Part 3 (1/2)
The rocks which issued from the centre of the earth in a state of fusion are found a.s.sociated or interstratified with ma.s.ses of every epoch, more especially with those of the more ancient strata. The formations which these rocks have originated possess great interest; first, because they enter into the composition of the terrestrial crust; secondly, because they have impressed on its surface, in the course of their eruption, some of the characteristics of its configuration and structure; finally, because, by their means, the metals which are the objects of human industry have been brought nearer to the surface. According to the order of their appearance, or as nearly so as can be ascertained, we shall cla.s.s the eruptive rocks in two groups:--
I. The _Volcanic Rocks_, of comparatively recent origin, which have given rise to a succession of trachytes, basalts, and modern lavas.
These, being of looser texture, are presumed to have cooled more rapidly than the Plutonic rocks, and at or near the surface.
II. The _Plutonic Rocks_, of older date, which are exemplified in the various kinds of granites, the syenites, the protogines, porphyries, &c.
These differ from the volcanic rocks in their more compact crystalline structure, in the absence of tufa, as well as of pores and cavities; from which it is inferred that they were formed at considerable depths in the earth, and that they have cooled and crystallised slowly under great pressure.
PLUTONIC ERUPTIONS.
The great eruptions of _ancient granite_ are supposed to have occurred during the primary epoch, and chiefly in the carboniferous period. They present themselves sometimes in considerable ma.s.ses, for the earth's crust being still thin and permeable, it was prepared as it were for absorbing the granite ma.s.ses. In consequence of its weak cohesion, the primitive crust of the globe would be rent and penetrated in all directions, as represented in the following section of Cape Wrath, in Sutherlands.h.i.+re, in which the veins of granite ramify in a very irregular manner across the gneiss and hornblende-schist, there a.s.sociated with it. (Fig. 3.)
[Ill.u.s.tration: Fig. 3.--Veins of granite traversing the gneiss of Cape Wrath.]
Granite, when it is sound, furnishes a fine building-stone, but we must not suppose that it deserves that character of extreme hardness with which the poets have gratuitously gifted it. Its granular texture renders it unfit for road-stone, where it gets crushed too quickly to dust. With his hammer the geologist easily shapes his specimens; and in the Russian War, at the bombardment of Bomarsund, the shot from our s.h.i.+ps demonstrated that ramparts of granite could be as easily demolished as those constructed of limestone.
The component minerals of granite are felspar, quartz, and mica, in varying proportions; felspar being generally the predominant ingredient, and quartz more plentiful than mica--the whole being united into a confusedly granular or crystalline ma.s.s. Occasionally it pa.s.ses insensibly from fine to coa.r.s.e-grained granite, and the finer grained is even sometimes found embedded in the more coa.r.s.ely granular variety: sometimes it a.s.sumes a porphyritic texture. Porphyritic granite is a variety of granite, the components of which--quartz, felspar, and mica--are set in a non-crystallised paste, uniting the ma.s.s in a manner which will be familiar to many of our readers who may have seen the granite of the Land's End, in Cornwall. Alongside these orthoclase crystals, quartz is implanted, usually in grains of irregular shape, more rarely crystallised, and seldom in the form of perfect crystals. To these ingredients are added thin scales or small hexagonal plates and crystals of white, brown, black, or greenish-coloured mica. Finally, the name of _quartziferous porphyry_ is reserved for those varieties which present crystals of quartz; the other varieties are simply called _porphyritic granite_. _True_ porphyry presents a paste essentially composed of compact felspar, in which the crystals of orthoclase--that is, felspar with a potash base--are abundantly disseminated, and sometimes with great regularity.
