are of
Upper Miocene date, as shown by the fossil shells included in the marine tuffs
which have been upraised at San Vicente, in the northern part of the island, to
the height of 1300 feet above the level of the sea. A similar marine and
volcanic formation constitutes the fundamental portion of the neighbouring
island of Porto Santo, forty miles distant from Madeira, and is there elevated
to an equal height, and covered, as in Madeira, with lavas of supra-marine
origin.
The largest number of fossils have been collected from the tuffs and
conglomerates and some beds of limestone in the island of Baixo, off the
southern extremity of Porto Santo. They amount in this single locality to more
than sixty in number, of which about fifty are mollusca, but many of these are
only casts. Some of the shells probably lived on the spot during the intervals
between eruptions, and some may have been cast up into the water or air together
with muddy ejections, and, falling down again, have been deposited on the bottom
of the sea. The hollows in some of the fragments of vesicular lava of which the
breccias and conglomerates are composed are partially filled with calc-sinter,
being thus half converted into amygdaloids. Among the fossil shells common to
Madeira and Porto Santo, large cones, strombs, and cowries are conspicuous among
the univalves, and Cardium, Spondylus, and Lithodomus among the
lamellibranchiate bivalves, and among the Echinoderms the large Clypeaster
called C. altus, an extinct European Miocene fossil.
The largest list of fossils has been published by Mr. Karl Meyer, in Hartung's
"Madeira;" but in the collection made by myself, and in a still larger one
formed by Mr. J. Yate Johnson, several remarkable forms not in Meyer's list
occur, as, for example, Pholadomya, and a large Terebra. Mr. Johnson also found
a fine specimen of Nautilus (Atruria) ziczac (Figure 211), a well-known Falunian
fossil of Europe; and in the same volcanic tuff of Baixo, the Echinoderm Brisus
Scillae, a living Mediterranean species, found fossil in the Miocene strata of
Malta. Mr. Meyer identifies one-third of the Madeira shells with known European
Miocene (or Falunian) forms. The huge Strombus of San Vicente and Porto Santo,
S. Italicus, is an extinct shell of the Sub-apennine or Older Pliocene
formations. The mollusca already obtained from various localities of Madeira and
Porto Santo are not less than one hundred in number, and, according to the late
Dr. S.P. Woodward, rather more than a third are of species still living, but
many of these are not now inhabitants of the neighbouring sea.
It has been remarked (Chapter 16), that in the Older Pliocene and Upper Miocene
deposits of Europe many forms occur of a more southern aspect than those now
inhabiting the nearest sea. In like manner the fossil corals, or Zoantharia, six
in number, which I obtained from Madeira, of the genera Astraea, Sarcinula,
Hydnophora, were pronounced by Mr. Lonsdale to be forms foreign to the adjacent
coasts, and agreeing with the fauna of a sea warmer than that now separating
Madeira from the nearest part of the African coast. We learn, indeed, from the
observations made in 1859, by the Reverend R.T. Lowe, that more than one-half,
or fifty-three in ninety, of the marine mollusks collected by him from the sandy
beach of Mogador are common British species, although Mogador is 18 1/2 degrees
south of the nearest shores of England. The living shells of Madeira and Porto
Santo are in like manner those of a temperate climate, although in great part
differing specifically from those of Mogador. (Linnean Proceedings Zoology
1860.)
GRAND CANARY.
In the Canaries, especially in the Grand Canary, the same marine Upper Miocene
formation is found. Stratified tuffs, with intercalated conglomerates and lavas,
are there seen in nearly horizontal layers in sea-cliffs about 300 feet high,
near Las Palmas. Mr. Hartung and I were unable to find marine shells in these
tuffs at a greater elevation than 400 feet above the sea; but as the deposit to
which they belong reaches to the height of 1100 feet or more in the interior, we
conceive that an upheaval of at least that amount has taken place. The
Clypeaster altus, Spondylus gaederopus, Pectunculus pilosus, Cardita calyculata,
and several other shells, serve to identify this formation with that of the
Madeiras, and Ancillaria glandiformis, which is not rare, and some other
fossils, remind us of the faluns of Touraine.
The sixty-two Miocene species which I collected in the Grand Canary were
referred by the late Dr. S.P. Woodward to forty-seven genera, ten of which are
no longer represented in the neighbouring sea, namely Corbis, an African form,
Hinnites, now living in Oregon, Thecidium (T. Mediterranean, identical with the
Miocene fossil of St. Juvat, in Brittany), Calyptraea, Hipponyx, Nerita, Erato,
Oliva, Ancillaria, and Fasciolaria.
These tuffs of the southern shores of the Grand Canary, containing the Upper
Miocene shells, appear to be about the same age as the most ancient volcanic
rocks of the island, composed of slaty diabase, phonolite, and trachyte. Over
the marine lavas and tuffs trachytic and basaltic products of subaerial volcanic
origin, between 4000 and 5000 feet in thickness, have been piled, the central
parts of the Grand Canary reaching the height of about 6000 feet above the level
of the sea. A large portion of this mass is of Pliocene date, and some of the
latest lavas have been poured out since the time when the valleys were already
excavated to within a few feet of their present depth.
On the whole, the rocks of the Grand Canary, an island of a nearly circular
shape, and 6 1/2 geographical miles diameter, exhibit proofs of a long series of
eruptions beginning like those of Madeira, Porto Santo, and the Azores, in the
Upper Miocene period, and continued to the Post-Pliocene. The building up of the
Grand Canary by subaerial eruptions, several thousand feet thick, went on
simultaneously with the gradual upheaval of the earliest products of submarine
eruptions, in the same manner as the Pliocene marine strata of the oldest parts
of Vesuvius and Etna have been upraised during eruptions of Post-tertiary date.
In proof that movements of elevation have actually continued down to Post-
tertiary times, I may remark that I found raised beaches containing shells of
the Recent Period in the Grand Canary, Teneriffe, and Porto Santo. The most
remarkable raised beach which I observed in the Grand Canary, in the study of
which I was assisted by Don Pedro Maffiotte, is situated in the north-eastern
part of the island at San Catalina, about a quarter of a mile north of Las
Palmas. It intervenes between the base of the high cliff formed of the tuffs
with Miocene shells and the sea-shore. From this beach, at an elevation of
twenty-five feet above high-water mark, and at a distance of about 150 feet from
the present shore, I obtained more than fifty species of living marine shells.
Many of them, according to Dr. S.P. Woodward, are no longer inhabitants of the
contiguous sea, as, for example, Strombus bubonius, which is still living on the
West Coast of Africa, and Cerithium procerum, found at Mozambique; others are
Mediterranean species, as Pecten Jacobaeus and P. polymorphus. Some of these
testacea, such as Cardita squamosa, are inhabitants of deep water, and the
deposit on the whole seems to indicate a depth of water exceeding a hundred
feet.
AZORES.
In the island of St. Mary's, one of the Azores, marine fossil shells have long
been known. They are found on the north-east coast on a small projecting
promontory called Ponta do Papagaio (or Point-Parrot), chiefly in a limestone
about twenty feet thick, which rests upon, and is again covered by, basaltic
lavas, scoriae, and conglomerates. The pebbles in the conglomerate are cemented
together with carbonate of lime.
Mr. Hartung, in his account of the Azores, published in 1860, describes twenty-
three shells from St. Mary's (Hartung Die Azoren 1860 also Insel Gran Canaria,
Madeira und Porto Santo 1864 Leipsig.), of which eight perhaps are identical
with living species, and twelve are with more or less certainty referred to
European Tertiary forms, chiefly Upper Miocene. One of the most characteristic
and abundant of the new species, Cardium Hartungi, not known as fossil in
Europe, is very common in Porto Santo and Baixo, and serves to connect the
Miocene fauna of the Azores and the Madeiras. In some of the Azores, as well as
in the Canary islands, the volcanic fires are not yet extinct, as the recorded
eruptions of Lanzerote, Teneriffe, Palma, St. Michael's, and others, attest.
LOWER MIOCENE VOLCANIC ROCKS.
ISLE OF MULL AND ANTRIM.
I may refer the reader to the account already given (Chapter 15) of leaf-beds at
Ardtun, in the Isle of Mull in the Hebrides, which bear a relation to the
associated volcanic rocks of Lower Miocene date analogous to that which the
Madeira leaf-bed, above described (Chapter 29), bears to the Pliocene lavas of
that island. Mr. Geikie has shown that the volcanic rocks in Mull are above 3000
feet in thickness. There seems little doubt that the well-known columnar basalt
of Staffa, as well as that of Antrim in Ireland, are of the same age, and not of
higher antiquity, as once suspected.
THE EIFEL.
A large portion of the volcanic rocks of the Lower Rhine and the Eifel are
coeval with the Lower Miocene deposits to which most of the "Brown-Coal" of
Germany belongs. The Tertiary strata of that age are seen on both sides of the
Rhine, in the neighbourhood of Bonn, resting unconformably on highly inclined
and vertical strata of Silurian and Devonian rocks. The Brown-Coal formation of
that region consists of beds of loose sand, sandstone, and conglomerate, clay
with nodules of clay-iron-stone, and occasionally silex. Layers of light brown
and sometimes black lignite are interstratified with the clays and sands, and
often irregularly diffused through them. They contain numerous impressions of
leaves and stems of trees, and are extensively worked for fuel, whence the name
of the formation. In several places layers of trachytic tuff are
interstratified, and in these tuffs are leaves of plants identical with those
found in the brown-coal, showing that, during the period of the accumulation of
the latter, some volcanic products were ejected. The igneous rocks of the
Westerwald, and of the mountains called the Siebengebirge, consist partly of
basaltic and partly of trachytic lavas, the latter being in general the more
ancient of the two. There are many varieties of trachyte, some of which are
highly crystalline, resembling a coarse-grained granite, with large separate
crystals of feldspar. Trachytic tuff is also very abundant.
M. Von Dechen, in his work on the Siebengebirge, has given a copious list of the
animal and vegetable remains of the fresh-water strata associated with the
brown-coal of that part of Germany. (Geognost. Beschreib. des Siebengebirges am
Rhein Bonn 1852.) Plants of the genera Flabellaria, Ceanothus, and Daphnogene,
including D. cinnamomifolia (Figure 155), occur in these beds, with nearly 150
other plants. The fishes of the brown-coal near Bonn are found in a bituminous
shale, called paper-coal, from being divisible into extremely thin leaves. The
individuals are very numerous; but they appear to belong to a small number of
species, some of which were referred by Agassiz to the genera Leuciscus, Aspius,
and Perca. The remains of frogs also, of extinct species, have been discovered
in the paper-coal; and a complete series may be seen in the museum at Bonn, from
the most imperfect state of the tadpole to that of the full-grown animal. With
these a salamander, scarcely distinguishable from the recent species, has been
found, and the remains of many insects.
UPPER AND LOWER MIOCENE VOLCANIC ROCKS OF AUVERGNE.
The extinct volcanoes of Auvergne and Cantal, in central France, seem to have
commenced their eruptions in the Lower Miocene period, but to have been most
active during the Upper Miocene and Pliocene eras. I have already alluded to the
grand succession of events of which there is evidence in Auvergne since the last
retreat of the sea (see Chapter 29).
The earliest monuments of the Tertiary Period in that region are lacustrine
deposits of great thickness, in the lowest conglomerates of which are rounded
pebbles of quartz, mica-schist, granite, and other non-volcanic rocks, without
the slightest intermixture of igneous products. To these conglomerates succeed
argillaceous and calcareous marls and limestones, containing Lower Miocene
shells and bones of mammalia, the higher beds of which sometimes alternate with
volcanic tuff of contemporaneous origin. After the filling up or drainage of the
ancient lakes, huge piles of trachytic and basaltic rocks, with volcanic
breccias, accumulated to a thickness of several thousand feet, and were
superimposed upon granite, or the contiguous lacustrine strata. The greater
portion of these igneous rocks appear to have originated during the Upper
Miocene and Pliocene periods; and extinct quadrupeds of those eras, belonging to
the genera Mastodon, Rhinoceros, and others, were buried in ashes and beds of
alluvial sand and gravel, which owe their preservation to overspreading sheets
of lava.
