Part 69 (1/2)
[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.