Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/247

Rh CRYSTALLINE ROCKS.] arrangement. Besides, a rock which, like obsidian, may appear quite simple at one place may, at a short distance, show the presence of other minerals entitling it to a place among the compound rocks. _ (a.) Jllassiue IEoclcs.—This important division consists almost entirely of rocks which have resulted from igneous fusion. Considered from a chemical point of view, these rocks may be described as mixtures in different propor- tions of silicates of alumina, magnesia, lime, potash, and soda, usually with magnetic iron and phosphate of lime, mid, in a great group of rocks, with an excess of silicic acid, existing as free quartz. Taking this last feature as a basis of arrangement, some petrographers have proposed to divide the igneous rocks into an acid group, including such rocks as granite, quartz-porphyry, and quartz-trachyte, where the percentage of silica ranges from 60 to 75, and a basic group, typiﬁed by such rocks as leiicite—lava and basalt, where the proportion of silica is only about 50 per cent. In the vast majority of igneous rocks the chief silicate is a felspar,—the number of rocks where the felspar is repre- sented by another silicate (as leucite or neplieline) being comparatively few and unimportant. As the felspars group themselves into two great series, the monoclinic or ortho- class, and the triclinic or plagioclase, the former with, on the whole, a preponderance of silica, and as the minerals occur under tolerably distinct and deﬁnite conditions, it has been proposed to divide the felspar-bearing massive rocks into two series,—( 1) the orthoclase rocks, having orthoclase a.s their chief silicate, and often with free silica in excess, and ( 2) the plagioclase rocks, where the chief silicate is some species of triclinic felspar. The former series corresponds generally to the acid group above mentioned, while the plagioclase rocks are on the whole decidedly basic. It has been objected to this arrangement that the so-called plagioclase felspars are in reality very distinct minerals, with proportions of silica, ranging from 43 to 69 per cent. ; soda from 0 to 12 ; and lime from O to 20. But in the state of minute subdivision in which the minerals occur in many igneous rocks, it is often scarcely possible to determine the species of felspar. Without attempting here any formal classiﬁcation. accord- ing either to relative proportion of silica or to the distin- guishing felspar, it may be suflicient to arrange the following description of the massive rocks in a continuous series, with the most typical acid or orthoclase rocks at the beginning, and the basic felsparless rocks at the end. Granite is a thoroughly crystalline-graniilar admixture of felspar, mica, and quartz. The felspar is chiefly orthoclase, but striated triclinic felspars (as oligoclase and albite) may often be observed in smaller quantity. The mica in most granites seems to be the potash or muscovite variety, usually of a white silvery aspect; sometimes it is dark brown or black, and belongs to biotite (mag- nesian mica) or lcpidoinelane. Dr Heddle ﬁnds the common mica of the granites in the Scottish Highlands to be a new variety, which he has called liauglitoiiite. The quartz may be observed to form a kind of paste or magma wrapping round the other ingredients (see ﬁg. 1). It is only in cavities of the granite that the component minerals occur as independent well-formed crystals, and there too the accessory minerals are chiefly found, such as beryl, topaz, tour- nia.liiie, &c. Microscopic examination of granite by Sorby and other later observers has shown that the quartz is full of cavities containing liquid, sometimes in such numbers as to a.mount to a thousand millions in a cubic inch. The liquid in these cavities appears usually to be water containing chlorides of soda and potash, with sulphates of potash, soda, and lime. _Granites vary in texture from extremely coarse crystalline masses, with crystals an inch or more in length, to ﬁne granular rocks which pass nito elvanite or felsite. They are sometimes porpliyritic, with large scattered orthoclase crystals. The variety known as graphic granite is distinguished by the way in which the quartz is crystal- liz_ed through the felspar in imperfect or liemihedral shells arranged with their longer axes generally parallel, so as to produce on cross fracture the appearance of Hebrew characters. The mean of eleven analyses of granites made by Dr Haugliton GEOLOGY 233 gave the following average composition :—Silica, 72 '07; alumina, 14'81; peroxidefof iron, 2'22; potash, 5'11; soda, 2'79; liiue, 1'63; magnesia, 0'33; loss by ignition, 1 '09; total, 100'05,—with a mean speciﬁc gravity of 2'66. Granite occurs (1) as an eruptive rock, forming huge bosses, which rise through other formations both stratiﬁed and unstratiﬁed, and sending out veins into the surrounding and overlying rocks, which usually show evidence of much alteration as they approach the granite ; (2) connected with true volcanic rocks, and forming pro- bably the lower portions of masses which ﬂowed out at the surface as lavas; and (3) in the heart of mountain chains and elsewhere, so intimately connected with metamorphic rocks as to suggest that it is itself a ﬁnal stage of the metamorphism of rocks. Granite is thus a decidedly plutonie rock; that is, it has consolidated at some depth beneath the surface, and in this respect differs from the superﬁcial volcanic rocks, such as lavas, which have ﬂowed out above ground from volcanic oriﬁces. Qzoartz-porphyry, Fclsitc-porphyry, Fclstone, Fclsz'te.