Granite is supposed to have been ”formed at considerable depths in the earth, where it has cooled and crystallised slowly under great pressure, where the contained gases could not expand.”[11] ”The influence,” says Lyell, ”of subterranean heat may extend downwards from the crater of every active volcano to a great depth below, perhaps several miles or leagues, and the effects which are produced deep in the bowels of the earth may, or rather must, be distinct; so that volcanic and plutonic rocks, each different in texture, and sometimes even in composition, may originate simultaneously, the one at the surface, the other far beneath it.” Other views, however, of its origin are not unknown to science: Professor Ramsay and some other geologists consider granite to be metamorphic. ”For my own part,” says the Professor, ”I believe that in one sense it is an igneous rock; that is to say, that it has been completely fused. But in another sense it is a metamorphic rock, partly because it is impossible in many cases to draw any definite line between gneiss and granite, for they pa.s.s into each other by insensible gradations; and granite frequently _occupies the s.p.a.ce that ought to be filled with gneiss_, were it not that the gneiss has been entirely fused. I believe therefore that granite and its allies are simply the effect of the extreme of metamorphism, brought about by great heat with presence of water. In other words, when the metamorphism has been so great that all traces of the semi-crystalline laminated structure have disappeared, a more perfect crystallisation has taken place.”[12] It is obvious that the very result on which the Professor founds his theory, namely, the difficulty ”in many cases,” of drawing a line between the granite and the gneiss, would be produced by the sudden injection of the fluid minerals into gneiss, composed of the same materials. Moreover, it is only in some cases that the difficulty exists; in many others the line of separation is definable enough.[13]
[11] Lyell's ”Elements of Geology,” p. 694.
[12] ”Physical Geology and Geography of Great Britain,” by A. C.
Ramsay, p. 38, 2nd ed.
[13] At the same time it may be safely a.s.sumed (as Professor Ramsay believes to be the case) that granite in most cases is a metamorphic rock; yet are there many instances in which it may with greater truth be considered as a true plutonic rock.
The granitic rock called _Syenite_, in which a part of the mica is replaced by hornblende or amphibole, has to all appearance been erupted to the surface subsequently to the granite, and very often alongside of it. Thus the two extremities of the Vosges, towards Belfort and Strasburg, are eminently syenitic, while the intermediate part, towards Colmar, is as markedly granitic. In the Lyonnais, the southern region is granitic; the northern region, from Arbresle, is in great part syenitic.
Syenite also makes its appearance in the Limousin.
Syenite, into which rose-coloured felspar often enters, forms a beautiful rock, because the green or nearly black hornblende heightens, by contrast, the effect of its colour. This rock is a valuable adjunct for architectural ornament; it is that out of which the ancient Egyptians shaped many of their monumental columns, sphinxes, and sarcophagi; the most perfect type of it is found in Egypt, not far from the city of Syene, from which it derives its name. The obelisk of Luxor now in Paris, several of the Egyptian obelisks in Rome, and the celebrated sphinxes, of which copies may be seen in front of the Egyptian Court at the Crystal Palace, the pedestal of the statue of Peter the Great at St. Petersburg, and the facing of the sub-bas.e.m.e.nt of the column in the Place Vendome in Paris, are of this stone, of which there are quarries in the neighbourhood of Plancher-les-Mines in the Vosges.
Syenite disintegrates more readily than granite, and it contains indurated nodular concretions, which often remain in the form of large spherical b.a.l.l.s, in the midst of the debris resulting from disintegration of the ma.s.s. It remains to be added that syenitic ma.s.ses are often very variable as regards their composition; the hornblende is sometimes wanting, in which case we can only recognise an ancient granite. In other instances the hornblende predominates to such a degree, that a large or small-grained _diorite_, or greenstone, results.
The geologist should be prepared to observe these transitions, which are apt to lead him into error if pa.s.sed over without being noticed.
_Protogine_ is a talcose granite, composed of felspar, quartz, and talc or _chlorite_, or decomposed mica, which take the place of the usual mica. Excessively variable in its texture, protogine pa.s.ses from the most perfect granitic aspect to that of a porphyry, in such a manner as to present continual subjects of uncertainty, rendering it very difficult to determine its geological age. Nevertheless, it is supposed to have come to the surface before and during the coal-period; in short, at Creusot, protogine covers the coal-fields so completely, that it is necessary to sink the pits through the protogine, in order to penetrate to the coal, and the rock has so manifestly acted on the coal-measure strata, as to have contorted and metamorphosed them. Something a.n.a.logous to this manifests itself near Mont Blanc, where the colossal ma.s.s which predominates in that chain, and the peaks which belong to it, consist of protogine. But as no such action can be perceived in the overlying rocks of the Tria.s.sic period, it may be a.s.sumed that at the time of the deposition of the New Red Sandstone the protoginous eruptions had ceased.
