The Student's Elements of Geology Part 72

The princ.i.p.al effect of subterranean movements during the Tertiary Period seems to have consisted in the upheaval 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 great 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.

PLUTONIC ROCKS OF CRETACEOUS PERIOD.

(FIGURE 618. Section through three layers (b, c, d) of the Cretaceous series over granite (A).)

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, can not easily be decided. Suppose b, c, d, Figure 618, 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. But as some Cretaceous and even Tertiary rocks 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 periods may not have been brought up and exposed by denudation, at the height of 2000 or 3000 feet on the flanks of that chain.

PLUTONIC ROCKS OF THE OOLITE AND LIAS.

(FIGURE 619. Junction of granite with Jura.s.sic or Oolite strata in the Alps, near Champoleon. (Granite over Altered Rocks over Secondary Schists.))

In the Department of the Hautes Alpes, in France, M. Eliede 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 feldspar is observed partly to overlie the secondary rocks, producing an alteration which extends for about 30 feet downward, diminishing in the beds which lie farthest from the granite. (See Figure 619.) 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 crystallised near the junction. (Elie de Beaumont sur les Montagnes de l'Oisans etc. Mem. de la Soc.

d'Hist. Nat. de Paris tome 5.) Although the granite is inc.u.mbent in the section (Figure 619), we can not a.s.sume that it overflowed the strata, for the disturbances of the rocks are so great in this part of the Alps that their original position is often inverted.

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. (Von Buch Annales de Chimie etc.)

PLUTONIC ROCKS OF 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 now known to be members of the true Carboniferous series. This granite, like the syenitic granite of Christiania, has broken through the stratified formations, on the north-west side of Dartmoor, the successive members of the culm-measures ab.u.t.ting against the granite, and becoming metamorphic as they approach. These strata are also penetrated by granite veins, and Plutonic dikes, called "elvans." (Proceedings of the Geological Society volume 2 page 562 and Transactions second series volume 5 page 686.) The granite of Cornwall is probably of the same date, and, therefore, as modern as the Carboniferous strata, if not newer.

PLUTONIC ROCKS OF SILURIAN PERIOD.

(FIGURE 620. Section through Silurian strata and Granite.)

It has long been known that a very ancient granite near Christiania, in Norway, is posterior in date to the Lower Silurian strata of that region, although its exact position in the Palaeozoic series can not be defined. Von Buch first announced, in 1813, that it was of newer origin than certain 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 Chapter 31.)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, Figure 620, 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. (See the Gaea Norvegica and other works of Keilhau with whom I examined this country.)

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.

(FIGURE 621. Granite sending veins into Silurian strata and gneiss. Christiania, Norway.

a. Inclined gneiss.

b. Silurian strata.)

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 denuded before the sedimentary beds were superimposed (see Figure 621). 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 these Silurian strata. Between the origin, therefore, of the gneiss and the granite there intervened, first, the period when the strata of gneiss were denuded; secondly, the period of the deposition of the Silurian deposits upon the denuded and inclined gneiss, a. 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. These appearances may probably be due to hydrothermal action (see Chapter 33). I shall merely observe in this place that 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 Chapter 1). 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 known fossiliferous deposits. Could we discover some Laurentian 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, as we have already pointed out (Chapter 26), 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 can not 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 Laurentian system, we may then feel a.s.sured that the parent granite was formed before the Laurentian formation. But if the inc.u.mbent strata be merely Cambrian or Silurian, 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 feldspar, 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. (Murchison Geological Transactions second series volume 2 page 307.) 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.

Mr. T. McKenney Hughes has suggested to me in explanation of these phenomena that they may be the effect of the a.s.sociation of more pliant strata with hard unyielding rocks, the whole of which were subjected simultaneously to great movements, whether of elevation or subsidence, and of lateral pressure, during which the more solid granite, being incapable of compression, was forced through the softer beds of shale, sandstone, and limestone. He remarks that similar breccias with slickensides are observed on a minor scale where rocks of different composition and rigidity are contorted together. Such protrusion may have been brought about by degrees by innumerable shocks of earthquakes repeated after long intervals of time along the same tract of country. The opening of new fissures in the hardest rocks is a frequent accompaniment of such convulsions, and during the consequent vibrations, breccias must often be caused. But these catastrophes, as we well know, do not imply that the land or sea of the disturbed region are rendered uninhabitable by living beings, and by no means indicate a state of things different from that witnessed in the ordinary course of nature.

CHAPTER x.x.xIII.

METAMORPHIC ROCKS.

General Character of Metamorphic Rocks.

Gneiss.

Hornblende-schist.

Serpentine.

Mica-schist.

Clay-slate.

Quartzite.

Chlorite-schist.

Metamorphic Limestone.

Origin of the metamorphic Strata.

Their Stratification.

Fossiliferous Strata near intrusive Ma.s.ses of Granite converted into Rocks identical with different Members of the metamorphic Series.

Arguments hence derived as to the Nature of Plutonic Action.

Hydrothermal Action, or the Influence of Steam and Gases in producing Metamorphism.

Objections to the metamorphic Theory considered.

We have now considered three distinct cla.s.ses of rocks: first, the aqueous, or fossiliferous; secondly, the volcanic; and, thirdly, the Plutonic; and it remains for us to examine those crystalline (or hypogene) strata to which the name of METAMORPHIC has been a.s.signed. The last-mentioned term expresses, as before explained, a theoretical opinion that such strata, after having been deposited from water, acquired, by the influence of heat and other causes, a highly crystalline texture. They who still question this opinion may call the rocks under consideration the stratified hypogene formations or crystalline schists.

