Page images
PDF

the vacancies or the shrinkage fissures caused by the contraction consequent on the consolidation of the cooled portion or crust of the mass. The granite in such veins would necessarily be more siliceous than the granite mass through which they traverse, as basic rocks, although they melt at a lower temperature than the more siliceous, cool sooner and at a higher temperature, losing their heat much more rapidly. Consequently, it appears evident that the portions of a mass of granite which remain longest fluid ought to be more siliceous than the parts that cool first. The dyke-like veins of segregation are undoubtedly more siliceous than the rock which they traverse. Moreover, they are more compact, and finely crystalline. These latter peculiarities may, however, be due to the final rapid cooling when in the veins.

Note.—M'Farlane thus writes on the cooling of siliceous rocks :—" The scoria of iron-furnaces are usually very acid, containing as much as sixty per cent, of silex. They generally fuse at a temperature of 1,450 C. As they flow out of the breast of the furnace, they may be observed to do so very leisurely, to be sluggish and viscid, but nevertheless to continue fluid a long time ; and even, in some cases, to flow out of the building in which they have been produced, before solidifying. On the other hand, slags from certain copper-furnaces, or from those used for puddling iron, are more or less basic, containing from thirty to forty-five per cent, of silica. As they flow out they are seen to be very fluid, and to run quickly, but they solidify much more rapidly than iron slag. Yet these basic slags fuse at 1,300° C, or about 150° less than the more acid slags. Those who have been accustomed to observe metallurgical processes will not find it difficult to conceive how a very siliceous slag might continue fluid at a temperature at which a more basic one might become solid."

They often are more or less felsytoid; that is, have the aspect of a felstone or felsyte; but they always contain mica as a constituent, although often only visible under the lens or microscope. On account of their similitude in aspect to felsyte, also on account of their usual purplish reddish colour, Delesse and others erroneously call them " eurites," although they are quite dissimilar to the original eurytes of Daubuisson. Rose, on the other hand, describes a granite apparently identical with them, and calls it granityte, a name to which there seems to be no objection.

D. Granityte; Geanitite (Rose).—A compact finely crystalline rock, usually purplish or reddish purple in colour. An aggregate of felspar (orthoclase), quartz, and mica. Black and white mica seem always to be present, but often in such minute flakes as to be only microscopically visible. Pyrite, and sometimes marcasite, are locally present.

Granityte * occurs as veins in granite from mere lines to two or more yards in thickness. It seems to be allied to elvanyte or quartz-porphyry, hereafter described, as sometimes part of the quartz in the granityte seems to have crystallized out prior to the other constituents. Moreover, granityte nearly always weathers evenly like those rocks, and not with the rugged uneven surface, so characteristic of typical granite.

Allied to the intrusive granite are rocks that to the naked eye seem not mineralogically granite, yet petrologically they seem to be part of the intrusive granite, as they always occur associated

* If granityte, as suggested, fills cracks and fissures in the granite, it and elvanyte must necessarily have cooled under somewhat similar conditions. Jukes has described some of the granitytes of the counties of Wicklow and Dublin, Ireland, as elvanytes.

with it, into which they seem to merge, and it into them. They are as follows :—

E. Felsitic Granite (King); FelsiteRock (Cotta).

—" A rock of compact texture, about the hardness of felspar, with dull or smooth conchoidal or fissile fracture; colour yellowish, reddish, grey, greyish, or bluish, weathering white."—Cotta.

To the naked eye this rock does not appear to contain either mica or quartz; however, with a lens, both of these substances can be detected, but usually they appear to be sparingly and partially developed.

F. Greissen; Quartzitic Granite.—Apparently a

crystalline granular aggregate of quartz and mica.

Greissen occurs associated with the intrusive granite, and merges into it. Sometimes, indeed, there is felspar developed, but so sparingly and at such wide intervals, that it seems to be more an accessory than an essential of the rock. In some places even the mica seems to be absent, or in such minute particles as only to be detected by a microscopical examination.

Structural Varieties.

G. Porphyritic Intrusive, Or Highly Siliceous

Granite. — In which crystals of felspar are largely and conspicuously developed. H. Pegmatyte; Pegmatite [Gr. pegma, a hardened mass]. — The second variety of the veins of segregation; very coarsely and irregularly crystallized, the conspicuous constituents, according to Cotta, being "orthoclase, quartz, and silvery-white mica." Nevertheless, it usually also contains more or less dark-coloured mica, also pyrite or marcasite, and locally tourmaline.

Pegmatyte sometimes appears in large, wellmarked veins, but more frequently it occurs in irregular veins, patches, and lentils, dying out in every direction. The dark-coloured mica, although the other constituents may be very coarsely crystalline, usually occurs in minute scales, but not always.

a. Graphic granite [Gr. grapho, I write].—A

variety of Pegmatyte, in which the constituents are so arranged as to produce figures resembling written characters.

b. Blumen granite.—" The felspar assumes a form

resembling flowering-stalks."—Cotta.

c. Plumose granite. — The mica assumes a plu

mose form, like Prince of Wales's feathers. —Jukes.

The granites belonging to this type [intrusive granite] in general are not foliated; nevertheless Forbes mentions a " foliated granite" at Edisvand, Norway; of whicli he says: "No doubt could be entertained of its true eruptive origin." And my colleague, Mr. R. Gr. Symes, describes veins of a foliated granite which he observed N.B. of Castlebar, in the co. Mayo, Ireland. In south-west Mayo, associated with very slightly metamorphosed rock, are wide dykes of foliated rock, some being typical gneiss (leaves of quartz, felspar, and mica), others hornblendic gneiss, while some apparently are formed of leaves of quartz, orthoclase, and amphibole.

Not uncommon in the intrusive granite is a structure having an aspect somewhat like foliation; and yet it cannot be so classed, as the constituents of the rock have no tendency to occur in leaves. Perhaps it may possibly be some kind of close, irregular, rudely parallel-jointed structure, induced during the cooling of the rock, somewhat similar to the structure called by Scrope "shrinkage fissures." This structure of the intrusive granite is scarcely perceptible in a hard specimen, but in rock masses it is conspicuous.

Granites more or less due to metamorphism, having (for the most part) been altered into granite while in their present positions as regards the associated rocks (see note, page 7).

These rocks appear to have been formed from previously existing sedimentary and igneous rocks. Those due to the metamorphism of the sedimentary and of the basic-igneous rocks, are often very similar in aspect and composition, while some of the acid-igneous rocks have changed into granite somewhat similar to the Intrusive Granite (A).

B. Granite for the most part non-intrusive; Basic or Oligoclasc Granite; Scandinavian Granite [in part metamorphic].—Usually a crystalline aggregate of quartz, black and white mica, orthoclase, and oligoclase. When typical, it is a quinary granite; however it is very variable in its composition, which in part seems due to its metamorphic origin. Many minerals occur locally as adjuncts; such as amphibole, titanite, ripidolite, pyrite, marcasite, &c, forming different varieties.

Note.—Haughton is of opinion that the normal constituents of this class of metamorphic granite in Ireland, Scotland, Norway, Sweden, and Finland, are Quartz, Orthoclase, Oligoclase, Margarodite, and Lepidomelane.

« PreviousContinue »