Part 67 (2/2)

[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.

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