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--Tertiary 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--On the
probable age of the granites of Arran, in Scotland.
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 all
these tests fail when we endeavour to fix the chronology of a rock which
has crystallized from a state of fusion in the bowels of the earth. In that
case, we are reduced to the following tests; 1st, relative position; 2dly,
intrusion, and alteration of the rocks in contact; 3dly, mineral
characters; 4thly, included fragments.
_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 Newer Pliocene deposits rest on granite;
in Auvergne, where the freshwater Eocene 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. 442.), 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 easily recognize 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 the same era.
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
generalizations to have been premature. The syenitic granite of Norway
already alluded to may be of the same age as the Silurian strata, which it
traverses and alters, or may belong to the Old Red sandstone period;
whereas the granite of Dartmoor, although consisting of mica, quartz, and
felspar, is newer than the coal. (See p. 456.)
_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[450-A], and thus the granite is shown to be newer than certain
superficial slaty and trappean formations.
_Recent and Pliocene plutonic rocks, why invisible._--The explanation
already given in the 29th 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
simultaneously, or soon afterwards. 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.[450-B] 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[451-A], 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
crystallizes 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 Newer Pliocene
granites laid open to view, unless we are prepared to assume that
sufficient time has elapsed since the commencement of the Newer 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 upwards 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.
In the accompanying diagram (fig. 501.), 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. 1., 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_ 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.
[Illustration: Fig. 501. Diagram showing the relative position which the
plutonic and sedimentary formations of different ages may occupy.
I. Primary plutonic. 4. Recent strata.
II. Secondary plutonic. 3. Tertiary strata.
III. Tertiary plutonic. 2. Secondary strata.
IV. Recent plutonic. 1. Primary fossiliferous strata.
The metamorphic rocks are not indicated in this diagram; but the student
will infer, from what has been said in Chap. XXXII., that some portions
of the stratified formations Nos. 1. and 2. invaded by granite will
have become metamorphic.]
_Eocene granite and plutonic rocks._--In a former part of this volume (p.
205.), the great nummulitic formation of the Alps and Pyrenees was
referred to the Eocene period, and it follows that those vast movements
which have raised 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 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, 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-Pliocene period. In some part, therefore, of this
chain, we may expect to discover tertiary plutonic rocks laid open to
view. What we already know of the structure of the Chilian Andes seems
to realize this expectation. In a transverse section, examined by Mr.
Darwin, between Valparaiso and Mendoza, the Cordillera was found to
consist of two separate and parallel chains, formed of sedimentary rocks
of different ages, the strata in both resting on plutonic rocks, by
which they have been altered. In the western or oldest range, called the
Peuquenes, are black calcareous clay-slates, rising to the height of
nearly 14,000 feet above the sea, in which are shells of the genera
_Gryphaea_, _Turritella_, _Terebratula_, and _Ammonite_. These rocks are
supposed to be of the age of the central parts of the secondary series
of Europe. They are penetrated and altered by dikes and mountain masses
of a plutonic rock, which has the texture of ordinary granite, but
rarely contains quartz, being a compound of albite and hornblende.
The second or eastern chain consists chiefly of sandstones and
conglomerates, of vast thickness, the materials of which are derived
from the ruins of the western chain. The pebbles of the conglomerates
are, for the most part, rounded fragments of the fossiliferous slates
before mentioned. The resemblance of the whole series to certain
tertiary deposits on the shores of the Pacific, not only in mineral
character, but in the imbedded lignite and silicified woods, leads to
the conjecture that they also are tertiary. Yet these strata are not
only associated with trap rocks and volcanic tuffs, but are also altered
by a granite consisting of quartz, felspar, and talc. They are
traversed, moreover, by dikes of the same granite, and by numerous veins
of iron, copper, arsenic, silver, and gold; all of which can be traced
to the underlying granite.[453-A] We have, therefore, strong ground to
presume that the plutonic rock, here exposed on a large scale in the
Chilian Andes, is of later date than certain 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 in the nether regions of plutonic and metamorphic
rocks, 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 usually have been
stripped off by denudation.
