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constituents 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-like 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. QUARTZITIC or QUARTZOSE FELSTONE, or FELSYTE; QUARTZIFEROUS 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 is 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 ; c. 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.

Structural Varieties.

f. Striped, or Ribaned 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. Variolitic, or Spotted quartzitic felstone, containing 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.

C. 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

or less quantity of mica. Pyrite and quartz occur as accessories."-Cotta.

NOTE.-MICA TRAP. If the grouping of some recent writers were followed, the subgroup just described, and all other highly micaceous rocks, should here be introduced and described as Mica traps. Under this name, however, have been included "micaceous elvanyte," "micaceous felstone or minette," "micaceous euryte," "micaceous diabase or kersantyte," "micaceous dioryte or kersanton," ‚”“micaceous melaphyre," "micaceous doleryte," and many micaceous varieties of the above subgroups; rocks that are quite dissimilar, except that mica is largely developed in each. Mineralogically, such a group may exist, but geologically or petrologically it does not, for the different rocks which must be included under the name are varieties or subgroups not only of quite distinct groups, but also of distinct classes, for under it would be included granitic, plutonic, and volcanic rocks. Such a group will not be introduced into this Manual, but each micaceous rock will be found described in its natural group and place.

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D. EURYTE, or EURITE [Gr. eureo, to flow well or easily]. A compact or granular felspathic rock, occasionally showing glistening specks of quartz in the mass, usually purplish or greenish in colour; may be vesicular or amygdaloidal; when heated before the blowpipe in thin pieces, it fuses with greater or less facility. It is often columnar.

In A.D. 1817 Daubuisson proposed the name Eurite for the basic felstones, on account of their fusibility. This name was afterwards, however, misapplied by Delesse and others, who called Granityte (veins of segregation in granite), by Daubuisson's name, apparently mistaking the rock, on account of Granityte being in colour and aspect somewhat similar to Euryte.*

*A Granityte to the eye may appear the same as a Petro-silex or a Euryte, but if carefully examined will always be found to

Subsequently Naumann has proposed for Daubuisson's Euryte the name "Porphyrite," and Cotta advocates its adoption. It however seems

to be a most objectionable name, as many authors use the term synonymously with porphyry.* Moreover, the name refers to a structure which is not an essential feature of the subgroup, or, to quote Cotta," The name of Porphyrite refers to a texture which is not an essential feature of these rocks, because the Porphyrites are not always in fact porphyritic."

NOTE.-Intermediate between the highly siliceous felstones, or felsytes, and the whinstones, are rocks that partake more or less of the nature of both. These intermediate rocks Jukes and other British geologists seem to class with the basic rocks under the general name of Greenstone; † a name derived from the prevailing colour of the rocks, as even the purplish varieties have a green tinge, or merge into a green. One reason for this classification seems to be, that all the rocks included, fuse with greater or less facility before the blowpipe. Within the last few years there appears to be a reaction among some of the British geologists, who seem inclined to go to the other extreme, and place not only the eurytes, but also all the micaceous dolerytes, diorytes, and diabases in the Felstone group. This is evidently erroneous, as the latter rocks ought to be put with the basic rocks; or they and the eurytes should be placed in an intermediate group (called Hybrid rocks by Durocher) between the highly siliceous and the basic plutonic rock.

Varieties in Composition.

a. Orthoclasic euryte; b. Oligoclasic; c. Micaceous; d. Hornblendic; e. Chloritic, or Ripidolitic; f. Quartzose.

contain minute flakes of mica and a granular structure, never having the compact or semivitreous appearance characteristic of a Euryte.

*Its advocate, Cotta, seems to use it in this sense.

66

Jukes, in his Manual, mentions that there are interme

diate rocks," but he does not name them.

The orthoclase and oligoclase appear as crystals, changing the rock into a Porphyry; the micaceous variety by some would be called a Mica-trap, and in the quartzose variety the quartz appears in blebs, globules, and crystals.

Structural Varieties.

9. Euryte porphyry; h. Amygdaloidal euryte; i. Vesicular; j. Compact; k. Slab or flaggy euryte; and l. Cleaved; according to the structure that gives a character to the rock

mass.

The compact varieties have a conchoidal or subconchoidal fracture, while some of the cleaved rocks make a coarse roofing slate.

Structural Varieties of the Felstones.

E. COMPACT FELSTONE, or FELSYTE.-Typical felstone or felsyte (see Petrosilex).

F. GRANULAR FELSTONE, or FELSYTE.

G. FRIABLE, or MEALY FELSTONE.-A more or less fissile, or friable felstone; usually of a pale greenish or greyish colour; tough, but splits easily; a mealy or scaly aspect; often has a soapy feel, showing a transition into Steatyte. This rock is undistinguishable in aspect and composition from some of the fine mealy Felstone tuff. It may possibly be a true tuff, but as it occurs in dykes and intrusive masses, and often graduates into a compact felstone, it is here described. H. RUBBLY or SHINGLY FELSTONE; THICKLY-JOINTED FELSTONE.-So traversed by three or more regular or irregular systems of joint-lines, that the rock breaks up into rubble or angular shingle, and is incapable of being quarried in large pieces.

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