A Manual of Elementary Geology Part 67

[425-A] For a view of Puy de Tartaret and Mont Dor, see Scrope's Volcanos of Central France.

[427-A] Scrope's Central France, p. 60., and plate.

[428-A] Daubeny on Volcanos, p. 14.

[428-B] Edin. Journ. of Sci., No. iv. N. S. p. 276. Figures of some of these remains are given by M. Bertrand de Doue, Ann. De la Soc.

d'Agricult. de Puy, 1828.

[429-A] Mem. de la Soc. Geol. de France, tom. i. p. 175.

[429-B] See Lyell and Murchison, Ann. de Sci. Nat., Oct. 1829.

[430-A] See Scrope's Central France, p. 21.

[430-B] Ibid, p. 7.

[431-A] Boblaye and Virlet, Morea, p. 23.

[432-A] De la Beche, Geol. Proceedings, No. 41. p. 196.

[432-B] "The rock," as English readers of Burn's poems may remember, is a Scotch term for distaff.

[435-A] Murchison, Silurian System, &c. p. 230.

[435-B] Ibid., p. 272.

[435-C] Ibid., p. 325.

[435-D] Chap. XXVII. p. 356.

[435-E] Geol. Trans., 2d series, vol. iv. p. 55.

CHAPTER x.x.xIII.

PLUTONIC ROCKS--GRANITE.

General aspect of granite--Decomposing into spherical ma.s.ses--Rude columnar structure--a.n.a.logy and difference of volcanic and plutonic formations--Minerals in granite, and their arrangement--Graphic and porphyritic granite--Mutual penetration of crystals of quartz and felspar--Occasional minerals--Syenite--Syenitic, talcose, and schorly granites--Eurite--Pa.s.sage of granite into trap--Examples near Christiania and in Aberdeens.h.i.+re--a.n.a.logy in composition of trachyte and granite--Granite veins in Glen Tilt, Cornwall, the Valorsine, and other countries--Different composition of veins from main body of granite--Metalliferous veins in strata near their junction with granite--Apparent isolation of nodules of granite--Quartz veins--Whether plutonic rocks are ever overlying--Their exposure at the surface due to denudation.

The plutonic rocks may be treated of next in order, as they are most nearly allied to the volcanic cla.s.s already considered. I have described, in the first chapter, these plutonic rocks as the unstratified division of the crystalline or hypogene formations, and have stated that they differ from the volcanic rocks, not only by their more crystalline texture, but also by the absence of tuffs and breccias, which are the products of eruptions at the earth's surface, or beneath seas of inconsiderable depth. They differ also by the absence of pores or cellular cavities, to which the expansion of the entangled gases gives rise in ordinary lava. From these and other peculiarities it has been inferred, that the granites have been formed at considerable depths in the earth, and have cooled and crystallized slowly under great pressure, where the contained gases could not expand. The volcanic rocks, on the contrary, although they also have risen up from below, have cooled from a melted state more rapidly upon or near the surface. From this hypothesis of the great depth at which the granites originated, has been derived the name of "Plutonic rocks." The beginner will easily conceive that the influence 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.

By some writers, all the rocks now under consideration have been comprehended under the name of granite, which is, then, understood to embrace a large family of crystalline and compound rocks, usually found underlying all other formations; whereas we have seen that trap very commonly overlies strata of different ages. Granite often preserves a very uniform character throughout a wide range of territory, forming hills of a peculiar rounded form, usually clad with a scanty vegetation. The surface of the rock is for the most part in a crumbling state, and the hills are often surmounted by piles of stones like the remains of a stratified ma.s.s, as in the annexed figure, and sometimes like heaps of boulders, for which they have been mistaken. The exterior of these stones, originally quadrangular, acquires a rounded form by the action of air and water, for the edges and angles waste away more rapidly than the sides. A similar spherical structure has already been described as characteristic of basalt and other volcanic formations, and it must be referred to a.n.a.logous causes, as yet but imperfectly understood.

[Ill.u.s.tration: Fig. 484. Ma.s.s of granite near the Sharp Tor, Cornwall.]

Although it is the general peculiarity of granite to a.s.sume no definite shapes, it is nevertheless occasionally subdivided by fissures, so as to a.s.sume a cuboidal, and even a columnar, structure. Examples of these appearances may be seen near the Land's End, in Cornwall. (See figure.)

[Ill.u.s.tration: Fig. 485. Granite having a cuboidal and rude columnar structure, Land's End, Cornwall.]