In Auvergne, the most ancient and conspicuous of the volcanic masses is Mont
Dor, which rests immediately on the granitic rocks standing apart from the
fresh-water strata. This great mountain rises suddenly to the height of several
thousand feet above the surrounding platform, and retains the shape of a
flattened and somewhat irregular cone, the slope of which is gradually lost in
the high plain around. This cone is composed of layers of scoriae, pumice-
stones, and their fine detritus, with interposed beds of trachyte and basalt,
which descend often in uninterrupted sheets until they reach and spread
themselves round the base of the mountain. (Scrope Central France page 98.)
Conglomerates, also, composed of angular and rounded fragments of igneous rocks,
are observed to alternate with the above; and the various masses are seen to dip
off from the central axis, and to lie parallel to the sloping flanks of the
mountain. The summit of Mont Dor terminates in seven or eight rocky peaks, where
no regular crater can now be traced, but where we may easily imagine one to have
existed, which may have been shattered by earthquakes, and have suffered
degradation by aqueous agents. Originally, perhaps, like the highest crater of
Etna, it may have formed an insignificant feature in the great pile, and, like
it, may frequently have been destroyed and renovated.
Respecting the age of the great mass of Mont Dor, we can not come at present to
any positive decision, because no organic remains have yet been found in the
tuffs, except impressions of the leaves of trees of species not yet determined.
It has already been stated (Chapter 15) that the earliest eruptions must have
been posterior in origin to those grits and conglomerates of the fresh-water
formation of the Limagne which contain no pebbles of volcanic rocks. But there
is evidence at a few points, as in the hill of Gergovia, presently to be
mentioned, that some eruptions took place before the great lakes were drained,
while others occurred after the desiccation of those lakes, and when deep
valleys had already been excavated through fresh-water strata.
The valley in which the cone of Tartaret, above-mentioned (Chapter 29), is
situated affords an impressive monument of the very different dates at which the
igneous eruptions of Auvergne have happened; for while the cone itself is of
Post-Pliocene date, the valley is bounded by lofty precipices composed of sheets
of ancient columnar trachyte and basalt, which once flowed from the summit of
Mont Dor in some part of the Miocene period. These Miocene lavas had accumulated
to a thickness of nearly 1000 feet before the ravine was cut down to the level
of the river Couze, a river which was at length dammed up by the modern cone and
the upper part of its course transformed into a lake.
GERGOVIA.
(FIGURE 604. Hill of Gergovia.
Section through (bottom to top) White and green marls: Altered Marl: Dike:
Altered Marl: Limestone and peperino: Tuffs: Blue marls: White and yellow marl:
Basaltic capping.)
It has been supposed by some observers that there is an alternation of a
contemporaneous sheet of lava with fresh-water strata in the hill of Gergovia,
near Clermont. But this idea has arisen from the intrusion of the dike
represented in Figure 604, which has altered the green and white marls both
above and below. Nevertheless, there is a real alternation of volcanic tuff with
strata containing Lower Miocene fresh-water shells, among others a Melania
allied to M. inquinata (Figure 217), with a Melanopsis and a Unio; there can,
therefore, be no doubt that in Auvergne some volcanic explosions took place
before the drainage of the lakes, and at a time when the Lower Miocene species
of animals and plants still flourished.
EOCENE VOLCANIC ROCKS.
MONTE BOLCA.
The fissile limestone of Monte Bolca, near Verona, has for many centuries been
celebrated in Italy for the number of perfect Ichthyolites which it contains.
Agassiz has described no less than 133 species of fossil fish from this single
deposit, and the multitude of individuals by which many of the species are
represented is attested by the variety of specimens treasured up in the
principal museums of Europe. They have been all obtained from quarries worked
exclusively by lovers of natural history, for the sake of the fossils. Had the
lithographic stone of Solenhofen, now regarded as so rich in fossils, been in
like manner quarried solely for scientific objects, it would have remained
almost a sealed book to palaeontologists, so sparsely are the organic remains
scattered through it. When I visited Monte Bolca, in company with Sir Roderick
Murchison, in 1828, we ascertained that the fish-bearing beds were of Eocene
date, containing well-known species of Nummulites, and that a long series of
submarine volcanic eruptions, evidently contemporaneous, had produced beds of
tuff, which are cut through by dikes of basalt. There is evidence here of a long
series of submarine volcanic eruptions of Eocene date, and during some of them,
as Sir R. Murchison has suggested, shoals of fish were probably destroyed by the
evolution of heat, noxious gases, and tufaceous mud, just as happened when
Graham's Island was thrown up between Sicily and Africa in 1831, at which time
the waters of the Mediterranean were seen to be charged with red mud, and
covered with dead fish over a wide area. (Principles of Geology chapter 26 9th
edition page 432.)
Associated with the marls and limestones of Monte Bolca are beds containing
lignite and shale with numerous plants, which have been described by Unger and
Massalongo, and referred by them to the Eocene period. I have already cited
(Chapter 16) Professor Heer's remark, that several of the species are common to
Monte Bolca and the white clay of Alum Bay, a Middle Eocene deposit; and the
same botanist dwells on the tropical character of the flora of Monte Bolca and
its distinctness from the sub-tropical flora of the Lower Miocene of Switzerland
and Italy, in which last there is a far more considerable mixture of forms of a
temperate climate, such as the willow, poplar, birch, elm, and others. That
scarcely any one of the Monte Bolca fish should have been found in any other
locality in Europe, is a striking illustration of the extreme imperfection of
the palaeontological record. We are in the habit of imagining that our insight
into the geology of the Eocene period is more than usually perfect, and we are
certainly acquainted with an almost unbroken succession of assemblages of shells
passing one into the other from the era of the Thanet sands to that of the
Bembridge beds or Paris gypsum. The general dearth, therefore, of fish in the
different members of the Eocene series, Upper, Middle, and Lower, might induce a
hasty reasoner to conclude that there was a poverty of ichthyic forms during
this period; but when a local accident, like the volcanic eruptions of Monte
Bolca, occurs, proofs are suddenly revealed to us of the richness and variety of
this great class of vertebrata in the Eocene sea. The number of genera of Monte
Bolca fish is, according to Agassiz, no less than seventy-five, twenty of them
peculiar to that locality, and only eight common to the antecedent Cretaceous
period. No less than forty-seven out of the seventy-five genera make their
appearance for the first time in the Monte Bolca rocks, none of them having been
met with as yet in the antecedent formations. They form a great contrast to the
fish of the secondary strata, as, with the exception of the Placoids, they are
all Teleosteans, only one genus, Pycnodus, belonging to the order of Ganoids,
which form, as before stated, the vast majority of the ichthyolites entombed in
the secondary are Mesozoic rocks.
CRETACEOUS PERIOD.
M. Virlet, in his account of the geology of the Morea, page 205, has clearly
shown that certain traps in Greece are of Cretaceous date; as those, for
example, which alternate conformably with cretaceous limestone and greensand
between Kastri and Damala, in the Morea. They consist in great part of diallage
rocks and serpentine, and of an amygdaloid with calcareous kernels, and a base
of serpentine. In certain parts of the Morea, the age of these volcanic rocks is
established by the following proofs: first, the lithographic limestones of the
Cretaceous era are cut through by trap, and then a conglomerate occurs, at
Nauplia and other places, containing in its calcareous cement many well-known
fossils of the chalk and greensand, together with pebbles formed of rolled
pieces of the same serpentinous trap, which appear in the dikes above alluded
to.
PERIOD OF OOLITE AND LIAS.
Although the green and serpentinous trap-rocks of the Morea belong chiefly to
the Cretaceous era, as before mentioned, yet it seems that some eruptions of
similar rocks began during the Oolitic period (Boblaye and Virlet Morea page
23.); and it is probable that a large part of the trappean masses, called
ophiolites in the Apennines, and associated with the limestone of that chain,
are of corresponding age.
TRAP OF THE NEW RED SANDSTONE PERIOD.
In the southern part of Devonshire, trappean rocks are associated with New Red
Sandstone, and, according to Sir H. De la Beche, have not been intruded
subsequently into the sandstone, but were produced by contemporaneous volcanic
action. Some beds of grit, mingled with ordinary red marl, resemble sands
ejected from a crater; and in the stratified conglomerates occurring near
Tiverton are many angular fragments of trap porphyry, some of them one or two
tons in weight, intermingled with pebbles of other rocks. These angular
fragments were probably thrown out from volcanic vents, and fell upon
sedimentary matter then in the course of deposition. (De la Beche Geological
Proceedings volume 2 page 198.)
TRAP OF THE PERMIAN PERIOD.
The recent investigations of Mr. Archibald Geikie in Ayrshire have shown that
some of the volcanic rocks in that county are of Permian age, and it appears
highly probable that the uppermost portion of Arthur's Seat in the suburbs of
Edinburgh marks the site of an eruption of the same era.
TRAP OF THE CARBONIFEROUS PERIOD.
Two classes of contemporaneous trap-rocks occur in the coal-field of the Forth,
in Scotland. The newest of these, connected with the higher series of coal-
measures, is well exhibited along the shores of the Forth, in Fifeshire, where
they consist of basalt with olivine, amygdaloid, greenstone, wacke, and tuff.
They appear to have been erupted while the sedimentary strata were in a
horizontal position, and to have suffered the same dislocations which those
strata have subsequently undergone. In the volcanic tuffs of this age are found
not only fragments of limestone, shale, flinty slate, and sandstone, but also
pieces of coal. The other or older class of carboniferous traps are traced along
the south margin of Stratheden, and constitute a ridge parallel with the Ochils,
and extending from Stirling to near St. Andrews. They consist almost exclusively
of greenstone, becoming, in a few instances, earthy and amygdaloidal. They are
regularly interstratified with the sandstone, shale, and iron-stone of the lower
coal-measures, and, on the East Lomond, with Mountain Limestone. I examined
these trap-rocks in 1838, in the cliffs south of St. Andrews, where they consist
in great part of stratified tuffs, which are curved, vertical, and contorted,
like the associated coal-measures. In the tuff I found fragments of
carboniferous shale and limestone, and intersecting veins of greenstone.
FIFE-- FLISK DIKE.
A trap dike was pointed out to me by Dr. Fleming, in the parish of Flisk, in the
northern part of the county of Fife, which cuts through the grey sandstone and
shale, forming the lowest part of the Old Red Sandstone, but which may probably
be of carboniferous date. It may be traced for many miles, passing through the
amygdaloidal and other traps of the hill called Norman's Law in that parish. In
its course it affords a good exemplification of the passage from the trappean
into the Plutonic, or highly crystalline texture. Professor Gustavus Rose, to
whom I submitted specimens of this dike, found it to be dolerite, and composed
of greenish black augite and Labrador feldspar, the latter being the most
abundant ingredient. A small quantity of magnetic iron, perhaps titaniferous, is
also present. The result of this analysis is interesting, because both the
ancient and modern lavas of Etna consist in like manner of augite, Labradorite,
and titaniferous iron.
ERECT TREES BURIED IN VOLCANIC ASH AT ARRAN.
An interesting discovery was made in 1867 by Mr. E.A. Wunsch in the
carboniferous strata of the north-eastern part of the island of Arran. In the
sea-cliff about five miles north of Corrie, near the village of Laggan, strata
of volcanic ash occur, forming a solid rock cemented by carbonate of lime and
enveloping trunks of trees, determined by Mr. Binney to belong to the genera
Sigillaria and Lepidodendron. Some of these trees are at right angles to the
planes of stratification, while others are prostrate and accompanied by leaves
and fruits of the same genera. I visited the spot in company with Mr. Wunsch in
1870, and saw that the trees with their roots, of which about fourteen had been
observed, occur at two distinct levels in volcanic tuffs parallel to each other,
and inclined at an angle of about 40 degrees, having between them beds of shale
and coaly matter seven feet thick. It is evident that the trees were overwhelmed
by a shower of ashes from some neighbouring volcanic vent, as Pompeii was buried
by matter ejected from Vesuvius. The trunks, several of them from three to five
feet in circumference, remained with their Stigmarian roots spreading through
the stratum below, which had served as a soil. The trees must have continued for
years in an upright position after they were killed by the shower of burning
ashes, giving time for a partial decay of the interior, so as to afford hollow
cylinders into which the spores of plants were wafted. These spores germinated
and grew, until finally their stems were petrified by carbonate of lime like
some of the remaining portions of the wood of the containing Sigillaria. Mr.