We know that in the Bay of Baiae, in 1538, in Cutch in 1819, and on several
occasions in Peru and Chili, since the commencement of the present century,
the permanent upheaval or subsidence of land has been accompanied by the
simultaneous emission of lava at one or more points in the same volcanic
region. From these and other examples it may be inferred that the rising or
sinking of the earth's crust, operations by which sea is converted into
land, and land into sea, are a part only of the consequences of
subterranean igneous action. It can scarcely be doubted that this action
consists, in a great degree, of the baking, and occasionally the
liquefaction, of rocks, causing them to assume, in some cases a larger, in
others a smaller volume than before the application of heat. It consists
also in the generation of gases, and their expansion by heat, and the
injection of liquid matter into rents formed in superincumbent rocks. The
prodigious scale on which these subterranean causes have operated in Sicily
since the deposition of the Newer Pliocene strata will be appreciated, when
we remember that throughout half the surface of that island such strata are
met with, raised to the height of from 50 to that of 2000 and even 3000
feet above the level of the sea. In the same island also the older rocks
which are contiguous to these marine tertiary strata must have undergone,
within the same period, a similar amount of upheaval.
The like observations may be extended to nearly the whole of Europe, for,
since the commencement of the Eocene period, the entire European area,
including some of the central and very lofty portions of the Alps
themselves, as I have elsewhere shown[454-A], has, with the exception of a
few districts, emerged from the deep to its present altitude; and even
those tracts, which were already dry land before the Eocene era, have
almost everywhere acquired additional height. A large amount of subsidence
has also occurred during the same period, so that the extent of the
subterranean spaces which have either become the receptacles of sunken
fragments of the earth's crust, or have been rendered capable of supporting
other fragments at a much greater height than before, must be so great that
they probably equal, if not exceed in volume, the entire continent of
Europe. We are entitled, therefore, to ask what amount of change of
equivalent importance can be proved to have occurred in the earth's crust
within an equal quantity of time anterior to the Eocene epoch. They who
contend for the more intense energy of subterranean causes in the remoter
eras of the earth's history, may find it more difficult to give an answer
to this question than they anticipated.
The principal effect of volcanic action in the nether regions, during
the tertiary period, seems to have consisted in the upheaval to the
surface 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 vast 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.
[Illustration: Fig. 502. Block section.]
_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_, 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_.
As some Cretaceous rocks, however, have been raised to the height of more
than 9000 feet in the Pyrenees, we must not assume that plutonic formations
of the same age may not have been brought up and exposed by denudation, at
the height of 2000 or 3000 feet on the flanks of that chain.
_Period of Oolite and Lias._--In the department of the Hautes Alpes, in
France, near Vizille, M. Elie 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 felspar, is
observed partly to overlie the secondary rocks, producing an alteration
which extends for about 30 feet downwards, diminishing in the beds which
lie farthest from the granite. (See fig. 503.) 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 crystallized
near the junction.[456-A]
[Illustration: Fig. 503. Junction of granite with Jurassic or Oolite strata
in the Alps, near Champoleon.]
Although the granite is incumbent in the above section (fig. 503.), we
cannot assume that it overflowed the strata, for the disturbances of the
rocks are so great in this part of the Alps that they seldom retain the
position which they must originally have occupied.
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.[456-B] 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.[456-C]
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.[456-D]
_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
regarded by Professor Sedgwick and Sir R. Murchison as members of the true
carboniferous series. This granite, like the syenitic granite of
Christiania, has broken through the stratified formations without much
changing their strike. Hence, on the north-west side of Dartmoor, the
successive members of the culm-measures abut against the granite, and
become metamorphic as they approach. These strata are also penetrated by
granite veins, and plutonic dikes, called "elvans."[457-A] The granite of
Cornwall is probably of the same date, and, therefore, as modern as the
Carboniferous strata, if not much newer.
