The Student's Elements of Geology Part 71

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

Von Oeynhausen and Von Dechen. (Philosophical Magazine and Annals No. 27 New Series March 1829.) 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.

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 ma.s.s, 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. (Necker Proceedings of the Geological Society No. 26 page 392.)

(FIGURE 615. a, b. Quartz vein pa.s.sing 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 2 1/2 feet wide; then the crack a-b pa.s.sed 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.

(FIGURE 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 ma.s.ses between the primary or palaeozoic 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 ma.s.s 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 pa.s.s into granite, and deserve, perhaps, to be cla.s.sed as Plutonic rather than trappean rocks, which may truly be described as interposed conformably between fossiliferous strata, as the porphyries (a, c, Figure 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 ma.s.ses 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 ill.u.s.tration 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 a.s.sociated 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 can not 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 cla.s.s of rocks a.n.a.logous 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 pa.s.sing 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 pa.s.sage 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.

CHAPTER x.x.xII.

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 (Chapter 31), 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 recognise 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 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 can not 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 (Silliman's Journal No. 69 page 123.), 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 cla.s.s 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 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. (See Principles Index Jorullo.) 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 (Ibid.

Volcanic Eruptions.), 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 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 a.s.sume 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 cla.s.s formed successively below-- subterposition in the Plutonic, like superposition in the sedimentary rocks, being usually characteristic of a newer origin.

(FIGURE 617. Diagram showing the relative position which the Plutonic and sedimentary formations of different ages may occupy.

I. Primary Plutonic rocks.

II. Secondary Plutonic rocks.

III. Tertiary Plutonic rocks.

IV. Post-tertiary Plutonic rocks.

1. Primary fossiliferous or Palaeozoic strata.

2. Secondary or Mesozoic strata.

3. Tertiary or Cainozoic strata.

4. Post-tertiary strata.

The metamorphic rocks are not indicated in this diagram: but the student will infer, from what is said in Chapters 31 and 33, that some portions of the stratified formations, Nos. 1 and 2, invaded by granite, will have become metamorphic.)

In Figure 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 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. 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. (Western Islands volume 1 page 330.) 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 cla.s.s. 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 a.s.signed 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 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, has, with the exception of a few districts, emerged from the deep to its present alt.i.tude. (See map of Europe, and explanation, in Principles book 1.) 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.