ON THE DIFFERENT AGES OF THE VOLCANIC ROCKS--_continued_.
Volcanic rocks of the Pliocene and Miocene periods
continued--Auvergne--Mont Dor--Breccias and alluviums of Mont Perrier,
with bones of quadrupeds--River dammed up by lava-current--Range of
minor cones from Auvergne to the Vivarais--Monts Dome--Puy de
Come--Puy de Pariou--Cones not denuded by general flood--Velay--Bones
of quadrupeds buried in scoriae--Cantal--Eocene volcanic rocks--Tuffs
near Clermont--Hill of Gergovia--Trap of Cretaceous period--Oolitic
period--New Red Sandstone period--Carboniferous period--Old Red
Sandstone period--"Rock and Spindle" near St. Andrews--Silurian
period--Cambrian volcanic rocks.
_Tertiary Volcanic Rocks.--Auvergne._--The extinct volcanos of Auvergne
and Cantal in Central France seem to have commenced their eruptions in
the Upper Eocene period, but to have been most active during the 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 p. 178.).
The earliest monuments of the tertiary period in that region are
lacustrine deposits of great thickness (2. fig. 480. p. 424.), 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 (3. fig. 480.)
containing Upper Eocene 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 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 freshwater strata.[422-A] 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, all the
sides sloping more or less rapidly, until their inclination 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, till
they reach and spread themselves round the base of the mountain.[423-A]
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 may frequently have been destroyed and renovated.
According to some geologists, this mountain, as well as Vesuvius, Etna, and
all large volcanos, has derived its dome-like form not from the
preponderance of eruptions from one or more central points, but from the
upheaval of horizontal beds of lava and scoriae. I have explained my reasons
for objecting to this view at the close of Chap. XXIX., when speaking of
Palma, and in the Principles of Geology.[423-B] The average inclination of
the dome-shaped mass of Mont Dor is 8 deg. 6', whereas in Mounts Loa and Kea,
before mentioned, in the Sandwich Islands (see fig. 457. p. 394.), the
flanks of which have been raised by recent lavas, we find from Mr. Dana's
description that the one has a slope of 6 deg. 30', the other of 7 deg. 46'. We
may, therefore, reasonably question whether there is any absolute necessity
for supposing that the basaltic currents of the ancient French volcano were
at first more horizontal than they are now. Nevertheless it is highly
probable that during the long series of eruptions required to give rise to
so vast a pile of volcanic matter, which is thickest at the summit or
centre of the dome, some dislocation and upheaval took place; and during
the distension of the mass, beds of lava and scoriae may, in some places,
have acquired a greater, in others a less inclination, than that which at
first belonged to them.
Respecting the age of the great mass of Mont Dor, we cannot 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. We may certainly conclude, that the earliest eruptions were
posterior in origin to those grits, and conglomerates of the freshwater
formation of the Limagne, which contain no pebbles of volcanic rocks;
while, on the other hand, some eruptions took place before the great lakes
were drained; and others occurred after the desiccation of those lakes, and
when deep valleys had already been excavated through freshwater strata.
In the annexed section, I have endeavoured to explain the geological
structure of a portion of Auvergne, which I re-examined in 1843.[423-C] It
may convey some idea to the reader of the long and complicated series of
events, which have occurred in that country, since the first lacustrine
strata (No. 2.) were deposited on the granite (No. 1.). The changes of
which we have evidence are the more striking, because they imply great
denudation, without there being any proofs of the intervention of the sea
during the whole period. It will be seen that the upper freshwater beds
(No. 3.), once formed in a lake, must have suffered great destruction
before the excavation of the valleys of the Couze and Allier had begun. In
these freshwater beds, Upper Eocene fossils, as described in Chap. XV.,
have been found. The basaltic dike 4' is one of many examples of the
intrusion of volcanic matter through the Eocene freshwater beds, and may
have been of Upper Eocene or Miocene date, giving rise, when it reached the
surface and overflowed, to such platforms of basalt, as often cap the
tertiary hills in Auvergne, and one of which (4) is seen on Mont Perrier.