Carruthers has discovered that sometimes the plants which had thus grown and
become fossil in the inside of a single trunk belonged to several distinct
genera. The fact that the tree-bearing deposits now dip at an angle of 40
degrees is the more striking, as they must clearly have remained horizontal and
undisturbed during a long period of intermittent and contemporaneous volcanic
action.
In some of the associated carboniferous shales, ferns and calamites occur, and
all the phenomena of the successive buried forests remind us of the sections in
Figures 439 and 440 of the Nova Scotia coal-measures, with this difference only,
that in the case of the South Joggins the fossilisation of the trees was
effected without the eruption of volcanic matter.
TRAP OF THE OLD RED SANDSTONE PERIOD.
By referring to the section explanatory of the structure of Forfarshire, already
given (Chapter 5), the reader will perceive that beds of conglomerate, No. 3,
occur in the middle of the Old Red Sandstone system, 1, 2, 3, 4. The pebbles in
these conglomerates are sometimes composed of granitic and quartzose rocks,
sometimes exclusively of different varieties of trap, which last, although
purposely omitted in the section referred to, is often found either intruding
itself in amorphous masses and dikes into the old fossiliferous tilestones, No.
4, or alternating with them in conformable beds. All the different divisions of
the red sandstone, 1, 2, 3, 4, are occasionally intersected by dikes, but they
are very rare in Nos. 1 and 2, the upper members of the group consisting of red
shale and red sandstone. These phenomena, which occur at the foot of the
Grampians, are repeated in the Sidlaw Hills; and it appears that in this part of
Scotland volcanic eruptions were most frequent in the earlier part of the Old
Red Sandstone period. The trap-rocks alluded to consist chiefly of feldspathic
porphyry and amygdaloid, the kernels of the latter being sometimes calcareous,
often chalcedonic, and forming beautiful agates. We meet also with claystone,
greenstone, compact feldspar, and tuff. Some of these rocks look as if they had
flowed as lavas over the bottom of the sea, and enveloped quartz pebbles which
were lying there, so as to form conglomerates with a base of greenstone, as is
seen in Lumley Den, in the Sidlaw Hills. On either side of the axis of this
chain of hills (see Figure 55), the beds of massive trap, and the tuffs composed
of volcanic sand and ashes, dip regularly to the south-east or north-west,
conformably with the shales and sandstones.
But the geological structure of the Pentland Hills, near Edinburgh, shows that
igneous rocks were there formed during the newer part of the Devonian or "Old
Red" period. These hills are 1900 feet high above the sea, and consist of
conglomerates and sandstones of Upper Devonian age, resting on the inclined
edges of grits and slates of Lower Devonian and Upper Silurian date. The
contemporaneous volcanic rocks intercalated in this Upper Old Red consist of
feldspathic lavas, or feldstones, with associated tuffs or ashy beds. The lavas
were some of them originally compact, others vesicular, and these last have been
converted into amygdaloids. They consist chiefly of feldstone or compact
feldspar. The Pentland Hills, say Messrs. Maclaren and Geikie, afford evidence
that at the time of the Upper Old Red Sandstone, the district to the south-west
of Edinburgh was for a long while the seat of a powerful volcano, which sent out
massive streams of lava and showers of ash, and continued active until well-nigh
the dawn of the Carboniferous period. (Maclaren Geology of Fife and Lothians.
Geikie Transactions of the Royal Society Edinburgh 1860-1861.)
SILURIAN VOLCANIC ROCKS.
It appears from the investigations of Sir R. Murchison in Shropshire, that when
the Lower Silurian strata of that country were accumulating, there were frequent
volcanic eruptions beneath the sea; and the ashes and scoriae then ejected gave
rise to a peculiar kind of tufaceous sandstone or grit, dissimilar to the other
rocks of the Silurian series, and only observable in places where syenitic and
other trap-rocks protrude. These tuffs occur on the flanks of the Wrekin and
Caer Caradoc, and contain Silurian fossils, such as casts of encrinites,
trilobites, and mollusca. Although fossiliferous, the stone resembles a sandy
claystone of the trap family. (Murchison Silurian System etc. page 230.)
Thin layers of trap, only a few inches thick, alternate in some parts of
Shropshire and Montgomeryshire with sedimentary strata of the Lower Silurian
system. This trap consists of slaty porphyry and granular feldspar rock, the
beds being traversed by joints like those in the associated sandstone,
limestone, and shale, and having the same strike and dip. (Ibid. page 212.)
In Radnorshire there is an example of twelve bands of stratified trap,
alternating with Silurian schists and flagstones, in a thickness of 350 feet.
The bedded traps consist of feldspar porphyry, and other varieties; and the
interposed Llandeilo flags are of sandstone and shale, with trilobites and
graptolites. (Murchison Silurian System etc. page 325.)
The Snowdonian hills in Carnarvonshire consist in great part of volcanic tuffs,
the oldest of which are interstratified with the Bala and Llandeilo beds. There
are some contemporaneous feldspathic lavas of this era, which, says Professor
Ramsay, alter the slates on which they repose, having doubtless been poured out
over them, in a melted state, whereas the slates which overlie them having been
subsequently deposited after the lava had cooled and consolidated, have entirely
escaped alteration. But there are greenstones associated with the same
formation, which, although they are often conformable to the slates, are in
reality intrusive rocks. They alter the stratified deposits both above and below
them, and when traced to great distances are sometimes seen to cut through the
slates, and to send off branches. Nevertheless, these greenstones appear to
belong, like the lavas, to the Lower Silurian period.
CAMBRIAN VOLCANIC ROCKS.
The Lingula beds in North Wales have been described as 5000 feet in thickness.
In the upper portion of these deposits volcanic tuffs or ashy materials are
interstratified with ordinary muddy sediment, and here and there associated with
thick beds of feldspathic lava. These rocks form the mountains called the Arans
and the Arenigs; numerous greenstones are associated with them, which are
intrusive, although they often run in the lines of bedding for a space. "Much of
the ash," says Professor Ramsay, "seems to have been subaerial. Islands, like
Graham's Island, may have sometimes raised their craters for various periods
above the water, and by the waste of such islands some of the ashy matter became
waterworn, whence the ashy conglomerate. Viscous matter seems also to have been
shot into the air as volcanic bombs, which fell among the dust and broken
crystals (that often form the ashes) before perfect cooling and consolidation
had taken place." (Quarterly Geological Journal volume 9 page 170 1852.)
LAURENTIAN VOLCANIC ROCKS.
The Laurentian rocks in Canada, especially in Ottawa and Argenteuil, are the
oldest intrusive masses yet known. They form a set of dikes of a fine-grained
dark greenstone or dolerite, composed of feldspar and pyroxene, with occasional
scales of mica and grains of pyrites. Their width varies from a few feet to a
hundred yards, and they have a columnar structure, the columns being truly at
right angles to the plane of the dike. Some of the dikes send off branches.
These dolerites are cut through by intrusive syenite, and this syenite, in its
turn, is again cut and penetrated by feldspar porphyry, the base of which
consists of petrosilex, or a mixture of orthoclase and quartz. All these trap-
rocks appear to be of Laurentian date, as the Cambrian and Huronian rocks rest
unconformably upon them. (Logan Geology of Canada 1863.) Whether some of the
various conformable crystalline rocks of the Laurentian series, such as the
coarse-grained granitoid and porphyritic varieties of gneiss, exhibiting
scarcely any signs of stratification, and some of the serpentines, may not also
be of volcanic origin, is a point very difficult to determine in a region which
has undergone so much metamorphic action.
CHAPTER XXXI.
PLUTONIC ROCKS.
General Aspect of Plutonic Rocks.
Granite and its Varieties.
Decomposing into Spherical Masses.
Rude columnar Structure.
Graphic Granite.
Mutual Penetration of Crystals of Quartz and Feldspar.
Glass Cavities in Quartz of Granite.
Porphyritic, talcose, and syenitic Granite.
Schorlrock and Eurite.
Syenite.
Connection of the Granites and Syenites with the Volcanic Rocks.
Analogy in Composition of Trachyte and Granite.
Granite Veins in Glen Tilt, Cape of Good Hope, and Cornwall.
Metalliferous Veins in Strata near their Junction with Granite.
Quartz Veins.
Exposure of Plutonic Rocks at the surface due to Denudation.
The Plutonic rocks may be treated of next in order, as they are most nearly
allied to the volcanic class already considered. I have described, in the first
chapter, these Plutonic rocks as the unstratified division of the crystalline or
hypogene formations, and have stated that they differ from the volcanic rocks,
not only by their more crystalline texture, but also by the absence of tuffs and
breccias, which are the products of eruptions at the earth's surface, whether
thrown up into the air or the sea. They differ also by the absence of pores or
cellular cavities, to which the expansion of the entangled gases gives rise in
ordinary lava, never being scoriaceous or amygdaloidal, and never forming a
porphyry with an uncrystalline base, nor alternating with tuffs.
From these and other peculiarities it has been inferred that the granites have
been formed at considerable depths in the earth, and have cooled and
crystallised slowly under great pressure, where the contained gases could not
expand. The volcanic rocks, on the contrary, although they also have risen up
from below, have cooled from a melted state more rapidly upon or near the
surface. From this hypothesis of the great depth at which the granites
originated, has been derived the name of "Plutonic rocks." The beginner will
easily conceive that the influence of subterranean heat may extend downward from
the crater of every active volcano to a great depth below, perhaps several miles
or leagues, and the effects which are produced deep in the bowels of the earth
may, or rather must, be distinct; so that volcanic and Plutonic rocks, each
different in texture, and sometimes even in composition, may originate
simultaneously, the one at the surface, the other far beneath it. The Plutonic
formations also agree with the volcanic in having veins or ramifications
proceeding from central masses into the adjoining rocks, and causing alterations
in these last, which will be presently described. They also resemble trap in
containing no organic remains; but they differ in being more uniform in texture,
whole mountain masses of indefinite extent appearing to have originated under
conditions precisely similar.
The two principal members of the Plutonic family of rocks are Granite and
Syenite, each of which, with their varieties, bear very much the same relation
to each other as the trachytes bear to the basalts. Granite is a compound of
feldspar, quartz, and mica, the feldspars being rich in silica, which forms from
60 to 70 per cent of the whole aggregate. In Syenite quartz is rare or wanting,
hornblende taking the place of mica, and the proportion of silica not exceeding
50 to 60 per cent.
(FIGURE 605. Mass of granite near the Sharp Tor, Cornwall.)
(FIGURE 606. Granite having a cuboidal and rude columnar structure, Land's End,
Cornwall.)
Granite often preserves a very uniform character throughout a wide range of
territory, forming hills of a peculiar rounded form, usually clad with a scanty
vegetation. The surface of the rock is for the most part in a crumbling state,
and the hills are often surmounted by piles of stones like the remains of a
stratified mass, as in Figure 605, and sometimes like heaps of boulders, for
which they have been mistaken. The exterior of these stones, originally
quadrangular, acquires a rounded form by the action of air and water, for the
edges and angles waste away more rapidly than the sides. A similar spherical
structure has already been described as characteristic of basalt and other
volcanic formations, and it must be referred to analogous causes, as yet but
imperfectly understood. Although it is the general peculiarity of granite to
assume no definite shapes, it is nevertheless occasionally subdivided by
fissures, so as to assume a cuboidal, and even a columnar, structure. Examples
of these appearances may be seen near the Land's End, in Cornwall. (See Figure
606.)
(FIGURES 607 and 608. Graphic granite.
(FIGURE 607. Graphic granite. Section parallel to the laminae.)
(FIGURE 608. Graphic granite. Section transverse to the laminae.))
