Test by Included Fragments.—We may sometimes discover the relative age

of two trap-rocks, or of an aqueous deposit and the trap on which it

rests, by finding fragments of one included in the other in cases such

as those before alluded to, where the evidence of superposition alone

would be insufficient. It is also not uncommon to find a conglomerate

almost exclusively composed of rolled pebbles of trap, associated with

some fossiliferous stratified formation in the neighbourhood of massive

trap. If the pebbles agree generally in mineral character with the

latter, we are then enabled to determine its relative age by knowing

that of the fossiliferous strata associated with the conglomerate. The

origin of such conglomerates is explained by observing the shingle

beaches composed of trap-pebbles in modern volcanoes, as at the base of

Etna.

Recent and Post-pliocene Volcanic Rocks.—I shall now select examples of

contemporaneous volcanic rocks of successive geological periods, to

show that igneous causes have been in activity in all past ages of the

world. They have been perpetually shifting the places where they have

broken out at the earth’s surface, and we can sometimes prove that

those areas which are now the great theatres of volcanic action were in

a state of perfect tranquillity at remote geological epochs, and that,

on the other hand, in places where at former periods the most violent

eruptions took place at the surface and continued for a great length of

time, there has been an entire suspension of igneous action in

historical times, and even, as in the British Isles, throughout a large

part of the antecedent Tertiary Period.

In the absence of British examples of volcanic rocks newer than the

Upper Miocene, I may state that in other parts of the world, especially

in those where volcanic eruptions are now taking place from time to

time, there are tuffs and lavas belonging to that part of the Tertiary

era the antiquity of which is proved by the presence of the bones of

extinct quadrupeds which co-existed with terrestrial, fresh-water, and

marine mollusca of species still living. One portion of the lavas,

tuffs, and trap-dikes of Etna, Vesuvius, and the island of Ischia has

been produced within the historical era; another and a far more

considerable part originated at times immediately antecedent, when the

waters of the Mediterranean were already inhabited by the existing

testacea, but when certain species of elephant, rhinoceros, and other

quadrupeds now extinct, inhabited Europe.

_Vesuvius._—I have traced in the “Principles of Geology” the history of

the changes which the volcanic region of Campania is known to have

undergone during the last 2000 years. The aggregate effect of igneous

operations during that period is far from insignificant, comprising as

it does the formation of the modern cone of Vesuvius since the year 79,

and the production of several minor cones in Ischia, together with that

of Monte Nuovo in the year 1538. Lava-currents have also flowed upon

the land and along the bottom of the sea—volcanic sand, pumice, and

scoriæ have been showered down so abundantly that whole cities were

buried—tracts of the sea have been filled up or converted into

shoals—and tufaceous sediment has been transported by rivers and

land-floods to the sea. There are also proofs, during the same recent

period, of a permanent alteration of the relative levels of the land

and sea in several places, and of the same tract having, near Puzzuoli,

been alternately upheaved and depressed to the amount of more than

twenty feet. In connection with these convulsions, there are found, on

the shores of the Bay of Baiæ, recent tufaceous strata, filled with

articles fabricated by the hands of man, and mingled with marine

shells.

It has also been stated (p. 206), that when we examine this same

region, it is found to consist largely of tufaceous strata, of a date

anterior to human history or tradition, which are of such thickness as

to constitute hills from 500 to more than 2000 feet in height. Some of

these strata contain marine shells which are exclusively of living

species, others contain a slight mixture, one or two per cent of

species not known as living.

The ancient part of Vesuvius is called Somma, and consists of the

remains of an older cone which appears to have been partly destroyed by

explosion. In the great escarpment which this remnant of the ancient

mountain presents towards the modern cone of Vesuvius, there are many

dikes which are for the most part vertical, and traverse the inclined

beds of lava and scoriæ which were successively superimposed during

those eruptions by which the old cone was formed. They project in

relief several inches, or sometimes feet, from the face of the cliff,

being extremely compact, and less destructible than the intersected

tuffs and porous lavas. In vertical extent they vary from a few yards

to 500 feet, and in breadth from one to twelve feet. Many of them cut

all the inclined beds in the escarpment of Somma from top to bottom,

others stop short before they ascend above halfway. In mineral

composition they scarcely differ from the lavas of Somma, the rock

consisting of a base of leucite and augite, through which large

crystals of augite and some of leucite are scattered.

