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Varieties in Composition.

A. Hornblendic Granite.—An oligoclase granite,

in which amphibole is a constituent.

a. Titanitic Granite; Furbogh Granite.

b. Chloritic or Ripidolitic Granite.

c. Talcose Granite.

d. Epidotic Granite.

With amphibole titanite sometimes occurs, and this variety has been called Furbogh Granite, from a district in the county Galway, Ireland. In places ripidolite, talc, or epidote may be present with, or in place of, the amphibole, making varieties. The last-named mineral is supposed to be due to secondary formation.

Note.Homblendic granite by some geologista has been called " Syenitic granite." This name, however, seems not only unscientific, but also inapplicable, as the rock is not an aggregate of granite and syenyte, but of granite and amphibole [hornblende], and it is to the latter mineral that the name is intended to direct attention. Chloritic granite and Talcose granite have erroneously been called "Protogene granite," also a most incorrect name, as protogene (quartz-\-felspar-\-talc) is a constituent of neither. Furthermore, in general chloritic granite can have no relation to protogene, not containing any talc.

B. Graphitic Granite.—With graphite in addition

to mica. 0. Hematitic Ob Ferruginous Oligoclase Geanite.

—With micaceous iron-ore in addition to the

mica. D. Pyritous Oligoclase Granite. — Containing

pyrite or marcasite, or even chalcopyrite, in

addition to the other constituents.

Structural Varieties.

E. Porphyritic Oligoclase Granite; Galway Gra

nite.—Having one of the felspars developed in crystals, larger aud more conspicuous than those of the other constituents. Generally, it is the orthoclase that is porphyritically developed; however, sometimes it is the oligoclase. It is the typical granite of county Galway, Ireland.

F. Gneissoid Granite; Foliated Granite; Gneissic

Granite (Cotta).—An oligoclase granite, in which there is more or less distinct foliation. Sometimes one of the felspars is porphyritically developed, when in general the mica does not curl round the large crystals of felspar, but the latter lie promiscuously about.

a. Nodular Granite.—Containing round or semi

angular pieces or nodules of a dissimilar granite or of a granitic rock. The inliers often are a hornblendic granite or a rock, like minette, or like a gneissyte, but they vary considerably.

b. Lenticular Granite.—A more or less gneissoid

granite, containing lenticular patches or lentils of granite or a rock, like a gneissyte or minette. The lentils sometimes occur closely together, at other times widely apart; but in all cases the foliation of the rock curls round them.

G. Intrusive Oligoclase Granite; Omey Granite.

—Very similar to the non-intrusive oligoclase granite; but such minerals as amphibole, titanite, ripidolite, &c, only occur locally, as accessories. It has been called " Omey granite," from an island on the west coast of Ireland. Besides the varieties mentioned above, others might be named, but those given seem to be the most important.

Veins of segregation occur in the oligoclase granite. Their composition appears very similar to the veins of segregation (granityte) that occur in the highly siliceous granite, being of a more or less siliceous nature. There are also irregular veins, nests, and lenticular patches, sometimes of considerable size, of a rock answering Cotta's description for the rock called Pegmatyte. The Pegmatyte found associated with the non-intrusive oligoclase granite is, at least in some instances, due to the metamorphism of irregular felspathic veins and lenticular patches that existed in the metamorphosed sedimentary and irruptive rocks prior to their being changed into granite.

Basic igneous rocks may be metamorphosed into rocks very similar to the Basic or oligoclase granite, or its varieties; and the Acid or highly siliceous igneous rocks may change into a rock like the finer varieties of the Intrusive or highly siliceous granite (A); while the basic felstones {Euryte and such rocks) may be altered into more or less hornblendic or chloritic, or even talcose granite.

S. Granitic Felstone.—A fine, more or less evengrained aggregate of quartz, felspar, and mica. In some varieties the felspar seems always to be orthoclase; the mica may be black or white, often both. Part of the mica may be replaced by amphibole or ripidolite. They weather neither like a true granite nor yet like a felstone, having a characteristic weathering of their own, partaking of the nature of both.

Granitic felstones probably are metamorphosed felstones. They are often banded, the lines sometimes being parallel, or nearly so, to the walls of the dykes, but at other times they are oblique. The bands are due to thin zones of different colour or texture, perhaps also of composition. In these bands, or between them, there is on rare occasions a structure like foliation.

All the metamorphosed igneous rocks are more or less granitoid, in accordance with the intensity of the metamorphism that has acted on them and the associated sedimentary rocks.

C. Protogene [Gr. protogenes, first formed; the rock, when first discovered, being supposed to be the oldest].—A granitoid rock; a crystalline aggregate of quartz, felspar, and talc.

Protogene, although not a true granite, seems allied to those of the Oligoclase type (B). The name is said to have been given originally to a metamorphic rock, but its original signification appears to have been lost, for Gotta thus describes it:—" A granite which contains talc or chlorite, or decomposed mica, instead of the usual mica." This is an erroneous definition, for if it contains either mica or ripidolite instead of talc, it cannot be Protogene.* However, Gotta would appear not to be the only geologist or petrologist who has found this rock a stumbling-block; as Haughton, after examining numerous specimens of the so-called Protogene from the Alps, states that most of them "contained not talc, but dull mica or ripidolite, or some kindred mineral." If a rock contains mica in addition to talc, or ripidolite in place of talc, it cannot be protogene, but is, in the one case, Talcose Granite, and in the other, Ghloritic or Ripidolitic Granite.

* Some basic felstones, when metamorphosed, appear to be true protogene (quartz+felspar+talc).

The formation of granite is not fully understood; yet I cannot but believe that while some are undoubtedly intrusive, others have been formed by extreme metamorphic action. However, between the two extremes there is a rock (previously mentioned and described as Intrusive oligoclase granite) that partakes of the nature of both, having constituents very similar to the ordinary oligoclase granite, while it is intrusive, like the highly siliceous granite. This at first may appear an anomaly; nevertheless it seems capable of a simple explanation.

All authorities infer that granites were formed at a considerable depth beneath the earth's surface, under pressure, by extreme heat. They cooled and consolidated, more or less gradually, long previous to coming near, or being exposed at the surface of the earth.

Eocks, under the above conditions, would melt more or less rapidly in accordance with the intensity of the heat to which they were subjected, the amount of pressure, and constituents of which they are composed. Moreover, the more acid portions would remain longer fluid, during refrigeration, than the basic (see page 28).

If, while in a state of fusion, the heating action were to cease or begin to subside (either of which must at some time have happened), the rocks or strata affected would be in the following conditions: Part would be entirely fused or liquefied; part would be semifused; while the rocks adjacent to the latter would be only more or less altered. Of the latter class, the least altered rock would be represented by rocks belonging to the Schist series; while the more altered rocks, according to the intensity of the metamorphic action, would form the Gneiss

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