[Illustration: Fig. 480. Section from the valley of the Couze at Nechers,
through Mont Perrier and Issoire to the Valley of the Allier, and the Tour
de Boulade, Auvergne.
10. Lava-current of Tartaret near its termination at Nechers.
9. Bone-bed, red sandy clay under the lava of Tartaret.
8. Bone-bed of the Tour de Boulade.
7. Alluvium newer than No. 6.
6. Alluvium with bones of hippopotamus.
5 _c._ Trachytic breccia resembling 5 _a._
5 _b._ Upper bone-bed of Perrier, gravel, &c.
5 _a._ Pumiceous breccia and conglomerate, angular masses of trachyte,
quartz, pebbles, &c.
5. Lower bone-bed of Perrier, ochreous sand and gravel.
4 _a._ Basaltic dyke.
4. Basaltic platform.
3. Upper freshwater beds, limestone, marl, gypsum, &c.
2. Lower freshwater formation, red clay, green sand, &c.
1. Granite.]
It not unfrequently happens that beds of gravel containing bones of extinct
mammalia are detected under these very ancient sheets of basalt, as between
No. 4. and the freshwater strata, No. 3., at A, from which it is clear that
the surface of 3 formed at that period the lowest level at which the waters
then draining the country flowed. Next in age to this basaltic platform
comes a patch of ochreous sand and gravel (No. 5.), containing many bones
of quadrupeds. Upon this rests a pumiceous breccia and conglomerate, with
angular masses of trachyte, and some quartz pebbles. This deposit is
followed by 5 _b_, which is similar to 5, and 5 _c_ similar to the
trachytic breccia 5 _a_. These two breccias are supposed, from their
similarity to others found on Mount Dor, to have descended from the flanks
of that mountain during eruptions; and the interstratified alluvial
deposits contain the remains of mastodon, rhinoceros, tapir, deer, beaver,
and quadrupeds of other genera referable to about forty species, all of
which are extinct. I formerly supposed them to belong to the same era as
the Miocene faluns of Touraine; but, whether they may not rather be
ascribed to the older Pliocene epoch is a question which farther inquiries
and comparisons must determine.
Whatever be their date in the tertiary series, they are quadrupeds which
inhabited the country when the formations 5 and 5 _c_ originated.
Probably they were drowned during floods, such as rush down the flanks
of volcanos during eruptions, when great bodies of steam are emitted
from the crater, or when, as we have seen, both on Etna and in Iceland
in modern times, large masses of snow are suddenly melted by lava,
causing a deluge of water to bear down fragments of igneous rocks mixed
with mud, to the valleys and plains below.
It will be seen that the valley of the Issoire, down which these ancient
inundations swept, was first excavated at the expense of the formations
2, 3, and 4, and then filled up by the masses 5 and 5 _c_, after which
it was re-excavated before the more modern alluviums (Nos. 6. and 7.)
were formed. In these again other fossil mammalia of distinct species
have been detected by M. Bravard, the bones of an hippopotamus having
been found among the rest.
At length, when the valley of the Allier was eroded at Issoire down to its
lowest level, a talus of angular fragments of basalt and freshwater
limestone (No. 8.) was formed, called the bone-bed of the Tour de Boulade,
from which a great many other mammalia have been collected by MM. Bravard
and Pomel. In this assemblage the _Elephas primigenius_, _Rhinoceros
tichorinus_, _Deer_ (including rein-deer), _Equus_, _Bos_, _Antelope_,
_Felis_, and _Canis_, were included. Even this deposit seems hardly to be
the newest in the neighbourhood, for if we cross from the town of Issoire
(see fig. 480.) over Mont Perrier to the adjoining valley of the Couze, we
find another bone-bed (No. 9.), overlaid by a current of lava (No. 10.).
