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series and the Gneissoid granite. The second class, or the semifluid rocks, would form the Oligoclase granite, or the Granite due to extreme metamorphisrn; while the first class, or the completely fluid mass, would form the different intrusive or irruptive rocks.

Of the completely fluid mass, part ought to find its way to, or close to, the earth's surface, and then cool under little or no pressure, forming the Plutonic and Volcanic rocks; a portion would consolidate between the source, the caldron, if it may be so called, and the surface of the earth, under more or less pressure, and form Elvanyte or Quartz-Porphyry, the granitic rocks next to be enumerated and described; while the residue would consolidate in or near the caldron.

The consolidation of the residue, that remained in or near the caldron or source, could scarcely have been simultaneous, as it is probable the outer or upper portion would consolidate sooner than the inner or lower part. Moreover the acid portion would consolidate slower than the basic part; consequently there ought to be two distinct rocks forming,—a more basic rock above, and a more acid or siliceous rock below. As the upper or more basic rock cooled and consolidated, it ought to shrink more or less, and thereby form cracks, fissures, and other vacancies, into which the underlying still fluid or semifluid matter would be squeezed; therefore at any time prior to their final consolidation, part of the oligoclase granite or part of the highly siliceous granite might be forced up into and occupy all vacancies in the supercrescent rocks, and thereby form intrusive granite of one or both types (Intrusive oligoclase granite and Intrusive highly siliceous granite);—subsequently, when the overlying rocks were denuded away, tracts, veins, &c, of these granites would be exposed.

The above suggestions, if correct, give a solution for the formation of the three different kinds of granite,—Highly siliceous granite, Intrusive oligoclase granite, and the Non-intrusive oligoclase granite. The Non-intrusive oligoclase granite would be represented by the semifluid portion that first consolidated; the Intrusive oligoclase granite by the more basic portion of the fluid mass that was forced out of its normal position; while the Highly siliceous granite would be the part that took longest to cool and consolidate. That granite must cool and consolidate under laws somewhat similar to the above appears highly probable, because when two or more granites come from one source, the last formed is more siliceous than the rock through which it breaks up, while the veins of segregation in the former are more siliceous than the parent rock in which they appear.

D. Elvanyte (Jukes); Quartz-Porphyry; Quartziferous Porphyry; Granitic Porphyry (Cotta), [Celtic, el or oil, rock, and van or ban, white, so called from the white crust of the rock when weathered]. —A granitic rock, having a granular or compact felsitic or felspathic matrix, inclosing globules, blebs, and crystals of quartz, crystals of one or more felspars, with more or less mica or amphibole, ripidolite, and the like. Pyrite and marcasite are often constituents. In many of the siliceous varieties, a considerable portion of the quartz crystallized out prior to the other minerals. The matrix may be siliceous {felsitic) or basic [felspathic), and of a greenish, greyish, brownish, blackish, bluish, or even yellowish colour: all of the yellowish, as also some of the blues, seem due to weathering.

Note. — Elvanyte naturally includes not only the quartzporphyries, but also the syenytes (quartz+felspar-(-amphibole) of some British geologists.

The free silica or quartz in the Elvanyte is for the most part in irregular blebs or globules, from minute specks to the size of a pea, or even larger. The quartz internally has a glassy, often radiated structure, and externally on some, the crystal faces can be recognized, while others appear to be regular coated minute balls. The quartz is scattered through the mass, sometimes sparingly, but at other times so thickly as to give the rock a pisolitic aspect. Elvanytes, except some of the very granitic varieties, weather with the even flowing surface so characteristic of the Felstones, and not with the rough rugged aspect of granite. They often near the edge of large masses, at the walls of dykes, and in small branch dykes or veins merge into a rock in appearance identical with felstone: this compact rock is called by Cotta the Base or Mother-rock. The Mother-rock at dyke walls is rarely more-than a few inches thick. Elvanytes being hypogene rocks, are never accompanied by tuff.

Note.—A true elvanyte is more crystalline at the centre of a dyke or mass than at the margins, while an elvanoid rock due to metamorphism is more crystalline at the margins than in the interior, the change in the latter taking place from the outside inwards.

Varieties in Composition.

A. Felsitic and Felspathic Elvanyte.—Respec

tively a siliceous or a basic elvanyte, in which the quartz, mica, and other minerals are more or less developed.

a. Orthoclasic Elvanyte. — When the felspar is

principally orthoclase.

b. Oligoclasic Elvanyte.—When the felspar crys

tals are principally oligoclase.

c. Rappakivi Elvanyte.—An elvanyte in which

many of the orthoclase crystals (often pink or flesh-colour) are inclosed in an envelope of oligoclase (often light green). In the Finland variety the orthoclase is generally in rounded masses inclosed in orbicular envelopes of oligoclase.

B. Quartzitic or Siliceous Elvanyte.—Having a

large percentage of siliceous matter. G. Micaceous Elvanyte.—Having a large percentage of mica.

Note.—These rocks would be called " mica traps" by some authors. (See Mica Trap, page 48.)

D. Pyeitous Elvanyte. — Having a large percent

age of either pyrite, marcasite, or chalcopyrite.

E. Hornblendic Elvanyte.—When typical, a gra

nitoid rock, an aggregate of quartz, felspar, and amphibole, usually with some pyrite or marcasite; it however graduates into more or less compact varieties.

Varieties in composition are, — a. Micaceous; b. Ohloritic; c. Felspathic; d. Pyritous; respectively, as different minerals give a marked character; some structural varieties are gneissoid.

Note.—Some authors would call homblendic elvanyte and its varieties "syenyte." Part of a mass may be a true syenyte (amp/jiioJe+orthoclase+quartz), but in general the rock contains two felspars.

F. Chloritic Ob Ripidolitic Elvanyte.—Having ripidolite replacing most or all of the mica or amphibole.

Structural Varieties.

0. Granular Elvanyte.—With a matrix somewhat

resembling granite; but distinct crystals of one or more felspars, with globules, blebs, and crystals of quartz, flakes of mica, or crystals of amphibole, are separately and prominently developed. In one variety ripidolite replaces the mica or amphibole. H. Compact Elvanyte, or Pontellaryte.—The matrix is more or less compact throughout, forming a paste, often dark-coloured. It contains separate crystals, globules, or flakes of one or more felspars, amphibole, quartz, mica, or ripidolite.

1. Gneissoid Elvanyte.—Elvanyte with a struc

ture that seems to be foliation, more or less well-defined. Some varieties are micaceous, others homblendic; while some may be chloritic. Some gneissoid elvanytes may be metamorphosed igneous rock. J. Granitoid Elvanyte.—For the most part an aggregate of quartz and felspar, with mica, or ripidolite, or amphibole. Part of the matrix appears as a sort of paste, while some of the

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