Feldspar, quartz, and mica are usually considered as the minerals essential to
granite, the feldspar being most abundant in quantity, and the proportion of
quartz exceeding that of mica. These minerals are united in what is termed a
confused crystallisation; that is to say, there is no regular arrangement of the
crystals in granite, as in gneiss (see Figure 622), except in the variety termed
graphic granite, which occurs mostly in granitic veins. This variety is a
compound of feldspar and quartz, so arranged as to produce an imperfect laminar
structure. The crystals of feldspar appear to have been first formed, leaving
between them the space now occupied by the darker-coloured quartz. This mineral,
when a section is made at right angles to the alternate plates of feldspar and
quartz, presents broken lines, which have been compared to Hebrew characters.
(See Figure 608.) The variety of granite called by the French Pegmatite, which
is a mixture of quartz and common feldspar, usually with some small admixture of
white silvery mica, often passes into graphic granite.
Ordinary granite, as well as syenite and eurite, usually contains two kinds of
feldspar: First, the common, or orthoclase, in which potash is the prevailing
alkali, and this generally occurs in large crystals of a white or flesh colour;
and secondly, feldspar in smaller crystals, in which soda predominates, usually
of a dead white or spotted, and striated like albite, but not the same in
composition. (Delesse Ann. des Mines 1852 tome 3 page 409 and 1848 tome 13 page
675.)
As a general rule, quartz, in a compact or amorphous state, forms a vitreous
mass, serving as the base in which feldspar and mica have crystallised; for
although these minerals are much more fusible than silex, they have often
imprinted their shapes upon the quartz. This fact, apparently so paradoxical,
has given rise to much ingenious speculation. We should naturally have
anticipated that, during the cooling of the mass, the flinty portion would be
the first to consolidate; and that the different varieties of feldspar, as well
as garnets and tourmalines, being more easily liquefied by heat, would be the
last. Precisely the reverse has taken place in the passage of most granite
aggregates from a fluid to a solid state, crystals of the more fusible minerals
being found enveloped in hard, transparent, glassy quartz, which has often taken
very faithful casts of each, so as to preserve even the microscopically minute
striations on the surface of prisms of tourmaline. Various explanations of this
phenomenon have been proposed by MM. de Beaumont, Fournet, and Durocher. They
refer to M. Gaudin's experiments on the fusion of quartz, which show that silex,
as it cools, has the property of remaining in a viscous state, whereas alumina
never does. This "gelatinous flint" is supposed to retain a considerable degree
of plasticity long after the granitic mixture has acquired a low temperature.
Occasionally we find the quartz and feldspar mutually imprinting their forms on
each other, affording evidence of the simultaneous crystallisation of both.
(Bulletin 2e serie 4 1304; and d'Archiac Hist. des Progres de la Geol. 1 38.)
According to the experiments and observations of Gustavus Rose, the quartz of
granite has the specific gravity of 2.6, which characterises silica when it is
precipitated from a liquid solvent, and not that inferior density, namely, 2.3,
which belongs to it when it cools in the laboratory from a state of fusion in
what is called the dry way. By some it had been rashly inferred that the manner
in which the consolidation of granite takes place is exceedingly different from
the cooling of lavas, and that the intense heat supposed to be necessary for the
production of mountain masses of Plutonic rocks might be dispensed with. But Mr.
David Forbes informs me that silica can crystallise in the dry way, and he has
found in quartz forming a constituent part of some trachytes, both from
Guadeloupe and Iceland, glass cavities quite similar to those met with in
genuine volcanic minerals.
These "glass cavities," which with many other kindred phenomena have been
carefully studied by Mr. Sorby, are those in which a liquid, on cooling, has
become first viscous and then solid without crystallising or undergoing a
definite change in its physical structure. Other cavities which, like those just
mentioned, are frequently discernible under the microscope in the minerals
composing granitic rocks, are filled, some of them with gas or vapour, others
with liquid, and by the movements of the bubbles thus included the distinctness
of such cavities from those filled with a glassy substance can be tested. Mr.
Sorby admits that the frequent occurrence of fluid cavities in the quartz of
granite implies that water was almost always present in the formation of this
rock; but the same may be said of almost all lavas, and it is now more than
forty years since Mr. Scrope insisted on the important part which water plays in
volcanic eruptions, being so intimately mixed up with the materials of the lava
that he supposed it to aid in giving mobility to the fluid mass. It is well
known that steam escapes for months, sometimes for years, from the cavities of
lava when it is cooling and consolidating. As to the result of Mr. Sorby's
experiments and speculations on this difficult subject, they may be stated in a
few words. He concludes that the physical conditions under which the volcanic
and granitic rocks originate are so far similar that in both cases they combine
igneous fusion, aqueous solution, and gaseous sublimation-- the proof, he says,
of the operation of water in the formation of granite being quite as strong as
of that of heat. (See Quarterly Geological Journal volume 14 pages 465, 488.)
When rocks are melted at great depths water must be present, for two reasons--
First, because rainwater and seawater are always descending through fissured and
porous rocks, and must at length find their way into the regions of subterranean
heat; and secondly, because in a state of combination water enters largely into
the composition of some of the most common minerals, especially those of the
aluminous class. But the existence of water under great pressure affords no
argument against our attributing an excessively high temperature to the mass
with which it is mixed up. Bunsen, indeed, imagines that in Iceland water
attains a white heat at a very moderate depth. To what extent some of the
metamorphic rocks containing the same minerals as the granites may have been
formed by hydrothermal action without the intervention of intense heat
comparable to that brought into play in a volcanic eruption, will be considered
when we treat of the metamorphic rocks in the thirty-third chapter.
PORPHYRITIC GRANITE.
(FIGURE 609. Porphyritic granite. Land's End, Cornwall.)
This name has been sometimes given to that variety in which large crystals of
common feldspar, sometimes more than three inches in length, are scattered
through an ordinary base of granite. An example of this texture may be seen in
the granite of the Land's End, in Cornwall (Figure 609). The two larger
prismatic crystals in this drawing represent feldspar, smaller crystals of which
are also seen, similar in form, scattered through the base. In this base also
appear black specks of mica, the crystals of which have a more or less perfect
hexagonal outline. The remainder of the mass is quartz, the translucency of
which is strongly contrasted to the opaqueness of the white feldspar and black
mica. But neither the transparency of the quartz nor the silvery lustre of the
mica can be expressed in the engraving.
The uniform mineral character of large masses of granite seems to indicate that
large quantities of the component elements were thoroughly mixed up together,
and then crystallised under precisely similar conditions. There are, however,
many accidental, or "occasional," minerals, as they are termed, which belong to
granite. Among these black schorl or tourmaline, actinolite, zircon, garnet, and
fluor spar are not uncommon; but they are too sparingly dispersed to modify the
general aspect of the rock. They show, nevertheless, that the ingredients were
not everywhere exactly the same; and a still greater difference may be traced in
the ever-varying proportions of the feldspar, quartz, and mica.
TALCOSE GRANITE
Talcose Granite, or Protogine of the French, is a mixture of feldspar, quartz,
and talc. It abounds in the Alps, and in some parts of Cornwall, producing by
its decomposition the kaolin or china clay, more than 12,000 tons of which are
annually exported from that country for the potteries.
SCHORL-ROCK, AND SCHORLY GRANITE.
The former of these is an aggregate of schorl, or tourmaline, and quartz. When
feldspar and mica are also present, it may be called schorly granite. This kind
of granite is comparatively rare.
EURITE, FELDSTONE.
Eurite is a rock in which the ingredients of granite are blended into a finely
granular mass, mica being usually absent, and, when present, in such minute
flakes as to be invisible to the naked eye. It is sometimes called FELDSTONE,
and when the crystals of feldspar are conspicuous it becomes FELDSPAR PORPHYRY.
All these and other varieties of granite pass into certain kinds of trap-- a
circumstance which affords one of many arguments in favour of what is now the
prevailing opinion, that the granites are also of igneous origin. The contrast
of the most crystalline form of granite to that of the most common and earthy
trap is undoubtedly great; but each member of the volcanic class is capable of
becoming porphyritic, and the base of the porphyry may be more and more
crystalline, until the mass passes to the kind of granite most nearly allied in
mineral composition.
SYENITIC GRANITE.
The quadruple compound of quartz, feldspar, mica, and hornblende, may be termed
Syenitic Granite, and forms a passage between the granites and the syenites.
This rock occurs in Scotland and in Guernsey.
SYENITE.
We now come to the second division of the Plutonic rocks, or those having less
than 60 per cent of silica, and which, as before stated, are usually called
syenitic. Syenite originally received its name from the celebrated ancient
quarries of Syene, in Egypt. It differs from granite in having hornblende as a
substitute for mica, and being without quartz. Werner at least considered
syenite as a binary compound of feldspar and hornblende, and regarded quartz as
merely one of its occasional minerals.
MIASCITE.
Miascite is one of the varieties of syenite most frequently spoken of; it is
composed chiefly of orthoclase and nepheline, with hornblende and quartz as
occasional accessory minerals. It derives its name from Miask, in the Ural
Mountains, where it was first discovered by Gustavus Rose. ZIRCON-SYENITE is
another variety closely allied to Miascite, but containing crystals of Zircon.
CONNECTION OF THE GRANITES AND SYENITES WITH THE VOLCANIC ROCKS.
The minerals which constitute alike the Plutonic and volcanic rocks consist,
almost exclusively, of seven elements, namely, silica, alumina, magnesia, lime,
soda, potash, and iron (see Table 28.1); and these may sometimes exist in about
the same proportions in a porous lava, a compact trap, and a crystalline
granite. The same lava, for example, may be glassy, or scoriaceous, or stony, or
porphyritic, according to the more or less rapid rate at which it cools.
It would be easy to multiply examples and authorities to prove the gradation of
the Plutonic into the trap rocks. On the western side of the Fiord of
Christiania, in Norway, there is a large district of trap, chiefly greenstone-
porphyry and syenitic-greenstone, resting on fossiliferous strata. To this, on
its southern limit, succeeds a region equally extensive of syenite, the passage
from the trappean to the crystalline Plutonic rock being so gradual that it is
impossible to draw a line of demarkation between them.
"The ordinary granite of Aberdeenshire," says Dr. MacCulloch, "is the usual
ternary compound of quartz, feldspar, and mica; though sometimes hornblende is
substituted for the mica. But in many places a variety occurs which is composed
simply of feldspar and hornblende; and in examining more minutely this duplicate
compound, it is observed in some places to assume a fine grain, and at length to
become undistinguishable from the greenstones of the trap family. It also passes
in the same uninterrupted manner into a basalt, and at length into a soft
claystone, with a schistose tendency on exposure, in no respect differing from
those of the trap islands of the western coast." The same author mentions, that
in Shetland a granite composed of hornblende, mica, feldspar, and quartz
graduates in an equally perfect manner into basalt. (System of Geology volume 1
pages 157 and 158.) In Hungary there are varieties of trachyte, which,
geologically speaking, are of modern origin, in which crystals, not only of
mica, but of quartz, are common, together with feldspar and hornblende. It is
easy to conceive how such volcanic masses may, at a certain depth from the
surface, pass downward into granite.
GRANITIC VEINS.
(Figures 610 and 611. Junction of granite and argillaceous schist in Glen Tilt.
(MacCulloch. (Geological Transactions First Series volume 3 plate 21.))
(FIGURE 610. Junction of granite and argillaceous schist in Glen Tilt.)
(FIGURE 611. Junction of granite and argillaceous schist in Glen Tilt.))
I have already hinted at the close analogy in the forms of certain granitic and
trappean veins; and it will be found that strata penetrated by Plutonic rocks
have suffered changes very similar to those exhibited near the contact of
volcanic dikes. Thus, in Glen Tilt, in Scotland, alternating strata of limestone
and argillaceous schist come in contact with a mass of granite. The contact does
not take place as might have been looked for if the granite had been formed
there before the strata were deposited, in which case the section would have
appeared as in Figure 610; but the union is as represented in Figure 611, the
undulating outline of the granite intersecting different strata, and
occasionally intruding itself in torturous veins into the beds of clay-slate and
limestone, from which it differs so remarkably in composition. The limestone is
sometimes changed in character by the proximity of the granitic mass or its
veins, and acquires a more compact texture, like that of hornstone or chert,
with a splintery fracture, and effervescing freely with acids.