Nothing is more remarkable than the usual parallelism of the opposite

sides of the dikes, which correspond almost as regularly as the two

opposite faces of a wall of masonry. This character appears at first

the more inexplicable, when we consider how jagged and uneven are the

rents caused by earthquakes in masses of heterogeneous composition,

like those composing the cone of Somma. In explanation of this

phenomenon, M. Necker refers us to Sir W. Hamilton’s account of an

eruption of Vesuvius in the year 1779, who records the following fact:

“The lavas, when they either boiled over the crater, or broke out from

the conical parts of the volcano, constantly formed channels as regular

as if they had been cut by art down the steep part of the mountain; and

whilst in a state of perfect fusion, continued their course in those

channels, which were sometimes full to the brim, and at other times

more or less so, according to the quantity of matter in motion.

”These channels (says the same observer), I have found, upon

examination after an eruption, to be in general from two to five or six

feet wide, and seven or eight feet deep. They were often hid from the

sight by a quantity of scoriæ that had formed a crust over them; and

the lava, having been conveyed in a covered way for some yards, came

out fresh again into an open channel. After an eruption, I have walked

in some of those subterraneous or covered galleries, which were

exceedingly curious, the sides, top, and bottom _being worn perfectly

smooth and even_ in most parts by the violence of the currents of the

red-hot lavas which they had conveyed for many weeks successively.” I

was able to verify this phenomenon in 1858, when a stream of lava

issued from a lateral cone.[3] Now, the walls of a vertical fissure,

through which lava has ascended in its way to a volcanic vent, must

have been exposed to the same erosion as the sides of the channels

before adverted to. The prolonged and uniform friction of the heavy

fluid, as it is forced and made to flow upward, cannot fail to wear and

smooth down the surfaces on which it rubs, and the intense heat must

melt all such masses as project and obstruct the passage of the

incandescent fluid.

The rock composing the dikes both in the modern and ancient part of

Vesuvius is far more compact than that of ordinary lava, for the

pressure of a column of melted matter in a fissure greatly exceeds that

in an ordinary stream of lava; and pressure checks the expansion of

those gases which give rise to vesicles in lava. There is a tendency in

almost all the Vesuvian dikes to divide into horizontal prisms, a

phenomenon in accordance with the formation of vertical columns in

horizontal beds of lava; for in both cases the divisions which give

rise to the prismatic structure are at right angles to the cooling

surfaces. (See p. 510.)

_Auvergne._—Although the latest eruptions in central France seem to

have long preceded the historical era, they are so modern as to have a

very intimate connection with the present superficial outline of the

country and with the existing valleys and river-courses. Among a great

number of cones with perfect craters, one called the Puy de Tartaret

sent forth a lava-current which can be traced up to its crater, and

which flowed for a distance of thirteen miles along the bottom of the

present valley to the village of Nechers, covering the alluvium of the

old valley in which were preserved the bones of an extinct species of

horse, and of a lagomys and other quadrupeds all closely allied to

recent animals, while the associated land-shells were of species now

living, such as _Cyclostoma elegans, Helix hortensis, H. nemoralis,_

_H. lapicida,_ and _Clausilia rugosa._ 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 only 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 fourteen 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.

While the river Couze has in most cases, as at the site of this ancient

bridge, been simply able to cut a deep channel through the lava, the

lower portion of which is shown to be columnar, the same torrent has in

other places, where the valley was contracted to a narrow gorge, had

power to remove the entire mass of basaltic rock, causing for a short

space a complete breach of continuity in the volcanic current. The work

of erosion has 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, occupying the lowest part of the great

valley descending from Mont Dor (see p. 542), and damming up the river

so as to cause the Lake of Chambon, 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.