The history of this lava-current, which terminates a few hundred yards
below the point No. 10., in the suburbs of the village of Nechers, is
interesting. It forms a long narrow stripe more than 13 miles in length, at
the bottom of the valley of the Couze, which flows out of a lake at the
foot of Mont Dor. This lake is caused by a barrier thrown across the
ancient channel of the Couze, consisting partly of the volcanic cone called
the Puy de Tartaret, formed of loose scoriae, from the base of which has
issued the lava-current before mentioned. The materials of the dam which
blocked up the river, and caused the Lac de Chambon, are also, in part,
derived from a land-slip which may have happened at the time of the great
eruption which formed the cone.
This cone of Tartaret affords an impressive monument of the very different
dates at which the igneous eruptions of Auvergne have happened; for it was
evidently thrown up at the bottom of the existing valley, which is bounded
by lofty precipices composed of sheets of ancient columnar trachyte and
basalt, which once flowed at very high levels from Mont Dor.[425-A]
When we follow the course of the river Couze, from its source in the
lake of Chambon, to the termination of the lava-current at Nechers, a
distance of thirteen miles, we find that the torrent has in most places
cut a deep channel through the lava, the lower portion of which is
columnar. In some narrow gorges it has even had power to remove the
entire mass of basaltic rock, though the work of erosion must have been
very slow, as the basalt is tough and hard, and one column after another
must have been undermined and reduced to pebbles, and then to sand.
During the time required for this operation, the perishable cone of
Tartaret, composed of sand and ashes, has stood uninjured, proving that
no great flood or deluge can have passed over this region in the
interval between the eruption of Tartaret and our own times.
If we now return to the section (fig. 480.), I may observe that the
lava-current of Tartaret, which has diminished greatly in height and
volume near its termination, presents here a steep and perpendicular
face 25 feet in height towards the river. Beneath it is the alluvium No.
9., consisting of a red sandy clay, which must have covered the bottom
of the valley when the current of melted rock flowed down. The bones
found in this alluvium, which I obtained myself, consisted of a species
of field-mouse, _Arvicola_, and the molar tooth of an extinct horse,
_Equus fossilis_. The other species, obtained from the same bed, are
referable to the genera _Sus_, _Bos_, _Cervus_, _Felis_, _Canis_,
_Martes_, _Talpa_, _Sorex_, _Lepus_, _Sciurus_, _Mus_, and _Lagomys_, in
all no less than forty-three species, all closely allied to recent
animals, yet nearly all of them, according to M. Bravard, showing some
points of difference, like those which Mr. Owen discovered in the case
of the horse above alluded to. The bones, also, of a frog, snake, and
lizard, and of several birds, were associated with the fossils before
enumerated, and several recent land shells, such as _Cyclostoma
elegans_, _Helix hortensis_, _H. nemoralis_, _H. lapicida_, and
_Clausilia rugosa_. If the animals were drowned by floods, which
accompanied the eruptions of the Puy de Tartaret, they would give an
exceedingly modern geological date to that event, which must, in that
case, have belonged to the Newer-Pliocene, or, perhaps, the
Post-Pliocene period. That the current, which has issued from the Puy de
Tartaret, may nevertheless be very ancient in reference to the events of
human history, we may conclude, not only from the divergence of the
mammiferous fauna from that of our day, but from the fact that a Roman
bridge of such form and construction as continued in use down to the
fifth century, but which may be older, is now seen at a place about a
mile and a half from St. Nectaire. This ancient bridge spans the river
Couze with two arches, each about 14 feet wide. These arches spring from
the lava of Tartaret, on both banks, showing that a ravine precisely
like that now existing, had already been excavated by the river through
that lava thirteen or fourteen centuries ago.
In Central France there are several hundred minor cones, like that of
Tartaret, a great number of which, like Monte Nuovo, near Naples, may have
been principally due to a single eruption. Most of these cones range in a
linear direction from Auvergne to the Vivarais, and they were faithfully
described so early as the year 1802, by M. de Montlosier. They have given
rise chiefly to currents of basaltic lava. Those of Auvergne called the
Monts Dome, placed on a granitic platform, form an irregular ridge (see
fig. 436.), about 18 miles in length, and 2 in breadth. They are usually
truncated at the summit, where the crater is often preserved entire, the
lava having issued from the base of the hill. But frequently the crater is
broken down on one side, where the lava has flowed out. The hills are
composed of loose scoriae, blocks of lava, lapilli, and pozzuolana, with
fragments of trachyte and granite.