The conversion of the limestone and these and many other instances into a
siliceous rock, effervescing slowly with acids, would be difficult of
explanation, were it not ascertained that such limestones are always impure,
containing grains of quartz, mica, or feldspar disseminated through them. The
elements of these minerals, when the rock has been subjected to great heat, may
have been fused, and so spread more uniformly through the whole mass.
(FIGURE 612. Granite veins traversing clay slate, Table Mountain, Cape of Good
Hope. (Captain B. Hall Transactions of the Royal Society of Edinburgh volume
7.))
(FIGURE 613. Granite veins traversing gneiss, Cape Wrath. (MacCulloch (Western
Islands plate 31.))
In the Plutonic, as in the volcanic rocks, there is every gradation from a
torturous vein to the most regular form of a dike, such as intersect the tuffs
and lavas of Vesuvius and Etna. Dikes of granite may be seen, among other
places, on the southern flank of Mount Battock, one of the Grampians, the
opposite walls sometimes preserving an exact parallelism for a considerable
distance. As a general rule, however, granite veins in all quarters of the globe
are more sinuous in their course than those of trap. They present similar shapes
at the most northern point of Scotland, and the southernmost extremity of
Africa, as Figures 612 and 613 will show.
It is not uncommon for one set of granite veins to intersect another; and
sometimes there are three sets, as in the environs of Heidelberg, where the
granite on the banks of the river Necker is seen to consist of three varieties,
differing in colour, grain, and various peculiarities of mineral composition.
One of these, which is evidently the second in age, is seen to cut through an
older granite; and another, still newer, traverses both the second and the
first. In Shetland there are two kinds of granite. One of them, composed of
hornblende, mica, feldspar, and quartz, is of a dark colour, and is seen
underlying gneiss. The other is a red granite, which penetrates the dark variety
everywhere in veins. (MacCulloch System of Geology volume 2 page 58.)
(FIGURE 614. Granite veins passing through hornblende slate, Carnsilver Cove,
Cornwall.)
Figure 614 is a sketch of a group of granite veins in Cornwall, given by Messrs.
Von Oeynhausen and Von Dechen. (Philosophical Magazine and Annals No. 27 New
Series March 1829.) The main body of the granite here is of a porphyritic
appearance, with large crystals of feldspar; but in the veins it is fine-
grained, and without these large crystals. The general height of the veins is
from 16 to 20 feet, but some are much higher.
Granite, syenite, and those porphyries which have a granitiform structure, in
short all Plutonic rocks, are frequently observed to contain metals, at or near
their junction with stratified formations. On the other hand, the veins which
traverse stratified rocks are, as a general law, more metalliferous near such
junctions than in other positions. Hence it has been inferred that these metals
may have been spread in a gaseous form through the fused mass, and that the
contact of another rock, in a different state of temperature, or sometimes the
existence of rents in other rocks in the vicinity, may have caused the
sublimation of the metals. (Necker Proceedings of the Geological Society No. 26
page 392.)
(FIGURE 615. a, b. Quartz vein passing through gneiss and greenstone. Tronstad
Strand, near Christiania.)
Veins of pure quartz are often found in granite as in many stratified rocks, but
they are not traceable, like veins of granite or trap, to large bodies of rock
of similar composition. They appear to have been cracks, into which siliceous
matter was infiltered. Such segregation, as it is called, can sometimes clearly
be shown to have taken place long subsequently to the original consolidation of
the containing rock. Thus, for example, I observed in the gneiss of Tronstad
Strand, near Drammen, in Norway, the section on the beach shown in Figure 615.
It appears that the alternating strata of whitish granitiform gneiss and black
hornblende-schist were first cut by a greenstone dike, about 2 1/2 feet wide;
then the crack a-b passed through all these rocks, and was filled up with
quartz. The opposite walls of the vein are in some parts incrusted with
transparent crystals of quartz, the middle of the vein being filled up with
common opaque white quartz.
(FIGURE 616. Euritic porphyry alternating with primary fossiliferous strata,
near Christiania.)
We have seen that the volcanic formations have been called overlying, because
they not only penetrate others but spread over them. M. Necker has proposed to
call the granites the underlying igneous rocks, and the distinction here
indicated is highly characteristic. It was, indeed, supposed by some of the
earlier observers that the granite of Christiania, in Norway, was intercalated
in mountain masses between the primary or palaeozoic strata of that country, so
as to overlie fossiliferous shale and limestone. But although the granite sends
veins into these fossiliferous rocks, and is decidedly posterior in origin, its
actual superposition in mass has been disproved by Professor Keilhau, whose
observations on this controverted point I had opportunities, in 1837, of
verifying. There are, however, on a smaller scale, certain beds of euritic
porphyry, some a few feet, others many yards in thickness, which pass into
granite, and deserve, perhaps, to be classed as Plutonic rather than trappean
rocks, which may truly be described as interposed conformably between
fossiliferous strata, as the porphyries (a, c, Figure 616) which divide the
bituminous shales and argillaceous limestones, f, f. But some of these same
porphyries are partially unconformable, as b, and may lead us to suspect that
the others also, notwithstanding their appearance of interstratification, have
been forcibly injected. Some of the porphyritic rocks above mentioned are highly
quartzose, others very feldspathic. In proportion as the masses are more
voluminous, they become more granitic in their texture, less conformable, and
even begin to send forth veins into contiguous strata. In a word, we have here a
beautiful illustration of the intermediate gradations between volcanic and
Plutonic rocks, not only in their mineralogical composition and structure, but
also in their relations of position to associated formations. If the term
"overlying" can in this instance be applied to a Plutonic rock, it is only in
proportion as that rock begins to acquire a trappean aspect.
It has been already hinted that the heat which in every active volcano extends
downward to indefinite depths must produce simultaneously very different effects
near the surface and far below it; and we can not suppose that rocks resulting
from the crystallising of fused matter under a pressure of several thousand
feet, much less several miles, of the earth's crust can exactly resemble those
formed at or near the surface. Hence the production at great depths of a class
of rocks analogous to the volcanic, and yet differing in many particulars, might
have been predicted, even had we no Plutonic formations to account for. How well
these agree, both in their positive and negative characters, with the theory of
their deep subterranean origin, the student will be able to judge by considering
the descriptions already given.
It has, however, been objected, that if the granitic and volcanic rocks were
simply different parts of one great series, we ought to find in mountain chains
volcanic dikes passing upward into lava and downward into granite. But we may
answer that our vertical sections are usually of small extent; and if we find in
certain places a transition from trap to porous lava, and in others a passage
from granite to trap, it is as much as could be expected of this evidence.
The prodigious extent of denudation which has been already demonstrated to have
occurred at former periods, will reconcile the student to the belief that
crystalline rocks of high antiquity, although deep in the earth's crust when
originally formed, may have become uncovered and exposed at the surface. Their
actual elevation above the sea may be referred to the same causes to which we
have attributed the upheaval of marine strata, even to the summits of some
mountain chains.
CHAPTER XXXII.
ON THE DIFFERENT AGES OF THE PLUTONIC ROCKS.
Difficulty in ascertaining the precise Age of a Plutonic Rock.
Test of Age by Relative Position.
Test by Intrusion and Alteration.
Test by Mineral Composition.
Test by included Fragments.
Recent and Pliocene Plutonic Rocks, why invisible.
Miocene Syenite of the Isle of Skye.
Eocene Plutonic Rocks in the Andes.
Granite altering Cretaceous Rocks.
Granite altering Lias in the Alps and in Skye.
Granite of Dartmoor altering Carboniferous Strata.
Granite of the Old Red Sandstone Period.
Syenite altering Silurian Strata in Norway.
Blending of the same with Gneiss.
Most ancient Plutonic Rocks.
Granite protruded in a solid Form.
When we adopt the igneous theory of granite, as explained in the last chapter,
and believe that different Plutonic rocks have originated at successive periods
beneath the surface of the planet, we must be prepared to encounter greater
difficulty in ascertaining the precise age of such rocks than in the case of
volcanic and fossiliferous formations. We must bear in mind that the evidence of
the age of each contemporaneous volcanic rock was derived either from lavas
poured out upon the ancient surface, whether in the sea or in the atmosphere, or
from tuffs and conglomerates, also deposited at the surface, and either
containing organic remains themselves or intercalated between strata containing
fossils. But the same tests entirely fail, or are only applicable in a modified
degree, when we endeavour to fix the chronology of a rock which has crystallised
from a state of fusion in the bowels of the earth. In that case we are reduced
to the tests of relative position, intrusion, alteration of the rocks in
contact, included fragments, and mineral character; but all these may yield at
best a somewhat ambiguous result.
TEST OF AGE BY RELATIVE POSITION.
Unaltered fossiliferous strata of every age are met with reposing immediately on
Plutonic rocks; as at Christiania, in Norway, where the Post-pliocene deposits
rest on granite; in Auvergne, where the fresh-water Miocene strata, and at
Heidelberg, on the Rhine, where the New Red sandstone occupy a similar place. In
all these, and similar instances, inferiority in position is connected with the
superior antiquity of granite. The crystalline rock was solid before the
sedimentary beds were superimposed, and the latter usually contain in them
rounded pebbles of the subjacent granite.
TEST BY INTRUSION AND ALTERATION.
But when Plutonic rocks send veins into strata, and alter them near the point of
contact, in the manner before described (Chapter 31), it is clear that, like
intrusive traps, they are newer than the strata which they invade and alter.
Examples of the application of this test will be given in the sequel.
TEST BY MINERAL COMPOSITION.
Notwithstanding a general uniformity in the aspect of Plutonic rocks, we have
seen in the last chapter that there are many varieties, such as syenite, talcose
granite, and others. One of these varieties is sometimes found exclusively
prevailing throughout an extensive region, where it preserves a homogeneous
character; so that, having ascertained its relative age in one place, we can
recognise its identity in others, and thus determine from a single section the
chronological relations of large mountain masses. Having observed, for example,
that the syenitic granite of Norway, in which the mineral called zircon abounds,
has altered the Silurian strata wherever it is in contact, we do not hesitate to
refer other masses of the same zircon-syenite in the south of Norway to a post-
Silurian date. Some have imagined that the age of different granites might, to a
great extent, be determined by their mineral characters alone; syenite, for
instance, or granite with hornblende, being more modern than common or micaceous
granite. But modern investigations have proved these generalisations to have
been premature.
TEST BY INCLUDED FRAGMENTS.
This criterion can rarely be of much importance, because the fragments involved
in granite are usually so much altered that they can not be referred with
certainty to the rocks whence they were derived. In the White Mountains, in
North America, according to Professor Hubbard, a granite vein, traversing
granite, contains fragments of slate and trap which must have fallen into the
fissure when the fused materials of the vein were injected from below
(Silliman's Journal No. 69 page 123.), and thus the granite is shown to be newer
than those slaty and trappean formations from which the fragments were derived.
RECENT AND PLIOCENE PLUTONIC ROCKS, WHY INVISIBLE.
The explanations already given in the 28th and in the last chapter of the
probable relation of the Plutonic to the volcanic formations, will naturally
lead the reader to infer that rocks of the one class can never be produced at or
near the surface without some members of the other being formed below. It is not
uncommon for lava-streams to require more than ten years to cool in the open
air; and where they are of great depth, a much longer period. The melted matter
poured from Jorullo, in Mexico, in the year 1759, which accumulated in some
places to the height of 550 feet, was found to retain a high temperature half a
century after the eruption. (See Principles Index Jorullo.) We may conceive,
therefore, that great masses of subterranean lava may remain in a red-hot or
incandescent state in the volcanic foci for immense periods, and the process of
refrigeration may be extremely gradual. Sometimes, indeed, this process may be
retarded for an indefinite period by the accession of fresh supplies of heat;
for we find that the lava in the crater of Stromboli, one of the Lipari Islands,
has been in a state of constant ebullition for the last two thousand years; and
we may suppose this fluid mass to communicate with some caldron or reservoir of
fused matter below. In the Isle of Bourbon, also, where there has been an
emission of lava once in every two years for a long period, the lava below can
scarcely fail to have been permanently in a state of liquefaction. If then it be
a reasonable conjecture, that about 2000 volcanic eruptions occur in the course
of every century, either above the waters of the sea or beneath them (Ibid.