_Puy de Côme._—The Puy de Côme and its lava-current, near Clermont, may

be mentioned as another minor volcano of about the same age. This

conical hill rises from the granitic platform, at an angle of between

30° and 40°, 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 fifty feet

high.[4]

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 Côme remains unimpaired, its loose materials

being protected by a dense vegetation, and the hill standing on a ridge

not commanded by any higher ground, so that no floods of rain-water can

descend upon it. There is no end to the waste which the hard basalt may

undergo in future, if the physical geography of the country continue

unchanged—no limit to the number of years during which the heap of

incoherent and transportable materials called the Puy de Côme may

remain in an almost stationary condition.

_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 scoriæ, as is remarkably the

case on Etna; and nothing but a water-spout breaking directly upon the

Puy de Pariou could carry away a portion of the hill, so long as it is

not rent or ingulfed by earthquakes.

Newer Pliocene Volcanic Rocks.—The more ancient portion of Vesuvius and

Etna originated at the close of the Newer Pliocene period, when less

than ten, sometimes only one, in a hundred of the shells differed from

those now living. In the case of Etna, it was before stated (p. 205)

that Post-pliocene formations occur in the neighbourhood of Catania,

while the oldest lavas of the great volcano are Pliocene. These last

are seen associated with sedimentary deposits at Trezza and other

places on the southern and eastern flanks of the great cone (see p.

205).

_Cyclopean Islands._—The Cyclopean Islands, called by the Sicilians Dei

Faraglioni, in the sea-cliffs of which these beds of clay, tuff, and

associated lava are laid open to view, are situated in the Bay of

Trezza, and may be regarded as the extremity of a promontory severed

from the main land. Here numerous proofs are seen of submarine

eruptions, by which the argillaceous and sandy strata were invaded and

cut through, and tufaceous breccias formed. Inclosed in these breccias

are many angular and hardened fragments of laminated clay in different

states of alteration by heat, and intermixed with volcanic sands.

Fig. 599: View of the Isle of Cyclops, in the Bay of Trezza.

The loftiest of the Cyclopean islets, or rather rocks, is about 200

feet in height, the summit being formed of a mass of stratified clay,

the laminæ of which are occasionally subdivided by thin arenaceous

layers. These strata dip to the N.W., and rest on a mass of columnar

lava (see Fig. 599) in which the tops of the pillars are weathered, and

so rounded as to be often hemispherical.

Fig. 600: Contortions of strata in the largest of the Cyclopean

Islands.

In some places in the adjoining and largest islet of the group, which

lies to the north-eastward of that represented in Figure 599), the

overlying clay has been greatly altered and hardened by the igneous

rock, and occasionally contorted in the most extraordinary manner; yet

the lamination has not been obliterated, but, on the contrary, rendered

much more conspicuous, by the indurating process.

In Fig. 600 I have represented a portion of the altered rock, a few

feet square, where the alternating thin laminæ of sand and clay are

contorted in a manner often observed in ancient metamorphic schists. A

great fissure, running from east to west, nearly divides this larger

island into two parts, and lays open its internal structure. In the

section thus exhibited, a dike of lava is seen, first cutting through

an older mass of lava, and then penetrating the superincumbent tertiary

strata. In one place the lava ramifies and terminates in thin veins,

from a few feet to a few inches in thickness (see Fig. 601). The

arenaceous laminæ are much hardened at the point of contact, and the

clays are converted into siliceous schist. In this island the altered

rocks assume a honey-comb structure on their weathered surface,

singularly contrasted with the smooth and even outline which the same

beds present in their usual soft and yielding state. The pores of the

lava are sometimes coated, or entirely filled 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.

Fig. 601: Newer pliocene strata invaded by lava. Isle of Cyclops

(horizontal section).

_Dikes of Palagonia._—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 scoriæ, 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

(Fig. 602) had cooled down, it must have been fractured and shifted

horizontally by a lateral movement.

In Fig. 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.

Figs. 602 and 603: Ground-plan of dikes near Palagonia.

