A Manual of Elementary Geology Part 71

_Chlorite-schist_ is a green slaty rock, in which chlorite is abundant in foliated plates, usually blended with minute grains of quartz, or sometimes with felspar or mica. Often a.s.sociated with, and graduating into, gneiss and clay-slate.

_Hypogene_, or _Metamorphic limestone_.--This rock, commonly called _primary limestone_, is sometimes a thick bedded white crystalline granular marble used in sculpture; but more frequently it occurs in thin beds, forming a foliated schist much resembling in colour and appearance certain varieties of gneiss and mica-schist. It alternates with both these rocks, and in like manner with argillaceous schist. It then usually contains some crystals of mica, and occasionally quartz, felspar, hornblende, and talc. This member of the metamorphic series enters sparingly into the structure of the hypogene districts of Norway, Sweden, and Scotland, but is largely developed in the Alps.

Before offering any farther observations on the probable origin of the metamorphic rocks, I subjoin, in the form of a glossary, a brief explanation of some of the princ.i.p.al varieties and their synonymies.

ACTINOLITE-SCHIST. A slaty foliated rock, composed chiefly of actinolite, (an emerald-green mineral, allied to hornblende,) with some admixture of felspar, or quartz, or mica.

AMPELITE. Aluminous slate (Brongniart); occurs both in the metamorphic and fossiliferous series.

AMPHIBOLITE. Hornblende rock, which see.

ARGILLACEOUS-SCHIST, or CLAY-SLATE. _See_ p. 465.

ARKOSE. Term used by Brongniart for granular Quartzite, which see.

CHIASTOLITE-SLATE scarcely differs from clay-slate, but includes numerous crystals of Chiastolite; in considerable thickness in c.u.mberland.

Chiastolite occurs in long slender rhomboidal crystals. For composition, see Table, p. 377.

CHLORITE-SCHIST. A green slaty rock, in which chlorite, a green scaly mineral, is abundant. _See_ p. 465.

CLAY-SLATE, or ARGILLACEOUS-SCHIST. _See_ p. 465.

EURITE and EURITIC PORPHYRY. A base of compact felspar, with grains of laminar felspar, and often mica and other minerals disseminated (Brongniart). M. D'Aubuisson regards eurite as an extremely fine-grained granite, in which felspar predominates, the whole forming an apparently h.o.m.ogeneous rock. Eurite has been already mentioned as a plutonic rock, but occurs also in beds subordinate to gneiss or mica-slate.

GNEISS. A stratified or laminated rock, same composition as granite.

_See_ p. 464.

HORNBLENDE ROCK, or AMPHIBOLITE. Composed of hornblende and felspar.

The same composition as hornblende-schist, stratified, but not fissile.

_See_ p. 376.

HORNBLENDE-SCHIST, or SLATE. Composed chiefly of hornblende, with occasionally some felspar. _See_ p. 464.

HORNBLENDIC or SYENITIC-GNEISS. Composed of felspar, quartz, and hornblende.

HYPOGENE LIMESTONE. _See_ p. 465.

MARBLE. _See_ p. 465.

MICA-SCHIST, or MICACEOUS-SCHIST. A slaty rock, composed of mica and quartz in variable proportions. _See_ p. 465.

MICA-SLATE. _See_ MICA-SCHIST, p. 465.

PHYLLADE. D'Aubuisson's term for clay-slate, from +phullas+, a heap of leaves.

PRIMARY LIMESTONE. _See_ HYPOGENE LIMESTONE, p. 465.

PROTOGINE. _See_ TALCOSE-GNEISS, p. 464.; when unstratified it is Talcose-granite.

QUARTZ ROCK, or QUARTZITE. A stratified rock; an aggregate of grains of quartz. _See_ p. 465.

SERPENTINE occurs in both divisions of the hypogene series, as a stratified or unstratified rock; contains much magnesia; is chiefly composed of the mineral called serpentine, mixed with diallage, talc, and steat.i.te. The pure varieties of this rock, called n.o.ble serpentine, consist of a hydrated silicate of magnesia, generally of a greenish colour: this base is commonly mixed with oxide of iron.