_Puy de Come._--The Puy de Come and its lava-current, near Clermont, may
be mentioned as one of these minor volcanos. This conical hill rises
from the granitic platform, at an angle of about 40 deg., to the height of
more than 900 feet. Its summit presents two distinct craters, one of
them with a vertical depth of 250 feet. A stream of lava takes its rise
at the western base of the hill, instead of issuing from either crater,
and descends the granitic slope towards the present site of the town of
Pont Gibaud. Thence it pours in a broad sheet down a steep declivity
into the valley of the Sioule, filling the ancient river-channel for the
distance of more than a mile. The Sioule, thus dispossessed of its bed,
has worked out a fresh one between the lava and the granite of its
western bank; and the excavation has disclosed, in one spot, a wall of
columnar basalt about 50 feet high.[427-A]
The excavation of the ravine is still in progress, every winter some
columns of basalt being undermined and carried down the channel of the
river, and in the course of a few miles rolled to sand and pebbles.
Meanwhile the cone of Come remains stationary, its loose materials being
protected by a dense vegetation, and the hill standing on a ridge not
commanded by any higher ground whence floods of rain-water may descend.
_Puy Rouge._--At another point, farther down the course of the Sioule,
we find a second illustration of the same phenomenon in the Puy Rouge, a
conical hill to the north of the village of Pranal. The cone is composed
entirely of red and black scoriae, tuff, and volcanic bombs. On its
western side there is a worn-down crater, whence a powerful stream of
lava has issued, and flowed into the valley of the Sioule. The river has
since excavated a ravine through the lava and subjacent gneiss, to the
depth of 400 feet.
On the upper part of the precipice forming the left side of this ravine,
we see a great mass of black and red scoriaceous lava; below this a thin
bed of gravel, evidently an ancient river-bed, now at an elevation of 50
feet above the channel of the Sioule. The gravel again rests upon
gneiss, which has been eroded to the depth of 50 feet. It is quite
evident in this case, that, while the basalt was gradually undermined
and carried away by the force of running water, the cone whence the lava
issued escaped destruction, because it stood upon a platform of gneiss
several hundred feet above the level of the valley in which the force of
running water was exerted.
_Puy de Pariou._--The brim of the crater of the Puy de Pariou, near
Clermont, is so sharp, and has been so little blunted by time, that it
scarcely affords room to stand upon. This and other cones in an equally
remarkable state of integrity have stood, I conceive uninjured, not _in
spite_ of their loose porous nature, as might at first be naturally
supposed, but in consequence of it. No rills can collect where all the rain
is instantly absorbed by the sand and scoriae, as is remarkably the case on
Etna; and nothing but a waterspout breaking directly upon the Puy de Pariou
could carry away a portion of the hill, so long as it is not rent or
engulphed by earthquakes.
Hence it is conceivable that even those cones which have the freshest
aspect, and most perfect shape, may lay claim to very high antiquity. Dr.
Daubeny has justly observed, that had any of these volcanos been in a state
of activity in the age of Julius Caesar, that general, who encamped upon the
plains of Auvergne, and laid siege to its principal city (Gergovia, near
Clermont), could hardly have failed to notice them. Had there been any
record of their eruptions in the time of Pliny or Sidonius Apollinaris, the
one would scarcely have omitted to make mention of it in his Natural
History, nor the other to introduce some allusion to it among the
descriptions of this his native province. This poet's residence was on the
borders of the Lake Aidat, which owed its very existence to the damming up
of a river by one of the most modern lava-currents.[428-A]
_Velay._--The observations of M. Bertrand de Doue have not yet established
that any of the most ancient volcanos of Velay were in action during the
Eocene period. There are beds of gravel in Velay, as in Auvergne, covered
by lava at different heights above the channels of the existing rivers. In
the highest and most ancient of these alluviums the pebbles are exclusively
of granitic rocks; but in the newer, which are found at lower levels, and
which originated when the valleys had been cut to a greater depth, an
intermixture of volcanic rocks has been observed.