_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 scoriæ, 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° 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 Caniçal 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 a former chapter (p. 143).

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[5] after my visit to Italy in 1828 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.[6] 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 scoriæ 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 scoriæ

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 scoriæ, 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 scoriæ 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

aëriform 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 (Fig. 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.

[1] Caldcleugh, Phil. Trans., 1836, p. 27.

[2] See Principles, _Index,_ “Skaptar Jokul.”

[3] Principles of Geology, vol. i, p. 626.

[4] Scrope’s Central France, p. 60, and plate.

[5] See 1st edit. of Principles of Geology, vol. iii, chaps. xiii and

xiv, 1833; and former editions of this work, chap. xxxi.

[6] Quart. Geol. Journ., vol. vi, p. 281.

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 (p. 532),

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_ (Fig. 211), a well-known Falunian fossil of Europe;

and in the same volcanic tuff of Baixo, the Echinoderm _Brissus

Scillæ,_ 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 (p. 212), 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 _Astræa, 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 18½ 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.[1]

_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 gæderopus, 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), _Calyptræa, 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 subaërial 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 6½ 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 subaërial

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 Jacobæus_ 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, scoriæ, 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,[2] 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 (p. 247) 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 (p. 532), 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,[3] 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. Plants of the

genera _Flabellaria, Ceanothus,_ and _ Daphnogene,_ including _D.

cinnamomifolia_ (Fig. 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 p. 527).

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 scoriæ, 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.[4] 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 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. It has already been stated (p. 234) 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 (p. 527), 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._—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 Fig. 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_ (Fig. 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.

Fig. 604: Hill of Gergovia.

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

palæontologists, 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.[5]

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 (p. 263) 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 palæontological 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, p. 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;[6] 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.[7]

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. Wünsch 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. Wünsch 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°, 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° 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 pp. 410 and 411 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 (p. 74), 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 Fig. 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.[8]

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 scoriæ 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.[9]

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.[10]

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.[11]

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

subaërial. 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.”[12]

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.[13] 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.

[1] Linnean Proceedings; Zoology, 1860.

[2] Hartung, Die Azoren, 1860; also Insel Gran Canaria, Madeira und

Porto Santo, 1864, Leipsig.

[3] Geognost. Beschreib. des Siebengebirges am Rhein. Bonn, 1852.

[4] Scrope’s Central France, p. 98.

[5] Principles of Geology, chap. xxvi, 9th ed., p. 432.

[6] Boblaye and Virlet, Morea, p. 23.

[7] De la Beche, Geol. Proceedings, vol. ii, p. 198.

[8] Maclaren, Geology of Fife and Lothians. Geikie, Trans. Royal Soc.

Edinburgh, 1860-1861.

[9] Murchison, Silurian System, etc., p. 230.

[10] Ibid., p. 212.

[11] Murchison, Silurian System, etc., p. 325.

[12] Quart. Geol. Journ., vol. ix, p. 170, 1852.

[13] Logan, Geology of Canada, 1863.

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.

Fig. 605: Mass of granite near the Sharp Tor, Cornwall.

Granite and its Varieties.—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 Fig.

606.)

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 Fig. 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 Fig. 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.

Fig. 606: Granite having a cuboidal and rude columnar structure, Land’s

End, Cornwall.

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.[1]

Graphic granite. Fig. 607: Section parallel to the laminæ. Fig. 608:

Section transverse to the laminæ.

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.[2]

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.[3]

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.

Fig. 609: Porphyritic granite. Land’s End, Cornwall.

_Porphyritic Granite._—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 (Fig. 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,_ 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 (p. 552), 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 p. 449); 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.[4] 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.—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 Fig. 610; but the union is as represented in Fig.

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.

Fig. 610 and Fig. 611: Junction of granite and arbillaceous schist in

Glen Tilt. (MacCulloch.) Fig. 610 and Fig. 611: Junction of granite and

arbillaceous schist in Glen Tilt. (MacCulloch.)[5]

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.

In the Plutonic, as in the volcanic rocks, there is every gradation

from a tortuous 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 Figs. 612 and 613 will show.