_Silurian period._--It has long been known that the granite near
Christiania, in Norway, is of newer origin than the Silurian strata
of that region. Von Buch first announced, in 1813, the discovery
of its posteriority in date to 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. 447.)
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. 504., 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.[457-B]
[Illustration: Fig. 504. Cross section.]
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.
[Illustration: Fig. 505. 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
disturbed and denuded before the sedimentary beds were superimposed (see
fig. 505.). 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 the transition strata. Between the origin, therefore,
of the gneiss and the granite there intervened, first, the period when the
strata of gneiss were inclined; secondly, the period when they were
denuded; thirdly, the period of the deposition of the transition deposits.
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. It seems necessary, therefore, to
conceive that the gneiss was softened and more or less melted when
penetrated by the granite. But had such junctions alone been visible, and
had we not learnt, from other sections, how long a period elapsed between
the consolidation of the gneiss and the injection of this granite, we might
have suspected that the gneiss was scarcely solidified, or had not yet
assumed its complete metamorphic character, when invaded by the plutonic
rock. From this example we may learn how impossible it is to conjecture
whether certain granites in Scotland, and other countries, which send veins
into gneiss and other metamorphic rocks, are primary, or whether they may
not belong to some secondary or tertiary period.
_Oldest granites._--It is not half a century since the doctrine was very
general that all granitic rocks were _primitive_, that is to say, that they
originated before the deposition of the first sedimentary strata, and
before the creation of organic beings (see above, p. 9.). 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 all the known
fossiliferous deposits. Could we discover some Lower Cambrian 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 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 Cambrian system, we may then feel
assured that the parent granite was formed before the Lower Cambrian
formation. But if the incumbent strata be merely Silurian or Upper
Cambrian, 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 felspar, 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.[459-A] 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.
If we admit that solid hypogene rocks, whether stratified or
unstratified, have in such cases been driven upwards so as to pierce
through yielding sedimentary deposits, we shall be enabled to account
for many geological appearances otherwise inexplicable. Thus, for
example, at Weinboehla and Hohnstein, near Meissen, in Saxony, a mass of
granite has been observed covering strata of the Cretaceous and Oolitic
periods for the space of between 300 and 400 yards square. It appears
clearly from a recent Memoir of Dr. B. Cotta on this subject[459-B],
that the granite was thrust into its actual position when solid. There
are no intersecting veins at the junction--no alteration as if by heat,
but evident signs of rubbing, and a breccia in some places, in which
pieces of granite are mingled with broken fragments of the secondary
rocks. As the granite overhangs both the lias and chalk, so the lias is
in some places bent over strata of the cretaceous era.
_Relative age of the granites of Arran._--In this island, the largest in
the Firth of Clyde, being twenty miles in length from north to south, the
four great classes of rocks, the fossiliferous, volcanic, plutonic, and
metamorphic, are all conspicuously displayed within a very small area, and
with their peculiar characters strongly contrasted. In the north of the
island the granite rises to the height of nearly 3000 feet above the sea,
terminating in mountainous peaks. (See section, fig. 506.) On the flanks of
the same mountains are chloritic-schists, blue roofing-slate, and other
rocks of the metamorphic order (No. 1.), into which the granite (No. 2.)
sends veins. This granite, therefore, is newer than the hypogene schists
(No. 1.), which it penetrates.