These rocks, when in their characteristic or normal state, are wholly devoid of organic remains, and contain no distinct fragments of other rocks, whether rounded or angular. They sometimes break out in the central parts of mountain chains, but in other cases extend over areas of vast dimensions, occupying, for example, nearly the whole of Norway and Sweden, where, as in Brazil, they appear alike in the lower and higher grounds. However crystalline these rocks may become in certain regions, they never, like granite or trap, send veins into contiguous formations. In Great Britain, those members of the series which approach most nearly to granite in their composition, as gneiss, mica-schist, and hornblende-schist, are confined to the country north of the rivers Forth and Clyde.

Many attempts have been made to trace a general order of succession or superposition in the members of this family; clay-slate, for example, having been often supposed to hold invariably a higher geological position than mica- schist, and mica-schist to overlie gneiss. But although such an order may prevail throughout limited districts, it is by no means universal. To this subject, however, I shall again revert, in Chapter 35, where the chronological relations of the metamorphic rocks are pointed out.

PRINc.i.p.aL METAMORPHIC ROCKS.

The following may be enumerated as the princ.i.p.al members of the metamorphic cla.s.s:-- gneiss, mica-schist, hornblende-schist, clay-slate, chlorite-schist, hypogene or metamorphic limestone, and certain kinds of quartz-rock or quartzite.

GNEISS.

(FIGURE 622. Fragment of gneiss, natural size; section made at right angles to the planes of foliation.)

The first of these, gneiss, may be called stratified-- or by those who object to that term, foliated-- granite, being formed of the same materials as granite, namely, feldspar, quartz, and mica. In the specimen in Figure 622, the white layers consist almost exclusively of granular feldspar, with here and there a speck of mica and grain of quartz. The dark layers are composed of grey quartz and black mica, with occasionally a grain of feldspar intermixed. The rock splits most easily in the plane of these darker layers, and the surface thus exposed is almost entirely covered with shining spangles of mica. The accompanying quartz, however, greatly predominates in quant.i.ty, but the most ready cleavage is determined by the abundance of mica in certain parts of the dark layer. Instead of consisting of these thin laminae, gneiss is sometimes simply divided into thick beds, in which the mica has only a slight degree of parallelism to the planes of stratification.

Hand specimens may often be obtained from such gneiss which are undistinguishable from granite, affording an argument to which we shall allude in the concluding part of this chapter, in favour of those who regard all granite and syenite not as igneous rocks, but as aqueous formations so altered as to have lost all signs of their original stratified arrangement. Gneiss in geology is commonly used to designate not merely stratified and foliated rocks having the same component materials as granite or syenite, but also in a wider sense to embrace the formation with which other members of the metamorphic series, such as hornblende-schist, may alternate, and which are then considered subordinate to the true gneiss.

The different varieties of rock allied to gneiss, into which feldspar enters as an essential ingredient, will be understood by referring to what was said of granite. Thus, for example, hornblende may be superadded to mica, quartz, and feldspar, forming a hornblendic or syenitic gneiss; or talc may be subst.i.tuted for mica, const.i.tuting talcose gneiss (called stratified protogine by the French), a rock composed of feldspar, quartz, and talc, in distinct crystals or grains.

EURITE, which has already been mentioned as a Plutonic rock, occurs also with precisely the same composition in beds subordinate to gneiss or mica-slate.

HORNBLENDE-SCHIST is usually black, and composed princ.i.p.ally of hornblende, with a variable quant.i.ty of feldspar, and sometimes grains of quartz. When the hornblende and feldspar are in nearly equal quant.i.ties, and the rock is not slaty, it corresponds in character with the greenstones of the trap family, and has been called "primitive greenstone." It may be termed hornblende rock, or amphibolite. Some of these hornblendic ma.s.ses may really have been volcanic rocks, which have since a.s.sumed a more crystalline or metamorphic texture.

SERPENTINE is a greenish rock, a silicate of magnesia, in which there is sometimes from 30 to 40 per cent of magnesia. It enters largely into the composition of a trap dike cutting through Old Red Sandstone in Forfarshire, and in that case is probably an altered basaltic dike which had contained much olivine. The theory of its having been originally a volcanic product subsequently altered by metamorphism may at first sight seem inconsistent with its occurrence in large and regularly stratified ma.s.ses in the metamorphic series in Scotland, as in Aberdeenshire. But it has been suggested in explanation that such serpentine may have been originally regularly-bedded trap tuff, and volcanic breccia, with much olivine, which would still retain a stratified appearance after their conversion into a metamorphic rock.

ACTINOLITE SCHIST is a slaty foliated rock, composed chiefly of actinolite, an emerald-green mineral, allied to hornblende, with some admixture of garnet, mica, and quartz.

MICA-SCHIST or MICACEOUS SCHIST is, next to gneiss, one of the most abundant rocks of the metamorphic series. It is slaty, essentially composed of mica and quartz, the mica sometimes appearing to const.i.tute the whole ma.s.s. Beds of pure quartz also occur in this formation. In some districts, garnets in regular twelve-sided crystals form an integrant part of mica-schist. This rock pa.s.ses by insensible gradations into clay-slate.

CLAY-SLATE-- ARGILLACEOUS SCHIST-- ARGILLITE.

This rock sometimes resembles an indurated clay or shale. It is for the most part extremely fissile, often affording good roofing-slate. Occasionally it derives a shining and silky l.u.s.tre from the minute particles of mica or talc which it contains. It varies from greenish or bluish-grey to a lead colour; and it may be said of this, more than of any other schist, that it is common to the metamorphic and fossiliferous series, for some clay-slates taken from each division would not be distinguishable by mineral characters alone. It is not uncommon to meet with an argillaceous rock having the same composition, without the slaty cleavage, which may be called argillite.