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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 elvanyte seems to be the passage-rock between granite and elvanyte.
K. PrROMEKiDe, 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."—Gotta.
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
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, oix 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 stome there is a platy arrangement, 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 niinera1constituents do not merge into one another, but are recognized by the eye, with or without the help of a magnifying-glass. Petrosilex is greyish, greenish, purplish, or bluish in colour; when yellowish or reddish, the colour seems due to weathering. Some are splintery, but usually they have from an uneven to a conchoidal fracture, and are translucent or subtranslucent. Some have a porcelanic, others a saccharoid aspect.
a. Felstone glass; Pitchstone felsyte.* — " The
principal mass is homogeneous; of vitreous pitch-iike appearance; conchoidal fracture; resinous lustre; translucent at the edges, and very variously coloured."—Cotta.
b. Ribaned petrosilex; Striped, or ribaned felsyte,
with bands of different colour, texture, and sometimes composition alternating. — In some varieties the rock splits into plates along the riban. B. Quaetzitic or Quaetzose Felstone, or Felsyte; Quaetzifeeous Felstone.—A. compact felsitic mass, inclosing crystals or crystalline grains of quartz. Of quartzitic felstone there are different varieties; some due to composition, others to structure. The compact matrix consists principally of felspar, and of it Cotta says: "Probably orthoclase; its proportion of silica is, however, too high even for orthoclase, and it is therefore probable that some quartz is intimately combined with the felspar." The colour of the matrix is greyish, greenish, purplish, or bluish, with the surface
* Pitchstone felsyte must not be confounded with the volcanic rock.
weathering a dirty white; when the rock is yellowish or reddish, the colour seems due to secondary action. The texture of the matrix varies considerably: sometimes it is compact like horn, with a smooth conchoidal fracture; at other times it ia granular, or saccharoid, or splintery. Usually it is compact, with a fracture from semiconchoidal to uneven, but sometimes it is vesicular or amygdaloidal.
Varieties in Composition.
a. Oligoclasic quartzitic felstone; b. Micaceous;
e. Hornblendic; d. Chloritic, or Ripidolitic;
e. Pyritous, according to whichever mineral
gives a character to the rock.
The micaceous varieties graduate into Minette,
while other varieties graduate into Felsitic elvanyte.
f. Striped, or Eibaned quartzitic felstone.—Thin
layers of somewhat dissimilar texture; hence the fracture appears to be striped like a riban, and the rock splits more easily in the direction of those layers than across them.
g. Viriolitic, or Spotted quartzitic felstone, con
taining in the matrix worm-shaped spots, or blotches of different colour, texture, and usually of composition.—In the latter case the rock weathers into pock-marked, or ovate hollows. G. Minette, Micaceous Felstone, or Felsyte.—A more or less felsitic or felspathic rock, containing much mica, and sometimes distinct crystals of orthoclase or oligoclase, or amphibole. a. Fraidronyte—Fraidronite.—" A greenish felspathic principal mass combined with a greater