It is necessary to add, however, that if the protogine rises in such bold pinnacles round Mont Blanc, the circ.u.mstance only applies to the more elevated parts of the mountain, and is influenced by the excessive rigour of the seasons, which demolishes and continually wears away all the parts of the rock which have been decomposed by atmospheric agency.
Where protogine occurs in milder climates--around Creusot, and at Pierre-sur-Autre, in the Forez chain, for instance--the mountains show none of the scarped and bristling peaks exhibited in the chain of Mont Blanc. Only single isolated ma.s.ses occasionally form _rocking-stones_, so called because, resting with a convex base upon a pedestal also convex, but in a contrary way, it is easy to move these naturally balanced blocks, although from their vast size it would require very considerable force to displace them. This tendency to fas.h.i.+on themselves into rounded or ellipsoidal forms belongs, also, to other granitic rocks, and even to some of the variegated sandstones. The rocking-stones have often given rise to legends and popular myths.
The great eruptions of granite, protogine, and porphyry took place, according to M. Fournet, during the carboniferous period, for porphyritic pebbles are found in the conglomerates of the Coal-measure period. ”The granite of Dartmoor, in Devons.h.i.+re,” says Lyell,[14] ”was formerly supposed to be one of the most ancient of the plutonic rocks, but it 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 Christiana, 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. The granite of Cornwall is probably of the same date, and therefore as modern as the Carboniferous strata, if not newer.”
[14] ”Elements of Geology,” p. 716, 6th edition.
The _ancient granites_ show themselves in France in the Vosges, in Auvergne, at Espinouse in Languedoc, at Plan-de-la-Tour in Provence, in the chain of the Cevennes, at Mont Pilat near Lyons, and in the southern part of the Lyonnaise chain. They rarely impart boldness or grandeur to the landscape, as might be expected from their crystallised texture and hardness; for having been exposed to the effects of atmospheric changes from the earliest times of the earth's consolidation, the rocks have become greatly worn away and rounded in the outlines of their ma.s.ses. It is only when recent dislocations have broken them up that they a.s.sume a picturesque character.
The Christiania granite alluded to above was at one time thought to have belonged to the Silurian period. But, in 1813, Von Buch announced that the strata in question consisted of limestones containing orthocerat.i.tes and trilobites; the shales and limestone being only penetrated by granite-veins, and altered for a considerable distance from the point of contact.[15] The same granite is found to penetrate the ancient gneiss of the country on which the fossiliferous beds rest--unconformably, as the geologists say; that is, they rest on the edges of the gneiss, from which other stratified deposits had been washed away, leaving the gneiss denuded before the sedimentary beds were deposited. ”Between the origin, therefore, of the gneiss and the granite,”[16] says Lyell, ”there intervened, first, the period when the strata of gneiss were denuded; secondly, the period of the deposition of the Silurian deposits. Yet the granite produced after this long interval is often so intimately blended with the ancient gneiss at the point of the 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.” From this example Sir Charles concludes that it is impossible to conjecture whether certain granites, which send veins into gneiss and other metamorphic rocks, have been so injected while the gneiss was scarcely solidified, or at some time during the Secondary or Tertiary period. As it is, no single ma.s.s of granite can be pointed out more ancient than the oldest known fossiliferous deposits; no Lower Cambrian stratum is known to rest immediately on granite; no pebbles of granite are found in the conglomerates of the Lower Cambrian. On the contrary, granite is usually found, as in the case of Dartmoor, in immediate contact with primary formations, with every sign of elevation subsequent to their deposition.
Porphyritic pebbles are found in the Coal-measures; porpyhries continue during the Tria.s.sic period; since, in some parts of Germany, veins of porphyry are found traversing the New Red Sandstone, or _gres bigarre_ of French geologists. Syenites have especially reacted upon the Silurian deposits and other old sedimentary rocks, up to those of the Lower Carboniferous period.