A Manual of Elementary Geology Part 69

[446-A] Necker, Proceedings of Geol. Soc., No. 26. p. 392.

[446-B] See Keilhau's Gaea Norvegica; Christiania, 1838.

CHAPTER x.x.xIV.

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 h.o.m.ogeneous character; so that having ascertained its relative age in one place, we can easily recognize its ident.i.ty in others, and thus determine from a single section the chronological relations of large mountain ma.s.ses. 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 ma.s.ses 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 cla.s.s 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 acc.u.mulated 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 ma.s.ses 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 r.e.t.a.r.ded 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 ma.s.s 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 quant.i.ty 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 a.s.sume 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 cla.s.s 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.

[Ill.u.s.tration: 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. x.x.xII., 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 cla.s.s. 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 a.s.signed 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 sh.e.l.ls 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 ma.s.ses 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 sh.o.r.es 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 a.s.sociated 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, a.r.s.enic, 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 a.s.sume, 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 superinc.u.mbent 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 alt.i.tude; 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 s.p.a.ces 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 ent.i.tled, therefore, to ask what amount of change of equivalent importance can be proved to have occurred in the earth's crust within an equal quant.i.ty 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 antic.i.p.ated.

The princ.i.p.al 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 repet.i.tion 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 a.s.sume 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.

[Ill.u.s.tration: 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 a.s.sume 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 ma.s.s 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 ma.s.s 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 circ.u.mstance 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]

[Ill.u.s.tration: Fig. 503. Junction of granite with Jura.s.sic or Oolite strata in the Alps, near Champoleon.]

Although the granite is inc.u.mbent in the above section (fig. 503.), we cannot a.s.sume 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 ma.s.s 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 sh.e.l.ls, 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 pa.s.ses 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 ma.s.s from which they emanate. (See p. 447.) Von Buch supposed that the plutonic rock alternated with the fossiliferous strata, and that large ma.s.ses of granite were sometimes inc.u.mbent 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 pa.s.s under it in ma.s.s, 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 ma.s.ses, as is so commonly the case with trappean formations.[457-B]

[Ill.u.s.tration: 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.

[Ill.u.s.tration: 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 pa.s.s freely through gneiss, ending sometimes in threads, as if the older rock had offered no resistance to their pa.s.sage. 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 a.s.sumed 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 ma.s.s 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 a.s.sume 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 a.s.sured that the parent granite was formed before the Lower Cambrian formation. But if the inc.u.mbent 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 Weinbohla and Hohnstein, near Meissen, in Saxony, a ma.s.s of granite has been observed covering strata of the Cretaceous and Oolitic periods for the s.p.a.ce 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.