TALCOSE-GNEISS. Same composition as talcose-granite or protogine, but either stratified or laminated. _See_ p. 464.

TALCOSE-SCHIST consists chiefly of talc, or of talc and quartz, or of talc and felspar, and has a texture something like that of clay-slate.

WHITESTONE. Same as Eurite.

_Origin of the Metamorphic Strata._

Having said thus much of the mineral composition of the metamorphic rocks, I may combine what remains to be said of their structure and history with an account of the opinions entertained of their probable origin. At the same time, it may be well to forewarn the reader that we are here entering upon ground of controversy, and soon reach the limits where positive induction ends, and beyond which we can only indulge in speculations. It was once a favourite doctrine, and is still maintained by many, that these rocks owe their crystalline texture, their want of all signs of a mechanical origin, or of fossil contents, to a peculiar and nascent condition of the planet at the period of their formation.

The arguments in refutation of this hypothesis will be more fully considered when I show, in the last chapter of this volume, to how many different ages the metamorphic formations are referable, and how gneiss, mica-schist, clay-slate, and hypogene limestone (that of Carrara for example), have been formed, not only since the first introduction of organic beings into this planet, but even long after many distinct races of plants and animals had pa.s.sed away in succession.

The doctrine respecting the crystalline strata, implied in the name metamorphic, may properly be treated of in this place; and we must first inquire whether these rocks are really ent.i.tled to be called stratified in the strict sense of having been originally deposited as sediment from water. The general adoption by geologists of the term stratified, as applied to these rocks, sufficiently attests their division into beds very a.n.a.logous, at least in form, to ordinary fossiliferous strata. This resemblance is by no means confined to the existence in both of an occasional slaty structure, but extends to every kind of arrangement which is compatible with the absence of fossils, and of sand, pebbles, ripple-mark, and other characters which the metamorphic theory supposes to have been obliterated by plutonic action. Thus, for example, we behold alike in the crystalline and fossiliferous formations an alternation of beds varying greatly in composition, colour, and thickness. We observe, for instance, gneiss alternating with layers of black hornblende-schist, or with granular quartz, or limestone; and the interchange of these different strata may be repeated for an indefinite number of times. In the like manner, mica-schist alternates with chlorite-schist, and with granular limestone in thin layers.

As in fossiliferous formations strata of pure siliceous sand alternate with micaceous sand and with layers of clay, so in the crystalline or metamorphic rocks we have beds of pure quartzite alternating with mica-schist and clay-slate. As in the secondary and tertiary series we meet with limestone alternating again and again with micaceous or argillaceous sand, so we find in the hypogene, gneiss and mica-schist alternating with pure and impure granular limestones.

It has also been shown that the ripple-mark is very commonly repeated throughout a considerable thickness of fossiliferous strata; so in mica-schist and gneiss, there is sometimes an undulation of the laminae on a minute scale, which may, perhaps, be a modification of similar inequalities in the original deposit.

In the crystalline formations also, as in many of the sedimentary before described, single strata are sometimes made up of laminae placed diagonally, such laminae not being regularly parallel to the planes of cleavage.

[Ill.u.s.tration: Fig. 509. Lamination of clay-slate, Montagne de Seguinat, near Gavarnie, in the Pyrenees.]

This disposition of the layers is ill.u.s.trated in the accompanying diagram, in which I have represented carefully the stratification of a coa.r.s.e argillaceous schist, which I examined in the Pyrenees, part of which approaches in character to a green and blue roofing slate, while part is extremely quartzose, the whole ma.s.s pa.s.sing downwards into micaceous schist. The vertical section here exhibited is about 3 feet in height, and the layers are sometimes so thin that fifty may be counted in the thickness of an inch. Some of them consist of pure quartz.

The inference drawn from the phenomena above described in favour of the aqueous origin of clay-slate and other crystalline strata, is greatly strengthened by the fact that many of these metamorphic rocks occasionally alternate with, and sometimes pa.s.s by intermediate gradations into, rocks of a decidedly mechanical origin, and exhibiting traces of organic remains.

The fossiliferous formations, moreover, into which this pa.s.sage is effected, are by no means invariably of the same age nor of the highest antiquity, as will be afterwards explained.