Volcanic Eruptions.), it will follow that the quantity of Plutonic rock
generated or in progress during the Recent epoch must already have been
considerable.
But as the Plutonic rocks originate at some depth in the earth's crust, they can
only be rendered accessible to human observation by subsequent upheaval and
denudation. Between the period when a Plutonic rock crystallises in the
subterranean regions and the era of its protrusion at any single point of the
surface, one or two geological periods must usually intervene. Hence, we must
not expect to find the Recent or even the Pliocene granites laid open to view,
unless we are prepared to assume that sufficient time has elapsed since the
commencement of the Pliocene period for great upheaval and denudation. A
Plutonic rock, therefore, must, in general, be of considerable antiquity
relatively to the fossiliferous and volcanic formations, before it becomes
extensively visible. As we know that the upheaval of land has been sometimes
accompanied in South America by volcanic eruptions and the emission of lava, we
may conceive the more ancient Plutonic rocks to be forced upward to the surface
by the newer rocks of the same class formed successively below-- subterposition
in the Plutonic, like superposition in the sedimentary rocks, being usually
characteristic of a newer origin.
(FIGURE 617. Diagram showing the relative position which the Plutonic and
sedimentary formations of different ages may occupy.
I. Primary Plutonic rocks.
II. Secondary Plutonic rocks.
III. Tertiary Plutonic rocks.
IV. Post-tertiary Plutonic rocks.
1. Primary fossiliferous or Palaeozoic strata.
2. Secondary or Mesozoic strata.
3. Tertiary or Cainozoic strata.
4. Post-tertiary strata.
The metamorphic rocks are not indicated in this diagram: but the student will
infer, from what is said in Chapters 31 and 33, that some portions of the
stratified formations, Nos. 1 and 2, invaded by granite, will have become
metamorphic.)
In Figure 617 an attempt is made to show the inverted order in which sedimentary
and Plutonic formations may occur in the earth's crust. The oldest Plutonic
rock, No. I, has been upheaved at successive periods until it has become exposed
to view in a mountain-chain. This protrusion of No. I has been caused by the
igneous agency which produced the newer Plutonic rocks Nos. II, III and IV. Part
of the primary fossiliferous strata, No. I, have also been raised to the surface
by the same gradual process. It will be observed that the Recent STRATA No. 4
and the Recent GRANITE or Plutonic rock No. IV are the most remote from each
other in position, although of contemporaneous date. According to this
hypothesis, the convulsions of many periods will be required before Recent or
Post-tertiary granite will be upraised so as to form the highest ridges and
central axes of mountain-chains. During that time the RECENT strata No. 4 might
be covered by a great many newer sedimentary formations.
MIOCENE PLUTONIC ROCKS.
A considerable mass of syenite, in the Isle of Skye, is described by Dr.
MacCulloch as intersecting limestone and shale, which are of the age of the
lias. The limestone, which at a greater distance from the granite contains
shells, exhibits no traces of them near its junction, where it has been
converted into a pure crystalline marble. (Western Islands volume 1 page 330.)
MacCulloch pointed out that the syenite here, as in Raasay, was newer than the
secondary rocks, and Mr. Geikie has since shown that there is a strong
probability that this Plutonic rock may be of Miocene age, because a similar
Syenite having a true granitic character in its crystallisation has modified the
Tertiary volcanic rocks of Ben More, in Mull, some of which have undergone
considerable metamorphism.
EOCENE PLUTONIC ROCKS.
In a former part of this volume (Chapter 16), the great nummulitic formation of
the Alps and Pyrenees was referred to the Eocene period, and it follows that
vast movements which have raised those fossiliferous rocks from the level of the
sea to the height of more than 10,000 feet above its level have taken place
since the commencement of the Tertiary epoch. Here, therefore, if anywhere, we
might expect to find hypogene formations of Eocene date breaking out in the
central axis or most disturbed region of the loftiest chain in Europe.
Accordingly, in the Swiss Alps, even the flysch, or upper portion of the
nummulitic series, has been occasionally invaded by Plutonic rocks, and
converted into crystalline schists of the hypogene class. There can be little
doubt that even the talcose granite or gneiss of Mont Blanc itself has been in a
fused or pasty state since the flysch was deposited at the bottom of the sea;
and the question as to its age is not so much whether it be a secondary or
tertiary granite or gneiss, as whether it should be assigned to the Eocene or
Miocene epoch.
Great upheaving movements have been experienced in the region of the Andes,
during the Post-tertiary period. In some part, therefore, of this chain, we may
expect to discover tertiary Plutonic rocks laid open to view; and Mr. Darwin's
account of the Chilian Andes, to which the reader may refer, fully realises this
expectation: for he shows that we have strong ground to presume that Plutonic
rocks there exposed on a large scale are of later date than certain Secondary
and Tertiary formations.
But the theory adopted in this work of the subterranean origin of the hypogene
formations would be untenable, if the supposed fact here alluded to, of the
appearance of tertiary granite at the surface, was not a rare exception to the
general rule. A considerable lapse of time must intervene between the formation
of Plutonic and metamorphic rocks in the nether regions and their emergence at
the surface. For a long series of subterranean movements must occur before such
rocks can be uplifted into the atmosphere or the ocean; and, before they can be
rendered visible to man, some strata which previously covered them must have
been stripped off by denudation.
We know that in the Bay of Baiae in 1538, in Cutch in 1819, and on several
occasions in Peru and Chili, since the commencement of the present century, the
permanent upheaval or subsidence of land has been accompanied by the
simultaneous emission of lava at one or more points in the same volcanic region.
From these and other examples it may be inferred that the rising or sinking of
the earth's crust, operations by which sea is converted into land, and land into
sea, are a part only of the consequences of subterranean igneous action. It can
scarcely be doubted that this action consists, in a great degree, of the baking,
and occasionally the liquefaction, of rocks, causing them to assume, in some
cases a larger, in others a smaller volume than before the application of heat.
It consists also in the generation of gases, and their expansion by heat, and
the injection of liquid matter into rents formed in superincumbent rocks. The
prodigious scale on which these subterranean causes have operated in Sicily
since the deposition of the Newer Pliocene strata will be appreciated when we
remember that throughout half the surface of that island such strata are met
with, raised to the height of from 50 to that of 2000 and even 3000 feet above
the level of the sea. In the same island also the older rocks which are
contiguous to these marine tertiary strata must have undergone, within the same
period, a similar amount of upheaval.
The like observations may be extended to nearly the whole of Europe, for, since
the commencement of the Eocene Period, the entire European area, including some
of the central and very lofty portions of the Alps themselves, as I have
elsewhere shown, has, with the exception of a few districts, emerged from the
deep to its present altitude. (See map of Europe, and explanation, in Principles
book 1.) There must, therefore, have been at great depths in the earth's crust,
within the same period, an amount of subterranean change corresponding to this
vast alteration of level affecting a whole continent.
The principal effect of subterranean movements during the Tertiary Period seems
to have consisted in the upheaval of hypogene formations of an age anterior to
the Carboniferous. The repetition of another series of movements, of equal
violence, might upraise the Plutonic and metamorphic rocks of many secondary
periods; and, if the same force should still continue to act, the next
convulsions might bring up to the day the TERTIARY and RECENT hypogene rocks. In
the course of such changes many of the existing sedimentary strata would suffer
greatly by denudation, others might assume a metamorphic structure, or become
melted down into Plutonic and volcanic rocks. Meanwhile the deposition of a
great thickness of new strata would not fail to take place during the upheaval
and partial destruction of the older rocks. But I must refer the reader to the
last chapter but one of this volume for a fuller explanation of these views.
PLUTONIC ROCKS OF CRETACEOUS PERIOD.
(FIGURE 618. Section through three layers (b, c, d) of the Cretaceous series
over granite (A).)
It will be shown in the next chapter that chalk, as well as lias, has been
altered by granite in the eastern Pyrenees. Whether such granite be cretaceous
or tertiary, can not easily be decided. Suppose b, c, d, Figure 618, to be three
members of the Cretaceous series, the lowest of which, b, has been altered by
the granite A, the modifying influence not having extended so far as c, or
having but slightly affected its lowest beds. Now it can rarely be possible for
the geologist to decide whether the beds d existed at the time of the intrusion
of A, and alteration of b and c, or whether they were subsequently thrown down
upon c. But as some Cretaceous and even Tertiary rocks have been raised to the
height of more than 9000 feet in the Pyrenees, we must not assume that plutonic
formations of the same periods may not have been brought up and exposed by
denudation, at the height of 2000 or 3000 feet on the flanks of that chain.
PLUTONIC ROCKS OF THE OOLITE AND LIAS.
(FIGURE 619. Junction of granite with Jurassic or Oolite strata in the Alps,
near Champoleon. (Granite over Altered Rocks over Secondary Schists.))
In the Department of the Hautes Alpes, in France, M. Eliede Beaumont traced a
black argillaceous limestone, charged with belemnites, to within a few yards of
a mass of granite. Here the limestone begins to put on a granular texture, but
is extremely fine-grained. When nearer the junction it becomes grey, and has a
saccharoid structure. In another locality, near Champoleon, a granite composed
of quartz, black mica, and rose-coloured feldspar is observed partly to overlie
the secondary rocks, producing an alteration which extends for about 30 feet
downward, diminishing in the beds which lie farthest from the granite. (See
Figure 619.) In the altered mass the argillaceous beds are hardened, the
limestone is saccharoid, the grits quartzose, and in the midst of them is a thin
layer of an imperfect granite. It is also an important circumstance that near
the point of contact, both the granite and the secondary rocks become
metalliferous, and contain nests and small veins of blende, galena, iron, and
copper pyrites. The stratified rocks become harder and more crystalline, but the
granite, on the contrary, softer and less perfectly crystallised near the
junction. (Elie de Beaumont sur les Montagnes de l'Oisans etc. Mem. de la Soc.
d'Hist. Nat. de Paris tome 5.) Although the granite is incumbent in the section
(Figure 619), we can not assume that it overflowed the strata, for the
disturbances of the rocks are so great in this part of the Alps that their
original position is often inverted.
At Predazzo, in the Tyrol, secondary strata, some of which are limestones of the
Oolitic period, have been traversed and altered by Plutonic rocks, one portion
of which is an augitic porphyry, which passes insensibly into granite. The
limestone is changed into granular marble, with a band of serpentine at the
junction. (Von Buch Annales de Chimie etc.)
PLUTONIC ROCKS OF CARBONIFEROUS PERIOD.
The granite of Dartmoor, in Devonshire, was formerly supposed to be one of the
most ancient of the Plutonic rocks, but is now ascertained to be posterior in
date to the culm-measures of that county, which from their position, and, as
containing true coal-plants, are now known to be members of the true
Carboniferous series. This granite, like the syenitic granite of Christiania,
has broken through the stratified formations, on the north-west side of
Dartmoor, the successive members of the culm-measures abutting against the
granite, and becoming metamorphic as they approach. These strata are also
penetrated by granite veins, and Plutonic dikes, called "elvans." (Proceedings
of the Geological Society volume 2 page 562 and Transactions second series
volume 5 page 686.) The granite of Cornwall is probably of the same date, and,
therefore, as modern as the Carboniferous strata, if not newer.
PLUTONIC ROCKS OF SILURIAN PERIOD.
(FIGURE 620. Section through Silurian strata and Granite.)
It has long been known that a very ancient granite near Christiania, in Norway,
is posterior in date to the Lower Silurian strata of that region, although its
exact position in the Palaeozoic series can not be defined. Von Buch first
announced, in 1813, that it was of newer origin than certain limestones
containing orthocerata and trilobites. The proofs consist in the penetration of
granite veins into the shale and limestone, and the alteration of the strata,
for a considerable distance from the point of contact, both of these veins and
the central mass from which they emanate. (See Chapter 31.)Von Buch supposed
that the Plutonic rock alternated with the fossiliferous strata, and that large
masses of granite were sometimes incumbent upon the strata; but this idea was
erroneous, and arose from the fact that the beds of shale and limestone often
dip towards the granite up to the point of contact, appearing as if they would
pass under it in mass, as at a, Figure 620, and then again on the opposite side
of the same mountain, as at b, dip away from the same granite. When the
junctions, however, are carefully examined, it is found that the Plutonic rock
intrudes itself in veins, and nowhere covers the fossiliferous strata in large
overlying masses, as is so commonly the case with trappean formations. (See the
Gaea Norvegica and other works of Keilhau with whom I examined this country.)
