Varieties in Composition.

A. FELSITIC and FELSPATHIC ELVANYTE.Respectively a siliceous or a basic elvanyte, in which the quartz, mica, and other minerals are more or less developed.

-When the felspar is

a. Orthoclasic Elvanyte. principally orthoclase.

b. Oligoclasic Elvanyte.-When the felspar crystals 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.

C. 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. PYRITOUS ELVANYTE. - Having a large percentage of either pyrite, marcasite, or chalcopyrite. E. HORNBLENDIC ELVANYTE.-When typical, a granitoid 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. Chloritic; c. Felspathic; d. Pyritous; respec

tively, as different minerals give a marked character; some structural varieties are gneissoid.

NOTE.-Some authors would call hornblendic elvanyte and its varieties "syenyte." Part of a mass may be a true syenyte (amphibole+orthoclase+quartz), but in general the rock contains two felspars.

F. CHLORITIC OR RIPIDOLITIC ELVANYTE.-Having ripidolite replacing most or all of the mica or amphibole.

Structural Varieties.

G. 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.

I. GNEISSOID ELVANYTE.-Elvanyte with a structure that seems to be foliation, more or less well-defined. Some varieties are micaceous, others hornblendic; 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

quartz occurs in globules, blebs, or irregular crystals, having crystallized out prior to the other minerals.

Most granitoid elvanytes weather more like a felstone than like a granite; some, however, do not. At the walls of dykes or masses they often merge into a more or less compact rock-" The Mother-rock, or Base," of Cotta. Granitoid elvan

yte seems to be the passage-rock between granite and elvanyte.

K. PYROMERIDE, or BALL ELVANYTE, Ball Porphyry.— "This rock, in addition to the usual quartz crystals, contains balls of felsite (either small and numerous, or large and isolated. The small balls are frequently marked internally with radial streaks. The interiors of the larger ones are usually split after the manner of septaria, or they contain a geodic cavity). The clefts or cavities in the balls are wholly or partly filled with hornstone, chalcedony, agate, quartz, amethyst, calcite, fluorite, &c. &c."-Cotta.

Class II.-PLUTONIC ROCKS.

THIS name (from Pluto, the god of the infernal regions) has been given to the rocks that at one time were buried beneath the earth or sea, as distinct from the Volcanic, or those that are thrown up and consolidated on the present surface of the earth. The Plutonic rocks were irrupted or intruded, and consolidated at or near a former surface of the earth, or at the bottom of a sea or lake, under, comparatively speaking, little pressure; but subsequently they were covered by successive

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deposits of "Derivate rocks." As they are not "hypogene rocks," they have their tuffs, agglomerates, and other mechanically-derived associates, with which they may be interstratified, as also with any class or variety of derivate or sedimentary rock.

In Nature there are no hard divisional lines between different kinds of rocks, more especially ingenite rocks; therefore the Plutonic rocks, on one hand merge into Granitic rocks, and on the other into Volcanic rocks. A boundary between the Granitic and Plutonic rocks can be defined, as the first are hypogene, while the latter are not; but the difference between the Plutonic and the Volcanic rocks is much more vague, we only knowing that the latter are intrusive rocks, erupted and consolidated at the present surface of the earth, while the former were formed in ages or geological periods long past, and subsequently were covered up by accumulations of strata that are now in part removed by the force of denudation, thereby bringing them again to the surface.

E. Felstone, Felsyte, or Felsite [Ger. felspath, rock spar]. A compact or granular, or splintery quartzitic felspathic rock-often porphyritic or quartzose; sometimes micaceous, or hornblendic, or ripidolitic; pyrite or marcasite, and sometimes chalcopyrite, may occur as constituents.

The Felstones, or Felsytes, include all the highly siliceous plutonic rocks, and are for the most part a felsitic compound. Various minerals, however, are locally ingredients, forming numerous subgroups and varieties. Felstones, or Felsytes, usually weather in flowing surfaces with even outlines. They are, however, affected by different structures. Some are homogeneous; in some there is a platy arrange

ment, rudely parallel to the walls of the dykes; while in others, sometimes, however, associated with the platy structure, is a spheroidal or concretionary arrangement. The platy structure may be due to the rock cooling in thin sections, or nearly vertical layers; or perhaps, as suggested by Scrope, to the differential motion of the vesicular and non-vesicular

portions of the cooling rock. The spheroidal or concretionary arrangement seems always to occur near the end of a dyke, and may be due to the cooling when the rock was in its last stage of motion Phillips, in his description of "Vesuvius," figures and describes a similar structure in some of the dykes of lava belonging to that volcano.

In other felstones, or felsytes, there are lines cutting obliquely across; while in some is a rudely columnar structure, except that the joint-planes are as often oblique as perpendicular to the walls of the dykes. Associated, in places, with the last-named is a very irregular platy development between the joint-lines usually perpendicular to them, but often more or less oblique.

The minerals most commonly appearing in Felstone or Felsyte, are-crystals of orthoclase, oligoclase, pyrite, marcasite, and amphibole, blebs or crystals of quartz, and flakes of black and white mica, and ripidolite. The quartz globules vary from minute particles to the size of a pea.

A. PETROSILEX (Brongniart) [Lat. petra, stone, and silex, flint, on account of the hard matrix and flint-like appearance of the rock], Felsyte, or Felsite. The type rock of the Felstones. A compact or granular, hard, siliceous felsitic rock; a granular mixture of felspar and quartz, in which, although fine-grained, the minera1