At St. Privat d'Allier a bed of volcanic scoriae and tuff was discovered
by Dr. Hibbert, inclosed between two sheets of basaltic lava; and in
this tuff were found the bones of several quadrupeds, some of them
adhering to masses of slaggy lava. Among other animals were _Rhinoceros
leptorhinus_, _Hyaena spelaea_, and a species allied to the spotted hyaena
of the Cape, together with four undetermined species of deer.[428-B] The
manner of the occurrence of these bones reminds us of the published
accounts of an eruption of Coseguina, 1835, in Central America (see p.
399.), during which hot cinders and scoriae fell and scorched to death
great numbers of wild and domestic animals and birds.
_Plomb du Cantal._--In regard to the age of the igneous rocks of the
Cantal, we can at present merely affirm, that they overlie the Eocene
lacustrine strata of that country (see Map, p. 179.). They form a great
dome-shaped mass, having an average slope of only 4 deg., which has evidently
been accumulated, like the cone of Etna, during a long series of eruptions.
It is composed of trachytic, phonolitic, and basaltic lavas, tuffs, and
conglomerates, or breccias, forming a mountain several thousand feet in
height. Dikes also of phonolite, trachyte, and basalt are numerous,
especially in the neighbourhood of the large cavity, probably once a
crater, around which the loftiest summits of the Cantal are ranged
circularly, few of them, except the Plomb du Cantal, rising far above the
border or ridge of this supposed crater. A pyramidal hill, called the Puy
Griou, occupies the middle of the cavity.[429-A] It is clear that the
volcano of the Cantal broke out precisely on the site of the lacustrine
deposit before described (p. 188.), which had accumulated in a depression
of a tract composed of micaceous schist. In the breccias, even to the very
summit of the mountain, we find ejected masses of the freshwater beds, and
sometimes fragments of flint, containing Eocene shells. Valleys radiate in
all directions from the central heights of the mountain, increasing in size
as they recede from those heights. Those of the Cer and Jourdanne, which
are more than 20 miles in length, are of great depth, and lay open the
geological structure of the mountain. No alternation of lavas with
undisturbed Eocene strata has been observed, nor any tuffs containing
freshwater shells, although some of these tuffs include fossil remains of
terrestrial plants, said to imply several distinct restorations of the
vegetation of the mountain in the intervals between great eruptions. On the
northern side of the Plomb du Cantal, at La Vissiere, near Murat, is a
spot, pointed out on the Map (p. 179.), where freshwater limestone and marl
are seen covered by a thickness of about 800 feet of volcanic rock. Shifts
are here seen in the strata of limestone and marl.[429-B]
_Eocene period._--In treating of the lacustrine deposits of Central
France, in the fifteenth chapter, it was stated that, in the arenaceous
and pebbly group of the lacustrine basins of Auvergne, Cantal, and
Velay, no volcanic pebbles had ever been detected, although massive
piles of igneous rocks are now found in the immediate vicinity. As this
observation has been confirmed by minute research, we are warranted in
inferring that the volcanic eruptions had not commenced when the older
subdivisions of the freshwater groups originated.
In Cantal and Velay no decisive proofs have yet been brought to light that
any of the igneous outbursts happened during the deposition of the
freshwater strata; but there can be no doubt that in Auvergne some volcanic
explosions took place before the drainage of the lakes, and at a time when
the Upper Eocene species of animals and plants still flourished. Thus, for
example, at Pont du Chateau, near Clermont, a section is seen in a
precipice on the right bank of the river Allier, in which beds of volcanic
tuff alternate with a freshwater limestone, which is in some places pure,
but in others spotted with fragments of volcanic matter, as if it were
deposited while showers of sand and scoriae were projected from a
neighbouring vent.[430-A]
Another example occurs in the Puy de Marmont, near Veyres, where a
freshwater marl alternates with volcanic tuff containing Eocene shells. The
tuff or breccia in this locality is precisely such as is known to result
from volcanic ashes falling into water, and subsiding together with ejected
fragments of marl and other stratified rocks. These tuffs and marls are
highly inclined, and traversed by a thick vein of basalt, which, as it
rises in the hill, divides into two branches.