These schists are highly inclined. Upon them rest beds of conglomerate
and sandstone (No. 3.), which are referable to the Old Red formation, to
which succeed various shales and limestones (No. 4.) containing the
fossils of the Carboniferous period, upon which are other strata of
sandstone and conglomerate (upper part of No. 4.), in which no fossils
have been met with, which it is conjectured may belong to the New Red
sandstone period. All the preceding formations are cut through by the
volcanic rocks (No. 5.), which consist of greenstone, basalt,
pitchstone, claystone-porphyry, and other varieties. These appear either
in the form of dikes, or in dense masses from 50 to 700 feet in
thickness, overlying the strata (No. 4.). They sometimes pass into
syenite of so crystalline a form, that it may rank as a plutonic
formation; and in one region, at Ploverfield, in Glen Cloy, a
fine-grained granite (6. _a_) is seen associated with the trap
formation, and sending veins into the sandstone or into the upper strata
of No. 4. This interesting discovery of granite in the southern region
of Arran, at a point where it is separated from the northern mass of
granite by a great thickness of secondary strata and overlying trap, was
made by Mr. L. A. Necker of Geneva, during his survey of Arran in 1839.
We also learn from the recent investigations of Prof. A. C. Ramsay, that
a similar fine-grained granite (No. 6. _b_) appears in the interior of
the northern granitic district, forming the nucleus of it, and sending
veins into the older coarse-grained granite (No. 2.). The trap dikes
which penetrate the older granite are cut off, according to Mr. Ramsay,
at the junction of the fine grained.
It is not improbable that the granite (No. 6. _b_) may be of the same age
as that of Ploverfield (No. 6. _a_), and this again may belong to the same
geological epoch as the trap formations (No. 5.). If there be any
difference of date, it would seem that the fine-grained granite must be
newer than the trappean rocks. But, on the other hand, the coarser granite
(No. 2.) may be the oldest rock in Arran, with the exception of the
hypogene slates (No. 1.), into which it sends veins.
[Illustration: Fig. 506. General Section of Arran from north to south.
1. Metamorphic or Hypogene schists, the oldest formations in Arran.
2. Coarse-grained granite sending veins into the schists, No. 1.
3. Old Red Sandstone and Conglomerate containing pebbles exclusively
derived from the rocks, No 1., without any intermixture of granitic
fragments.
4. Carboniferous strata and red sandstone (New Red?).
5. Trap, overlying and in dikes, passing occasionally into Syenites of
the Plutonic class.
6. _a._ Fine-grained granite, associated with the overlying trap, No. 5.
6. _b._ Similar fine-grained granite, sending veins into the older
granite, No. 2., and cutting off the trappean dikes, _c_, _d_.[461-A]]
An objection may perhaps at first be started to this conclusion, derived
from the curious and striking fact, the importance of which was first
emphatically pointed out by Dr. MacCulloch, that no pebbles of granite
occur in the conglomerates of the red sandstone in Arran, although these
conglomerates are several hundred feet in thickness, and lie at the foot of
lofty granite mountains, which tower above them. As a general rule, all
such aggregates of pebbles and sand are mainly composed of the wreck of
pre-existing rocks occurring in the immediate vicinity. The total absence
therefore of granitic pebbles has justly been a theme of wonder to those
geologists who have successively visited Arran, and they have carefully
searched there, as I have done myself, to find an exception, but in vain.
The rounded masses consist exclusively of quartz, chlorite-schist, and
other members of the metamorphic series; nor in the newer conglomerates of
No. 4. have any granitic fragments been discovered. Are we then entitled to
affirm that the coarse-grained granite (No. 2.), like the fine-grained
variety (No. 6. _a_), is more modern than all the other rocks of the
island? This we cannot assume at present, but we may confidently infer that
when the various beds of sandstone and conglomerate were formed, no granite
had reached the surface, or had been exposed to denudation in Arran. It is
clear that the crystalline schists were ground into sand and shingle when
the strata No. 3. were deposited, and at that time the waves had never
acted upon the granite, which now sends its veins into the schist. May we
then conclude, that the schists suffered denudation before they were
invaded by granite? This opinion, although not inadmissible, is by no
means fully borne out by the evidence. For at the time when the Old Red
sandstone originated, the metamorphic strata may have formed islands in the
sea, as in fig. 507., over which the breakers rolled, or from which
torrents and rivers descended, carrying down gravel and sand. The plutonic
rock or granite (B) may even then have been previously injected at a
certain depth below, and yet may never have been exposed to denudation.