—These names are given to different conditions under which orthoclase and quartz unite to form a massive eruptive rock. Vhen the quartz forms well-niarked grains, blebs, or crystals, the rock is a quartz-porphyry; when the felspar and quartz are so intimately mixed as to appear to the eye as a homogeneous matrix, the rock is called by one of the other epithets. The base of the rock, whether in the form of quartz-porphyry or felstone, is usually exceedingly compact or even ﬂinty in texture. Under the microscope it shows the microfelsitie character, the true nature and origin of which is still the subject of much discussion among petrographers. When the quartz occurs in forms visible to the naked eye it usually apripars as round or irregular grains, varying in size from mere spec s up to pieces as large as a pea or larger. Less frequently it occurs deﬁnitely crystallized, and some- times in perfect doubly terminated pyramids. Besides the othoclase of the diffused matrix, this mineral occurs in crystalline fragments and crystals, which sometimes reach to the lengtli of an inch or more, so as to give a markedly porphyritic character to the rock. 'l‘riclinic felspars usually occur, though perhaps not so commonly as in granite. Mica and hornblende are among the most frequent of the minerals which accompany the two essential constituents, while apatite, magnetite, and pyrite are not infrequent accessories. The colours of qiiartz-porphyry and felstone depend chieﬂy upon those of the felspar,—ﬂesli-red, reddisli-brown, purple, yellow, bluish or slate-grey, and even white, being in different places characteristic. The presence of much mica or hornblende gives dark grey, brown, or greenish tints. It will be observed in this, as in other rocks containing much felspar, that the colour, besides depending on the hue of that mineral, is greatly regulated by the nature and stage of decomposition. A rock weathering externally with a pale yellow or white crust may be found to be quite dark in the central un- decayed ortion. The esli-red qiiartz-porphyry of Dobritz, near Meissen, in Saxony, was found by Rentzseh to have the following chemical composition :—Silica, 76'92; alumina, 12'89; potash, 4'27; soda, 0'68; lime, 0'68; magnesia, 0'98; oxide of iron, 1'15; water, 1'97 ; total, 99 '54,—speciﬁc gravity, 2'49. Besides the differences of colour already referred to, minor varieties in composition are produced by the relative abundance and size of the felspar crystals, and by the presence of mica (micaccous quart:'- pm-phyry, micaceous felstone, or felsite), hornblende (hornblcndic quartz-porphyry or fclstone), or other accessory ingredient. The variety called minette consists of a felsite base with crystals of orthoclase and dark mica, and may be regarded as a micaceoiis fel- stone, bearing the same relation to the acid felspar-rocks (felstones) that mica-porphyrite does to the more basic forms (porphyrites). Vhen the base is very compact, and the felspar-crystals well deﬁned and of a different colour from the base, the rock sometimes takes a good polish, and may be used with effect as an ornamental stone. In ordinary language such a stone is classed with the “ marbles," under the name of “ porphyry.” Closely related to the qiiartz-porphyries and felstones, of which, indeed, it can be regarded only as a variety, comes the rock known as clean or el'va7L-itc. This is a Cornish term for a crystal- line-granular mixture of quartz and orthoclase, forming veins which proceed from granite, or occur only in its neighbourhood and are evidently associated with it. It forms an immediate stage between granite and qiiartz-porphyry. Quartz-porphyry and the other varieties included under this species occur (1) with plutonie rocks, as eruptive bosses or veins, often associated with granite, from which, indeed, the clvanite, as just stated, may be seen to proceed directly; of frequent occurrence also as veins and irregularly intruded masses among highly con- voluted rocks, especially when these have been more _or less meta- morphosed ; (2) in the chimneys of old volcanic oriﬁces, forming there the “ neck" or plug by which the vent is ﬁlled up; and _(3) as truly volcanic rocks which have been erupted at_ the surface in the form of ﬂows of lava, either (a) submarine, as in the felstones of . — 3o