Part 69 (2/2)
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, diminis.h.i.+ng 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 Devons.h.i.+re, 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 Sutherlands.h.i.+re, 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.
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