Fig. 612: Granite veins traversing clay slate, Table Mountain, Cape of

Good Hope. Fig. 612: Granite veins traversing clay slate, Table

Mountain, Cape of Good Hope.[6]

Fig. 613: Granite veins traversing gneiss, Cape Wrath. Fig. 613:

Granite veins traversing gneiss, Cape Wrath.[7]

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.[8]

Fig. 614 is a sketch of a group of granite veins in Cornwall, given by

Messrs. Von Oeynhausen and Von Dechen.[9] 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.

Fig. 614: Granite veins passing through hornblende slate, Carnsilver

Cove, Cornwall.

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.[10]

Fig. 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 2½

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.

Fig. 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 palæozoic 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,_ Fig. 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 cannot

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.

[1] Delesse, Ann. des Mines, 1852, tome iii, p. 409, and 1848, tome

xiii, p. 675.

[2] Bulletin, 2e série, iv, 1304; and D’Archiac, Hist. des Progrès de

la Géol., i, 38.

[3] See Quart. Geol. Journ., vol. xiv, pp. 465, 488.

[4] Syst. of Geol., vol. i, pp. 157 and 158.

[5] Geol. Trans., First Series, vol. iii, pl. 21.

[6] Captain B. Hall, Trans. Roy. Soc. Edinburgh, vol. vii.

[7] Western Islands, pl. 31.

[8] MacCulloch, Syst. of Geol., vol. ii, p. 58.

[9] Phil. Mag. and Annals, No. 27, New Series, March, 1829.

[10] Necker, Proceedings of the Geol. Soc., No. 26, p. 392.

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 (p. 559), 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 cannot 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,[1] 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.[2] 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,[3] 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.

Fig. 617: Diagram showing the relative position which the Plutonic and

sedimentary formations of different ages may occupy.

In Fig. 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.[4] 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 Baiæ 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,[5] has, with the exception of a

few districts, emerged from the deep to its present altitude. 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.

Fig. 618: Section through three layers (b, c, d) of the Cretaceous

series over granite (A).

Plutonic Rocks of Cretaceous Period.—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,

cannot easily be decided. Suppose _b, c, d,_ Fig. 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.

Fig. 619: Junction of granite with Jurassic or Oolite strata in the

Alps, near Champoleon.

Plutonic Rocks of the Oolite and Lias.—In the Department of the Hautes

Alpes, in France, M. Élie de 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 Fig. 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.[6] Although the granite is incumbent in the section

(Fig. 619), we cannot 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.[7]

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.”[8] The granite of Cornwall is probably

of the same date, and, therefore, as modern as the Carboniferous

strata, if not newer.

Fig. 620: Section through Silurian strata and Granite.

Plutonic Rocks of Silurian Period.—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 Palæozoic series cannot 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 p. 562)

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,_ Fig. 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.[9]

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.

Fig. 621: Granite sending veins into Silurian strata and gneiss.

Christiania, Norway.

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 p. 584). 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 p. 34). 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 (p. 464), 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 cannot 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.[10]

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.

[1] Silliman’s Journ., No. 69, p. 123.

[2] See “Principles,” _Index,_ “Jorullo.”

[3] Ibid., “Volcanic Eruptions.”

[4] “Western Islands,” vol. i, p. 330.

[5] See map of Europe, and explanation, in Principles, book i.

[6] Élie de Beaumont sur les Montagnes de l’Oisans, etc. Mém. de la

Soc. d’Hist. Nat. de Paris, tome v.

[7] Von Buch, Annales de Chimie, etc.

[8] Proceed. Geol. Soc., vol. ii, p. 562; and Trans., 2nd series, vol.

v, p. 686.

[9] See the Gæa Norvegica and other works of Keilhau, with whom I

examined this country.

[10] Murchison, Geol. Trans., 2nd series, vol. ii, p. 307.

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 XXXV, 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.

Fig. 622: Fragment of gneiss; section made at right angles to the

planes of foliation.

_Gneiss._—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 Fig. 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 laminæ,

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.