Now this granite, which is more modern than the Silurian strata of Norway, also
sends veins in the same country into an ancient formation of gneiss; and the
relations of the Plutonic rock and the gneiss, at their junction, are full of
interest when we duly consider the wide difference of epoch which must have
separated their origin.
(FIGURE 621. Granite sending veins into Silurian strata and gneiss. Christiania,
Norway.
a. Inclined gneiss.
b. Silurian strata.)
The length of this interval of time is attested by the following facts: The
fossiliferous, or Silurian, beds rest unconformably upon the truncated edges of
the gneiss, the inclined strata of which had been denuded before the sedimentary
beds were superimposed (see Figure 621). The signs of denudation are twofold;
first, the surface of the gneiss is seen occasionally, on the removal of the
newer beds containing organic remains, to be worn and smoothed; secondly,
pebbles of gneiss have been found in some of these Silurian strata. Between the
origin, therefore, of the gneiss and the granite there intervened, first, the
period when the strata of gneiss were denuded; secondly, the period of the
deposition of the Silurian deposits upon the denuded and inclined gneiss, a. Yet
the granite produced after this long interval is often so intimately blended
with the ancient gneiss, at the point of junction, that it is impossible to draw
any other than an arbitrary line of separation between them; and where this is
not the case, tortuous veins of granite pass freely through gneiss, ending
sometimes in threads, as if the older rock had offered no resistance to their
passage. These appearances may probably be due to hydrothermal action (see
Chapter 33). I shall merely observe in this place that had such junctions alone
been visible, and had we not learnt, from other sections, how long a period
elapsed between the consolidation of the gneiss and the injection of this
granite, we might have suspected that the gneiss was scarcely solidified, or had
not yet assumed its complete metamorphic character when invaded by the Plutonic
rock. From this example we may learn how impossible it is to conjecture whether
certain granites in Scotland, and other countries, which send veins into gneiss
and other metamorphic rocks, are primary, or whether they may not belong to some
secondary or tertiary period.
OLDEST GRANITES.
It is not half a century since the doctrine was very general that all granitic
rocks were PRIMITIVE, that is to say, that they originated before the deposition
of the first sedimentary strata, and before the creation of organic beings (see
above Chapter 1). But so greatly are our views now changed, that we find it no
easy task to point out a single mass of granite demonstrably more ancient than
known fossiliferous deposits. Could we discover some Laurentian strata resting
immediately on granite, there being no alterations at the point of contact, nor
any intersecting granitic veins, we might then affirm the Plutonic rock to have
originated before the oldest known fossiliferous strata. Still it would be
presumptuous, as we have already pointed out (Chapter 26), to suppose that when
a small part only of the globe has been investigated, we are acquainted with the
oldest fossiliferous strata in the crust of our planet. Even when these are
found, we can not assume that there never were any antecedent strata containing
organic remains, which may have become metamorphic. If we find pebbles of
granite in a conglomerate of the Lower Laurentian system, we may then feel
assured that the parent granite was formed before the Laurentian formation. But
if the incumbent strata be merely Cambrian or Silurian, the fundamental granite,
although of high antiquity, may be posterior in date to KNOWN fossiliferous
formations.
PROTRUSION OF SOLID GRANITE.
In part of Sutherlandshire, near Brora, common granite, composed of feldspar,
quartz, and mica is in immediate contact with Oolitic strata, and has clearly
been elevated to the surface at a period subsequent to the deposition of those
strata. (Murchison Geological Transactions second series volume 2 page 307.)
Professor Sedgwick and Sir R. Murchison conceive that this granite has been
upheaved in a solid form; and that in breaking through the submarine deposits,
with which it was not perhaps originally in contact, it has fractured them so as
to form a breccia along the line of junction. This breccia consists of fragments
of shale, sandstone, and limestone, with fossils of the oolite, all united
together by a calcareous cement. The secondary strata at some distance from the
granite are but slightly disturbed, but in proportion to their proximity the
amount of dislocation becomes greater.
Mr. T. McKenney Hughes has suggested to me in explanation of these phenomena
that they may be the effect of the association of more pliant strata with hard
unyielding rocks, the whole of which were subjected simultaneously to great
movements, whether of elevation or subsidence, and of lateral pressure, during
which the more solid granite, being incapable of compression, was forced through
the softer beds of shale, sandstone, and limestone. He remarks that similar
breccias with slickensides are observed on a minor scale where rocks of
different composition and rigidity are contorted together. Such protrusion may
have been brought about by degrees by innumerable shocks of earthquakes repeated
after long intervals of time along the same tract of country. The opening of new
fissures in the hardest rocks is a frequent accompaniment of such convulsions,
and during the consequent vibrations, breccias must often be caused. But these
catastrophes, as we well know, do not imply that the land or sea of the
disturbed region are rendered uninhabitable by living beings, and by no means
indicate a state of things different from that witnessed in the ordinary course
of nature.
CHAPTER XXXIII.
METAMORPHIC ROCKS.
General Character of Metamorphic Rocks.
Gneiss.
Hornblende-schist.
Serpentine.
Mica-schist.
Clay-slate.
Quartzite.
Chlorite-schist.
Metamorphic Limestone.
Origin of the metamorphic Strata.
Their Stratification.
Fossiliferous Strata near intrusive Masses of Granite converted into Rocks
identical with different Members of the metamorphic Series.
Arguments hence derived as to the Nature of Plutonic Action.
Hydrothermal Action, or the Influence of Steam and Gases in producing
Metamorphism.
Objections to the metamorphic Theory considered.
We have now considered three distinct classes of rocks: first, the aqueous, or
fossiliferous; secondly, the volcanic; and, thirdly, the Plutonic; and it
remains for us to examine those crystalline (or hypogene) strata to which the
name of METAMORPHIC has been assigned. The last-mentioned term expresses, as
before explained, a theoretical opinion that such strata, after having been
deposited from water, acquired, by the influence of heat and other causes, a
highly crystalline texture. They who still question this opinion may call the
rocks under consideration the stratified hypogene formations or crystalline
schists.
These rocks, when in their characteristic or normal state, are wholly devoid of
organic remains, and contain no distinct fragments of other rocks, whether
rounded or angular. They sometimes break out in the central parts of mountain
chains, but in other cases extend over areas of vast dimensions, occupying, for
example, nearly the whole of Norway and Sweden, where, as in Brazil, they appear
alike in the lower and higher grounds. However crystalline these rocks may
become in certain regions, they never, like granite or trap, send veins into
contiguous formations. In Great Britain, those members of the series which
approach most nearly to granite in their composition, as gneiss, mica-schist,
and hornblende-schist, are confined to the country north of the rivers Forth and
Clyde.
Many attempts have been made to trace a general order of succession or
superposition in the members of this family; clay-slate, for example, having
been often supposed to hold invariably a higher geological position than mica-
schist, and mica-schist to overlie gneiss. But although such an order may
prevail throughout limited districts, it is by no means universal. To this
subject, however, I shall again revert, in Chapter 35, where the chronological
relations of the metamorphic rocks are pointed out.
PRINCIPAL METAMORPHIC ROCKS.
The following may be enumerated as the principal members of the metamorphic
class:-- gneiss, mica-schist, hornblende-schist, clay-slate, chlorite-schist,
hypogene or metamorphic limestone, and certain kinds of quartz-rock or
quartzite.
GNEISS.
(FIGURE 622. Fragment of gneiss, natural size; section made at right angles to
the planes of foliation.)
The first of these, gneiss, may be called stratified-- or by those who object to
that term, foliated-- granite, being formed of the same materials as granite,
namely, feldspar, quartz, and mica. In the specimen in Figure 622, the white
layers consist almost exclusively of granular feldspar, with here and there a
speck of mica and grain of quartz. The dark layers are composed of grey quartz
and black mica, with occasionally a grain of feldspar intermixed. The rock
splits most easily in the plane of these darker layers, and the surface thus
exposed is almost entirely covered with shining spangles of mica. The
accompanying quartz, however, greatly predominates in quantity, but the most
ready cleavage is determined by the abundance of mica in certain parts of the
dark layer. Instead of consisting of these thin laminae, gneiss is sometimes
simply divided into thick beds, in which the mica has only a slight degree of
parallelism to the planes of stratification.
Hand specimens may often be obtained from such gneiss which are
undistinguishable from granite, affording an argument to which we shall allude
in the concluding part of this chapter, in favour of those who regard all
granite and syenite not as igneous rocks, but as aqueous formations so altered
as to have lost all signs of their original stratified arrangement. Gneiss in
geology is commonly used to designate not merely stratified and foliated rocks
having the same component materials as granite or syenite, but also in a wider
sense to embrace the formation with which other members of the metamorphic
series, such as hornblende-schist, may alternate, and which are then considered
subordinate to the true gneiss.
The different varieties of rock allied to gneiss, into which feldspar enters as
an essential ingredient, will be understood by referring to what was said of
granite. Thus, for example, hornblende may be superadded to mica, quartz, and
feldspar, forming a hornblendic or syenitic gneiss; or talc may be substituted
for mica, constituting talcose gneiss (called stratified protogine by the
French), a rock composed of feldspar, quartz, and talc, in distinct crystals or
grains.
EURITE, which has already been mentioned as a Plutonic rock, occurs also with
precisely the same composition in beds subordinate to gneiss or mica-slate.
HORNBLENDE-SCHIST is usually black, and composed principally of hornblende, with
a variable quantity of feldspar, and sometimes grains of quartz. When the
hornblende and feldspar are in nearly equal quantities, and the rock is not
slaty, it corresponds in character with the greenstones of the trap family, and
has been called "primitive greenstone." It may be termed hornblende rock, or
amphibolite. Some of these hornblendic masses may really have been volcanic
rocks, which have since assumed a more crystalline or metamorphic texture.
SERPENTINE is a greenish rock, a silicate of magnesia, in which there is
sometimes from 30 to 40 per cent of magnesia. It enters largely into the
composition of a trap dike cutting through Old Red Sandstone in Forfarshire, and
in that case is probably an altered basaltic dike which had contained much
olivine. The theory of its having been originally a volcanic product
subsequently altered by metamorphism may at first sight seem inconsistent with
its occurrence in large and regularly stratified masses in the metamorphic
series in Scotland, as in Aberdeenshire. But it has been suggested in
explanation that such serpentine may have been originally regularly-bedded trap
tuff, and volcanic breccia, with much olivine, which would still retain a
stratified appearance after their conversion into a metamorphic rock.
ACTINOLITE SCHIST is a slaty foliated rock, composed chiefly of actinolite, an
emerald-green mineral, allied to hornblende, with some admixture of garnet,
mica, and quartz.
MICA-SCHIST or MICACEOUS SCHIST is, next to gneiss, one of the most abundant
rocks of the metamorphic series. It is slaty, essentially composed of mica and
quartz, the mica sometimes appearing to constitute the whole mass. Beds of pure
quartz also occur in this formation. In some districts, garnets in regular
twelve-sided crystals form an integrant part of mica-schist. This rock passes by
insensible gradations into clay-slate.
CLAY-SLATE-- ARGILLACEOUS SCHIST-- ARGILLITE.
This rock sometimes resembles an indurated clay or shale. It is for the most
part extremely fissile, often affording good roofing-slate. Occasionally it
derives a shining and silky lustre from the minute particles of mica or talc
which it contains. It varies from greenish or bluish-grey to a lead colour; and
it may be said of this, more than of any other schist, that it is common to the
metamorphic and fossiliferous series, for some clay-slates taken from each
division would not be distinguishable by mineral characters alone. It is not
uncommon to meet with an argillaceous rock having the same composition, without
the slaty cleavage, which may be called argillite.