The plutonic formations also agree with the volcanic, in having veins or ramifications proceeding from central ma.s.ses into the adjoining rocks, and causing alterations in these last, which will be presently described. They also resemble trap in containing no organic remains; but they differ in being more uniform in texture, whole mountain ma.s.ses of indefinite extent appearing to have originated under conditions precisely similar. They also differ in never being scoriaceous or amygdaloidal, and never forming a porphyry with an uncrystalline base, or alternating with tuffs. Nor do they form conglomerates, although there is sometimes an insensible pa.s.sage from a fine to a coa.r.s.e-grained granite, and occasionally patches of a fine texture are imbedded in a coa.r.s.er variety.

[Ill.u.s.tration: Fig. 486. Gneiss. (See description, p. 464.)]

Felspar, quartz, and mica are usually considered as the minerals essential to granite, the felspar being most abundant in quant.i.ty, and the proportion of quartz exceeding that of mica. These minerals are united in what is termed a confused crystallization; that is to say, there is no regular arrangement of the crystals in granite, as in gneiss (see fig. 486.), except in the variety termed graphic granite, which occurs mostly in granitic veins. This variety is a compound of felspar and quartz, so arranged as to produce an imperfect laminar structure. The crystals of felspar appear to have been first formed, leaving between them the s.p.a.ce now occupied by the darker-coloured quartz. This mineral, when a section is made at right angles to the alternate plates of felspar and quartz, presents broken lines, which have been compared to Hebrew characters.

[2 Ill.u.s.trations: Graphic granite.

Fig. 487. Section parallel to the laminae.

Fig. 488. Section transverse to the laminae.]

As a general rule, quartz, in a compact or amorphous state, forms a vitreous ma.s.s, serving as the base in which felspar and mica have crystallized; for although these minerals are much more fusible than silex, they have often imprinted their shapes upon the quartz. This fact, apparently so paradoxical, has given rise to much ingenious speculation. We should naturally have antic.i.p.ated that, during the cooling of the ma.s.s, the flinty portion would be the first to consolidate; and that the different varieties of felspar, as well as garnets and tourmalines, being more easily liquefied by heat, would be the last. Precisely the reverse has taken place in the pa.s.sage of most granitic aggregates from a fluid to a solid state, crystals of the more fusible minerals being found enveloped in hard, transparent, gla.s.sy quartz, which has often taken very faithful casts of each, so as to preserve even the microscopically minute striations on the surface of prisms of tourmaline. Various explanations of this phenomenon have been proposed by MM. de Beaumont, Fournet, and Durocher. They refer to M. Gaudin's experiments on the fusion of quartz, which show that silex, as it cools, has the property of remaining in a viscous state, whereas alumina never does. This "gelatinous flint" is supposed to retain a considerable degree of plasticity long after the granitic mixture has acquired a low temperature; and M. E. de Beaumont suggests, that electric action may prolong the duration of the viscosity of silex. Occasionally, however, we find the quartz and felspar mutually imprinting their forms on each other, affording evidence of the simultaneous crystallization of both.[439-A]

[Ill.u.s.tration: Fig. 489. Porphyritic granite. Land's End, Cornwall.]

_Porphyritic granite._--This name has been sometimes given to that variety in which large crystals of felspar, sometimes more than 3 inches in length, are scattered through an ordinary base of granite. An example of this texture may be seen in the granite of the Land's End, in Cornwall (fig.

489.). The two larger prismatic crystals in this drawing represent felspar, smaller crystals of which are also seen, similar in form, scattered through the base. In this base also appear black specks of mica, the crystals of which have a more or less perfect hexagonal outline. The remainder of the ma.s.s is quartz, the translucency of which is strongly contrasted to the opaqueness of the white felspar and black mica. But neither the transparency of the quartz, nor the silvery l.u.s.tre of the mica, can be expressed in the engraving.

The uniform mineral character of large ma.s.ses of granite seems to indicate that large quant.i.ties of the component elements were thoroughly mixed up together, and then crystallized under precisely similar conditions. There are, however, many accidental, or "occasional," minerals, as they are termed, which belong to granite. Among these black schorl or tourmaline, actinolite, zircon, garnet, and fluor spar, are not uncommon; but they are too sparingly dispersed to modify the general aspect of the rock. They show, nevertheless, that the ingredients were not everywhere exactly the same; and a still greater variation may be traced in the ever-varying proportions of the felspar, quartz, and mica.