_Gergovia._--The hill of Gergovia, near Clermont, affords a third example.
I agree with MM. Dufrenoy and Jobert that there is no alternation here of a
contemporaneous sheet of lava with freshwater strata, in the manner
supposed by some other observers[430-B]; but the position and contents of
some of the associated tuffs, prove them to have been derived from volcanic
eruptions which occurred during the deposition of the lacustrine strata.
[Illustration: Fig. 481. Hill of Gergovia.]
The bottom of the hill consists of slightly inclined beds of white and
greenish marls, more than 300 feet in thickness, intersected by a dike of
basalt, which may be studied in the ravine above the village of Merdogne.
The dike here cuts through the marly strata at a considerable angle,
producing, in general, great alteration and confusion in them for some
distance from the point of contact. Above the white and green marls, a
series of beds of limestone and marl, containing freshwater shells, are
seen to alternate with volcanic tuff. In the lowest part of this division,
beds of pure marl alternate with compact fissile tuff, resembling some of
the subaqueous tuffs of Italy and Sicily called _peperinos_. Occasionally
fragments of scoriae are visible in this rock. Still higher is seen another
group of some thickness, consisting exclusively of tuff, upon which lie
other marly strata intermixed with volcanic matter. Among the species of
fossil shells which I found in these strata were _Melania inquinata_, a
_Unio_, and a _Melanopsis_, but they were not sufficient to enable me to
determine with precision the age of the formation.
There are many points in Auvergne where igneous rocks have been forced by
subsequent injection through clays and marly limestones, in such a manner
that the whole has become blended in one confused and brecciated mass,
between which and the basalt there is sometimes no very distinct line of
demarcation. In the cavities of such mixed rocks we often find calcedony,
and crystals of mesotype, stilbite, and arragonite. To formations of this
class may belong some of the breccias immediately adjoining the dike in the
hill of Gergovia; but it cannot be contended that the volcanic sand and
scoriae interstratified with the marls and limestones in the upper part of
that hill were introduced, like the dike, subsequently, by intrusion from
below. They must have been thrown down like sediment from water, and can
only have resulted from igneous action, which was going on
contemporaneously with the deposition of the lacustrine strata.
The reader will bear in mind that this conclusion agrees well with the
proofs, adverted to in the fifteenth chapter, of the abundance of silex,
travertin, and gypsum precipitated when the upper lacustrine strata were
formed; for these rocks are such as the waters of mineral and thermal
springs might generate.
_Cretaceous period._--Although we have no proof of volcanic rocks
erupted in England during the deposition of the chalk and greensand, it
would be an error to suppose that no theatres of igneous action existed
in the cretaceous period. M. Virlet, in his account of the geology of
the Morea, p. 205., has clearly shown that certain traps in Greece,
called by him ophiolites, are of this 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 ophiolite, 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[431-A]; 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.
That part of the volcanic rocks of the Hebrides, in our own country,
originated contemporaneously with the Oolite which they traverse and
overlie, has been ascertained by Prof. E. Forbes, in 1850.
_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.[432-A]
_Carboniferous period._--Two classes of contemporaneous trap rocks have
been ascertained by Dr. Fleming to 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 ironstone 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. At one spot, about two miles from St. Andrews, the
encroachment of the sea on the cliffs has isolated several masses of trap,
one of which (fig. 482.) is aptly called the "rock and spindle,"[432-B] for
it consists of a pinnacle of tuff, which may be compared to a distaff, and
near the base is a mass of columnar greenstone, in which the pillars
radiate from a centre, and appear at a distance like the spokes of a wheel.
The largest diameter of this wheel is about twelve feet, and the polygonal
terminations of the columns are seen round the circumference (or tire, as
it were, of the wheel), as in the accompanying figure. I conceive this mass
to be the extremity of a string or vein of greenstone, which penetrated the
tuff. The prisms point in every direction, because they were surrounded on
all sides by cooling surfaces, to which they always, arrange themselves at
right angles, as before explained (p. 385.).