CHLORITE SCHIST is a green slaty rock, in which chlorite is abundant in
foliated plates, usually blended with minute grains of quartz, or sometimes with
feldspar or mica; often associated with, and graduating into, gneiss and clay-
slate.
QUARTZITE, or QUARTZ ROCK, is an aggregate of grains of quartz which are either
in minute crystals, or in many cases slightly rounded, occurring in regular
strata, associated with gneiss or other metamorphic rocks. Compact quartz, like
that so frequently found in veins, is also found together with granular
quartzite. Both of these alternate with gneiss or mica-schist, or pass into
those rocks by the addition of mica, or of feldspar and mica.
CRYSTALLINE, OR METAMORPHIC LIMESTONE.
This hypogene rock, called by the earlier geologists PRIMARY LIMESTONE, is
sometimes a white crystalline granular marble, which when in thick beds can be
used in sculpture; but more frequently it occurs in thin beds, forming a
foliated schist much resembling in colour and arrangement certain varieties of
gneiss and mica-schist. When it alternates with these rocks, it often contains
some crystals of mica, and occasionally quartz, feldspar, hornblende, talc,
chlorite, garnet, and other minerals. It enters sparingly into the structure of
the hypogene districts of Norway, Sweden, and Scotland, but is largely developed
in the Alps.
ORIGIN OF THE METAMORPHIC STRATA.
Having said thus much of the mineral composition of the metamorphic rocks, I may
combine what remains to be said of their structure and history with an account
of the opinions entertained of their probable origin. At the same time, it may
be well to forewarn the reader that we are here entering upon ground of
controversy, and soon reach the limits where positive induction ends, and beyond
which we can only indulge in speculations. It was once a favourite doctrine, and
is still maintained by many, that these rocks owe their crystalline texture,
their want of all signs of a mechanical origin, or of fossil contents, to a
peculiar and nascent condition of the planet at the period of their formation.
The arguments in refutation of this hypothesis will be more fully considered
when I show, in Chapter 35, to how many different ages the metamorphic
formations are referable, and how gneiss, mica-schist, clay-slate, and hypogene
limestone (that of Carrara, for example) have been formed, not only since the
first introduction of organic beings into this planet, but even long after many
distinct races of plants and animals had flourished and passed away in
succession.
The doctrine respecting the crystalline strata implied in the name metamorphic
may properly be treated of in this place; and we must first inquire whether
these rocks are really entitled to be called stratified in the strict sense of
having been originally deposited as sediment from water. The general adoption by
geologists of the term stratified, as applied to these rocks, sufficiently
attests their division into beds very analogous, at least in form, to ordinary
fossiliferous strata. This resemblance is by no means confined to the existence
in both occasionally of a laminated structure, but extends to every kind of
arrangement which is compatible with the absence of fossils, and of sand,
pebbles, ripple-mark, and other characters which the metamorphic theory supposes
to have been obliterated by Plutonic action. Thus, for example, we behold alike
in the crystalline and fossiliferous formations an alternation of beds varying
greatly in composition, colour, and thickness. We observe, for instance, gneiss
alternating with layers of black hornblende-schist or of green chlorite-schist,
or with granular quartz or limestone; and the interchange of these different
strata may be repeated for an indefinite number of times. In the like manner,
mica-schist alternates with chlorite-schist, and with beds of pure quartz or of
granular limestone. We have already seen that, near the immediate contact of
granitic veins and volcanic dikes, very extraordinary alterations in rocks have
taken place, more especially in the neighbourhood of granite. It will be useful
here to add other illustrations, showing that a texture undistinguishable from
that which characterises the more crystalline metamorphic formations has
actually been superinduced in strata once fossiliferous.
FOSSILIFEROUS STRATA RENDERED METAMORPHIC BY INTRUSIVE MASSES OF GRANITE.
(FIGURE 623. Ground-plan of altered slate and limestone near granite.
Christiania. The arrows indicate the dip, and the oblique lines the strike of
the beds.)
In the southern extremity of Norway there is a large district, on the west side
of the fiord of Christiania, which I visited in 1837 with the late Professor
Keilhau, in which syenitic granite protrudes in mountain masses through
fossiliferous strata, and usually sends veins into them at the point of contact.
The stratified rocks, replete with shells and zoophytes, consist chiefly of
shale, limestone, and some sandstone, and all these are invariably altered near
the granite for a distance of from 50 to 400 yards. The aluminous shales are
hardened, and have become flinty. Sometimes they resemble jasper. Ribboned
jasper is produced by the hardening of alternate layers of green and chocolate-
coloured schist, each stripe faithfully representing the original lines of
stratification. Nearer the granite the schist often contains crystals of
hornblende, which are even met with in some places for a distance of several
hundred yards from the junction; and this black hornblende is so abundant that
eminent geologists, when passing through the country, have confounded it with
the ancient hornblende-schist, subordinate to the great gneiss formation of
Norway. Frequently, between the granite and the hornblende-slate above-
mentioned, grains of mica and crystalline feldspar appear in the schist, so that
rocks resembling gneiss and mica-schist are produced. Fossils can rarely be
detected in these schists, and they are more completely effaced in proportion to
the more crystalline texture of the beds, and their vicinity to the granite. In
some places the siliceous matter of the schist becomes a granular quartz; and
when hornblende and mica are added, the altered rock loses its stratification,
and passes into a kind of granite. The limestone, which at points remote from
the granite is of an earthy texture and blue colour, and often abounds in
corals, becomes a white granular marble near the granite, sometimes siliceous,
the granular structure extending occasionally upward of 400 yards from the
junction; the corals being for the most part obliterated, though sometimes
preserved, even in the white marble. Both the altered limestone and hardened
slate contain garnets in many places, also ores of iron, lead, and copper, with
some silver. These alterations occur equally whether the granite invades the
strata in a line parallel to the general strike of the fossiliferous beds, or in
a line at right angles to their strike, both of which modes of junction will be
seen by the ground-plan in Figure 623. (Keilhau Gaea Norvegica pages 61-63.)
The granite of Cornwall sends forth veins into a coarse argillaceous-schist,
provincially termed killas. This killas is converted into hornblende-schist near
the contact with the veins. These appearances are well seen at the junction of
the granite and killas, in St. Michael's Mount, a small island nearly 300 feet
high, situated in the bay, at a distance of about three miles from Penzance. The
granite of Dartmoor, in Devonshire, says Sir H. De la Beche, has intruded itself
into the Carboniferous slate and slaty sandstone, twisting and contorting the
strata, and sending veins into them. Hence some of the slate rocks have become
"micaceous; others more indurated, and with the characters of mica-slate and
gneiss; while others again appear converted into a hard zoned rock strongly
impregnated with feldspar." (Geological Manual page 479.)
We learn from the investigation of M. Dufrenoy that in the eastern Pyrenees
there are mountain masses of granite posterior in date to the formations called
lias and chalk of that district, and that these fossiliferous rocks are greatly
altered in texture, and often charged with iron-ore, in the neighbourhood of the
granite. Thus in the environs of St. Martin, near St. Paul de Fenouillet, the
chalky limestone becomes more crystalline and saccharoid as it approaches the
granite, and loses all trace of the fossils which it previously contained in
abundance. At some points, also, it becomes dolomitic, and filled with small
veins of carbonate of iron, and spots of red iron-ore. At Rancie the lias
nearest the granite is not only filled with iron-ore, but charged with pyrites,
tremolite, garnet, and a new mineral somewhat allied to feldspar, called, from
the place in the Pyrenees where it occurs, "couzeranite."
"Hornblende-schist," says Dr. MacCulloch, "may at first have been mere clay; for
clay or shale is found altered by trap into Lydian stone, a substance differing
from hornblende-schist almost solely in compactness and uniformity of texture."
(System of Geology volume 1 pages 210, 211.) "In Shetland," remarks the same
author, "argillaceous-schist (or clay-slate), when in contact with granite, is
sometimes converted into hornblende-schist, the schist becoming first siliceous,
and ultimately, at the contact, hornblende-schist." In like manner gneiss and
mica-schist may be nothing more than altered micaceous and argillaceous
sandstones, granular quartz may have been derived from siliceous sandstone, and
compact quartz from the same materials. Clay-slate may be altered shale, and
granular marble may have originated in the form of ordinary limestone, replete
with shells and corals, which have since been obliterated; and, lastly,
calcareous sands and marls may have been changed into impure crystalline
limestones.
The anthracite and plumbago associated with hypogene rocks may have been coal;
for not only is coal converted into anthracite in the vicinity of some trap
dikes, but we have seen that a like change has taken place generally even far
from the contact of igneous rocks, in the disturbed region of the Appalachians.
At Worcester, in the State of Massachusetts, 45 miles due west of Boston, a bed
of plumbago and impure anthracite occurs, interstratified with mica-schist. It
is about two feet in thickness, and has been made use of both as fuel, and in
the manufacture of lead pencils. At the distance of 30 miles from the plumbago,
there occurs, on the borders of Rhode Island, an impure anthracite in slates
containing impressions of coal-plants of the genera Pecopteris, Neuropteris,
Calamites, etc. This anthracite is intermediate in character between that of
Pennsylvania and the plumbago of Worcester, in which last the gaseous or
volatile matter (hydrogen, oxygen, and nitrogen) is to the carbon only in the
proportion of three per cent. After traversing the country in various
directions, I came to the conclusion that the carboniferous shales or slates
with anthracite and plants, which in Rhode Island often pass into mica-schists,
have at Worcester assumed a perfectly crystalline and metamorphic texture; the
anthracite having been nearly transmuted into that state of pure carbon which is
called plumbago or graphite. (See Lyell Quarterly Geological Journal volume 1
page 199.)
Now the alterations above described as superinduced in rocks by volcanic dikes
and granite veins prove incontestably that powers exist in nature capable of
transforming fossiliferous into crystalline strata, a very few simple elements
constituting the component materials common to both classes of rocks. These
elements, which are enumerated in Table 28.1, may be made to form new
combinations by what has been termed Plutonic action, or those chemical changes
which are no doubt connected with the passage of heat, unusually heated steam
and waters, through the strata.
HYDROTHERMAL ACTION, OR THE INFLUENCE OF STEAM AND GASES IN PRODUCING
METAMORPHISM.
The experiments of Gregory Watt, in fusing rocks in the laboratory, and allowing
them to consolidate by slow cooling, prove distinctly that a rock need not be
perfectly melted in order that a re-arrangement of its component particles
should take place, and a partial crystallisation ensue. (Philosophical
Transactions 1804.) We may easily suppose, therefore, that all traces of shells
and other organic remains may be destroyed, and that new chemical combinations
may arise, without the mass being so fused as that the lines of stratification
should be wholly obliterated. We must not, however, imagine that heat alone,
such as may be applied to a stone in the open air, can constitute all that is
comprised in Plutonic action. We know that volcanoes in eruption not only emit
fluid lava, but give off steam and other heated gases, which rush out in
enormous volume, for days, weeks, or years continuously, and are even disengaged
from lava during its consolidation.
We also know that long after volcanoes have spent their force, hot springs
continue for ages to flow out at various points in the same area. In regions,
also, subject to violent earthquakes such springs are frequently observed
issuing from rents, usually along lines of fault or displacement of the rocks.
These thermal waters are most commonly charged with a variety of mineral
ingredients, and they retain a remarkable uniformity of temperature from century
to century. A like uniformity is also persistent in the nature of the earthy,
metallic, and gaseous substances with which they are impregnated. It is well
ascertained that springs, whether hot or cold, charged with carbonic acid,
especially with hydrofluoric acid, which is often present in small quantities,
are powerful causes of decomposition and chemical reaction in rocks through
which they percolate.
The changes which Daubree has shown to have been produced by the alkaline waters
of Plombieres in the Vosges, are more especially instructive. (Daubree Sur le
Metamorphisme Paris 1860.) These waters have a heat of 160 degrees F., or an
excess of 109 degrees above the average temperature of ordinary springs in that
district. They were conveyed by the Romans to baths through long conduits or
aqueducts. The foundations of some of their works consisted of a bed of concrete
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