_Stratification of the metamorphic rocks distinct from cleavage._--The beds into which gneiss, mica-schist, and hypogene limestone divide, exhibit most commonly, like ordinary strata, a want of perfect geometrical parallelism.

For this reason, therefore, in addition to the alternate recurrence of layers of distinct materials, the stratified arrangement of the crystalline rocks cannot be explained away by supposing it to be simply a divisional structure like that to which we owe some of the slates used for writing and roofing. _Slaty cleavage_, as it has been called, has in many cases been produced by the regular deposition of thin plates of fine sediment one upon another; but there are many instances where it is decidedly unconnected with such a mode of origin, and where it is not even confined to the aqueous formations. Some kinds of trap, for example, as clinkstone, split into laminae, and are used for roofing.

There are, says Professor Sedgwick, three distinct forms of structure exhibited in certain rocks throughout large districts: viz.--First, stratification; secondly, joints; and thirdly, slaty cleavage; the two last having no connection with true bedding, and having been superinduced by causes absolutely independent of gravitation. All these different structures must have different names, even though there be some cases where it is impossible, after carefully studying the appearances, to decide upon the cla.s.s to which they belong.[469-A]

_Joints._--Now, in regard to the second of these forms of structure or joints, they are natural fissures which often traverse rocks in straight and well-determined lines. They afford to the quarryman, as Sir R.

Murchison observes, when speaking of the phenomena, as exhibited in Shropshire and the neighbouring counties, the greatest aid in the extraction of blocks of stone; and, if a sufficient number cross each other, the whole ma.s.s of rock is split into symmetrical blocks.[469-B] The faces of the joints are for the most part smoother and more regular than the surfaces of true strata. The joints are straight-cut c.h.i.n.ks, often slightly open, often pa.s.sing, not only through layers of successive deposition, but also through b.a.l.l.s of limestone or other matter which have been formed by concretionary action, since the original acc.u.mulation of the strata. Such joints, therefore, must often have resulted from one of the last changes superinduced upon sedimentary deposits.[469-C]

In the annexed diagram the flat surfaces of rock A, B, C, represent exposed faces of joints, to which the walls of other joints, J J, are parallel. S S are the lines of stratification; D D are lines of slaty cleavage, which intersect the rock at a considerable angle to the planes of stratification.

[Ill.u.s.tration: Fig. 510. Stratification, joints, and cleavage.]

Joints, according to Professor Sedgwick, are distinguishable from lines of slaty cleavage in this, that the rock intervening between two joints has no tendency to cleave in a direction parallel to the planes of the joints, whereas a rock is capable of indefinite subdivision in the direction of its slaty cleavage. In some cases where the strata are curved, the planes of cleavage are still perfectly parallel. This has been observed in the slate rocks of part of Wales (see fig. 511.), which consist of a hard greenish slate. The true bedding is there indicated by a number of parallel stripes, some of a lighter and some of a darker colour than the general ma.s.s. Such stripes are found to be parallel to the true planes of stratification, wherever these are manifested by ripple-mark, or by beds containing peculiar organic remains. Some of the contorted strata are of a coa.r.s.e mechanical structure, alternating with fine-grained crystalline chloritic slates, in which case the same slaty cleavage extends through the coa.r.s.er and finer beds, though it is brought out in greater perfection in proportion as the materials of the rock are fine and h.o.m.ogeneous. It is only when these are very coa.r.s.e that the cleavage planes entirely vanish.

These planes are usually inclined at a very considerable angle to the planes of the strata. In the Welsh chains, for example, the average angle is as much as from 30 to 40. Sometimes the cleavage planes dip towards the same point of the compa.s.s as those of stratification, but more frequently to opposite points. It may be stated as a general rule, that when beds of coa.r.s.er materials alternate with those composed of finer particles, the slaty cleavage is either entirely confined to the fine-grained rock, or is very imperfectly exhibited in that of coa.r.s.er texture. This rule holds, whether the cleavage is parallel to the planes of stratification or not.

[Ill.u.s.tration: Fig. 511. Parallel planes of cleavage intersecting curved strata. (Sedgwick.)]