[Illustration: Fig. 482. Rock and Spindle, St. Andrews.
_a._ Unstratified tuff.
_b._ Columnar greenstone.
_c._ Stratified tuff.]
[Illustration: Fig. 483. Columns of Greenstone, seen endwise.]
A trap dike was pointed out to me by Dr. Fleming, in the parish of Flisk,
in the northern part of Fifeshire, which cuts through the grey sandstone
and shale, forming the lowest part of the Old Red Sandstone. It may be
traced for many miles, passing through the amygdaloidal and other traps of
the hill called Normans Law. 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, finds the rock, which he calls dolerite, to consist
of greenish black augite and Labrador felspar, 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.
_Trap of the Old Red sandstone period._--By referring to the section
explanatory of the structure of Forfarshire, already given (p. 48.), 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 quartz rocks,
sometimes exclusively of different varieties of trap, which, although
purposely omitted in the above section, are often found either intruding
themselves 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 felspathic porphyry and
amygdaloid, the kernels of the latter being sometimes calcareous, often
calcedonic, and forming beautiful agates. We meet also with claystone,
clinkstone, greenstone, compact felspar, and tuff. Some of these rocks
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 section, p. 48.), 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.
_Silurian period._--It appears from the investigations of Sir R.
Murchison in Shropshire, that when the lower Silurian strata of that
county 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.[435-A]
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 felspar
rock, the beds being traversed by joints like those in the associated
sandstone, limestone, and shale, and having the same strike and dip.[435-B]
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 felspar-porphyry, clinkstone, and other
varieties; and the interposed Llandeilo flags are of sandstone and shale,
with trilobites and graptolites.[435-C]
The vast thickness of contemporaneous trappean rocks of lower Silurian
date in North Wales, explored by our government surveyors, has been
already alluded to.[435-D]
_Cambrian volcanic rocks._--Professor Sedgwick, in his account of the
geology of Cumberland, has described various trap rocks which accompany the
green slates of the Cambrian system, beneath all the rocks containing
organic remains. Different felspathic and porphyritic rocks and greenstones
occur, not only in dikes, but in conformable beds; and there is
occasionally a passage from these igneous rocks to some of the green
quartzose slates. Professor Sedgwick supposes these porphyries to have
originated contemporaneously with the stratified chloritic slates, the
materials of the slates having been supplied, in part at least, by
submarine eruptions oftentimes repeated.[435-E]
FOOTNOTES:
[422-A] See the map, p. 179.
[423-A] Scrope's Central France, p. 98.
[423-B] See chaps. xxiv., xxv., and xxvi., 7th and 8th editions.
[423-C] See Quarterly Geol. Journ., vol. ii. p. 77.
[425-A] For a view of Puy de Tartaret and Mont Dor, see Scrope's Volcanos
of Central France.
[427-A] Scrope's Central France, p. 60., and plate.
[428-A] Daubeny on Volcanos, p. 14.
[428-B] Edin. Journ. of Sci., No. iv. N. S. p. 276. Figures of some
of these remains are given by M. Bertrand de Doue, Ann. De la Soc.
d'Agricult. de Puy, 1828.
[429-A] Mem. de la Soc. Geol. de France, tom. i. p. 175.
[429-B] See Lyell and Murchison, Ann. de Sci. Nat., Oct. 1829.
[430-A] See Scrope's Central France, p. 21.
[430-B] Ibid, p. 7.
[431-A] Boblaye and Virlet, Morea, p. 23.
[432-A] De la Beche, Geol. Proceedings, No. 41. p. 196.
[432-B] "The rock," as English readers of Burn's poems may remember, is a
Scotch term for distaff.
[435-A] Murchison, Silurian System, &c. p. 230.
[435-B] Ibid., p. 272.
[435-C] Ibid., p. 325.
[435-D] Chap. XXVII. p. 356.
[435-E] Geol. Trans., 2d series, vol. iv. p. 55.