[Illustration: Fig. 507. Cross section.]
As to the time and manner of the subsequent protrusion of the
coarse-grained granite (No. 2.), this rock may have been thrust up
bodily, in a solid form, during that long series of igneous operations
which produced the trappean and plutonic formations (Nos. 5., 6.
_a_, and 6. _b_).
We have shown that these eruptions, whatever their date, were posterior to
the deposition of all the fossiliferous strata of Arran. We can also prove
that subsequently both the granitic and trappean rocks underwent great
aqueous denudation, which they probably suffered during their emergence
from the sea. The fact is demonstrated by the abrupt truncation of numerous
dikes, such as those at _c_, _d_, _e_, which are cut off on the surface of
the granite and trap. The overlying trap also ceases very abruptly on
approaching the boundary of the great hypogene region, and terminates in a
steep escarpment facing towards it as at _f_, fig. 506. When in its
original fluid state it could not have come thus suddenly to an end, but
must have filled up the hollow now separating it from the hypogene rocks,
had such a hollow then existed. This necessity of supposing that both the
trap and the conglomerate once extended farther, and that veins such as
_c_, _d_, fig. 506., were once prolonged farther upwards, prepares us to
believe that the whole of the northern granite may at one time have been
covered by newer formations, under the pressure of which, before its
protrusion, it assumed its highly crystalline texture.
The theory of the protrusion in a solid form of the northern nucleus of
granite is confirmed by the manner in which the hypogene slates (No. 1.)
and the beds of conglomerate (No. 3.) dip away from it on all sides. In
some places indeed the slates are inclined towards the granite, but this
exception might have been looked for, because these hypogene strata have
undergone disturbances at more than one geological epoch, and may at some
points, perhaps, have their original order of position inverted. The high
inclination, therefore, and the quaquaversal dip of the beds around the
borders of the granitic boss, and the comparative horizontality of the
fossiliferous strata in the southern part of the island, are facts which
all accord with the hypothesis of a great amount of movement at that point
where the granite is supposed to have been thrust up bodily, and where we
may conceive it to have been distended laterally by the repeated injection
of fresh supplies of melted materials.[463-A]
FOOTNOTES:
[450-A] Silliman's Journ., No. 69. p. 123.
[450-B] See "Principles," _Index_, "Jorullo."
[451-A] "Principles," _Index_, "Volcanic Eruptions."
[453-A] Darwin, pp. 390. 406.; second edition, p. 319.
[454-A] See map of Europe and explanation, in Principles, book i.
[456-A] Elie de Beaumont, sur les Montagnes de l'Oisans, &c. Mem. de la
Soc. d'Hist. Nat. de Paris, tom. v.
[456-B] See Murchison, Geol. Trans., 2d series, vol. ii. part ii.
pp. 311-321.
[456-C] Western Islands, vol. i. p. 330. plate 18., figs. 3, 4.
[456-D] Von Buch, Annales de Chimie, &c.
[457-A] Proceedings of Geol. Soc., vol. ii. p. 562.
[457-B] See the Gaea Norvegica and other works of Keilhau, with whom I
examined this country.
[459-A] Murchison, Geol. Trans., 2d series, vol. ii. p. 307.
[459-B] Geognostische Wanderungen, Leipzig, 1838.
[461-A] In the above section I have attempted to represent the new
discoveries made since 1839, by Mr. Necker and Mr. A. C. Ramsay, in regard
to the plutonic formations, 6. _a_, and 6. _b_.
[463-A] For the geology of Arran consult the works of Drs. Hutton and
MacCulloch, the Memoirs of Messrs. Von Dechen and Oeynhausen, that of
Professor Sedgwick and Sir R. Murchison (Geol. Trans. 2d series), Mr. L.
A. Necker's Memoir, read to the Royal Soc. of Edin. 20th April, 1840,
and Mr. Ramsay's Geol. of Arran, 1841. I examined myself a large part
of Arran in 1836.