_Syenite._--When hornblende is the subst.i.tute for mica, which is very commonly the case, the rock becomes Syenite: so called from the celebrated ancient quarries of Syene in Egypt. It has all the appearance of ordinary granite, except when mineralogically examined in hand specimens, and is fully ent.i.tled to rank as a geological member of the same plutonic family as granite. Syenite, however, after maintaining the granitic character throughout extensive regions, is not uncommonly found to lose its quartz, and to pa.s.s insensibly into syenitic greenstone, a rock of the trap family.

Werner considered syenite as a binary compound of felspar and hornblende, and regarded quartz as merely one of its occasional minerals.

_Syenitic-granite._--The quadruple compound of quartz, felspar, mica, and hornblende, may be so termed. This rock occurs in Scotland and in Guernsey.

_Talcose granite_, or Protogine of the French, is a mixture of felspar, quartz, and talc. It abounds in the Alps, and in some parts of Cornwall, producing by its decomposition the china clay, more than 12,000 tons of which are annually exported from that country for the potteries.[440-A]

_Schorl rock, and schorly granite._--The former of these is an aggregate of schorl, or tourmaline, and quartz. When felspar and mica are also present, it may be called schorly granite. This kind of granite is comparatively rare.

_Eurite._--A rock in which all the ingredients of granite are blended into a finely granular ma.s.s. Crystals of quartz and mica are sometimes scattered through the base of Eurite.

_Pegmat.i.te._--A name given by French writers to a variety of granite; a granular mixture of quartz and felspar; frequent in granite veins; pa.s.ses into graphic granite.

All these granites pa.s.s into certain kinds of trap, a circ.u.mstance which affords one of many arguments in favour of what is now the prevailing opinion, that the granites are also of igneous origin. The contrast of the most crystalline form of granite, to that of the most common and earthy trap, is undoubtedly great; but each member of the volcanic cla.s.s is capable of becoming porphyritic, and the base of the porphyry may be more and more crystalline, until the ma.s.s pa.s.ses to the kind of granite most nearly allied in mineral composition.

The minerals which const.i.tute alike the granitic and volcanic rocks consist, almost exclusively, of seven elements, namely, silica, alumina, magnesia, lime, soda, potash, and iron; and these may sometimes exist in about the same proportions in a porous lava, a compact trap, or a crystalline granite. It may perhaps be found, on farther examination--for on this subject we have yet much to learn--that the presence of these elements in certain proportions is more favourable than in others to their a.s.suming a crystalline or true granitic structure; but it is also ascertained by experiment, that the same materials may, under different circ.u.mstances, form very different rocks. The same lava, for example, may be gla.s.sy, or scoriaceous, or stony, or porphyritic, according to the more or less rapid rate at which it cools; and some trachytes and syenitic-greenstones may doubtless form granite and syenite, if the crystallization take place slowly.

It has also been suggested that the peculiar nature and structure of granite may be due to its retaining in it that water which is seen to escape from lavas when they cool slowly, and consolidate in the atmosphere.

Boutigny's experiments have shown that melted matter, at a white heat, requires to have its temperature lowered before it can vapourize water; and such discoveries, if they fail to explain the manner in which granites have been formed, serve at least to remind us of the entire distinctness of the conditions under which plutonic and volcanic rocks must be produced.[441-A]

It would be easy to multiply examples and authorities to prove the gradation of the granitic into the trap rocks. On the western side of the fiord of Christiania, in Norway, there is a large district of trap, chiefly greenstone-porphyry, and syenitic-greenstone, resting on fossiliferous strata. To this, on its southern limit, succeeds a region equally extensive of syenite, the pa.s.sage from the volcanic to the plutonic rock being so gradual that it is impossible to draw a line of demarcation between them.

"The ordinary granite of Aberdeens.h.i.+re," says Dr. MacCulloch, "is the usual ternary compound of quartz, felspar, and mica; but sometimes hornblende is subst.i.tuted for the mica. But in many places a variety occurs which is composed simply of felspar and hornblende; and in examining more minutely this duplicate compound, it is observed in some places to a.s.sume a fine grain, and at length to become undistinguishable from the greenstones of the trap family. It also pa.s.ses in the same uninterrupted manner into a basalt, and at length into a soft claystone, with a schistose tendency on exposure, in no respect differing from those of the trap islands of the western coast."[441-B] The same author mentions, that in Shetland, a granite composed of hornblende, mica, felspar, and quartz, graduates in an equally perfect manner into basalt.[441-C]

In Hungary there are varieties of trachyte, which, geologically speaking, are of modern origin, in which crystals, not only of mica, but of quartz, are common, together with felspar and hornblende. It is easy to conceive how such volcanic ma.s.ses may, at a certain depth from the surface, pa.s.s downwards into granite.