EOCKS : THEIR CONSTITUENTS AND KINDS. 83 



— especially the two former — in large crystalline masses, b. Porphyritic ; has the ortho- 

 clase in defined crystals, and may be (a) small-porphyritic, or (/3) large-porphyritic, and 

 have the base (7) coarse granular, or (5) fine, and even subaphanitic. c. Alhitic; con- 

 tains some albite, which is usually white, d. Oligoclase granite {Miarolyte); contains 

 oligoclase. e. Microcline granite; contains the potash triclinic feldspar, microcline. 

 f . Hornhlendic ; contains black or greenish black hornblende, along with the other con- 

 stituents of granite, g. Black micaceous granite; consists largely of mica, with defined 

 crystals of feldspar (porphyritic), and but little quartz, h. Chloritic. i. Zirconitic. 

 ]'. lolitic. k. Spherophyric or glohuliferous ; contains concretions which consist of mica, 

 or of feldspar and mica. 1. Gneissoid ; a granite in which there are traces of stratification ; 

 graduates into giieiss. 



Granultte (Leptynyte') . — Metamorphic and eruptive. Like granite, but containing 

 no mica, or only traces. 



Varieties. — a. Common granulyte ; white and usually fine granular, b. Flesh- 

 colored; usually coarsely crystalline, granular, and flesh-colored, c. Garnetiferous. 



d. Hornhlendic ; containing a little hornblende — a variety that graduates into syenyte. 



e. Magnetitic ; containing disseminated grains of magnetite, f. Graphic; quartzophyric 

 (Pegmatyte) 1 the quartz looking like Persian cuneiform characters over the cleavage- 

 surface of the feldspar ; sometimes coarse crystallizations of mica. 



Gneiss. — Metamorphic ; may be also altered eruptive. Like granite in constituents, 

 but with the mica and other ingredients more or less distinctly in layers, gneiss and 

 granite being closely related rocks. Gneiss breaks most readily in the direction of the 

 mica layers, and thus affords slabs, or is schistose in structure. 



Varieties. — Most of them are similar to those under granite. a. Granitoid. 

 b. Strongly schistose and micaceous, c. Muscovite gneiss; not common, d. 3Iuscovite- 

 biotite gneiss, e. Biotite gneiss, f. Albitic. g. Oligoclase-bearing. h. Hornblendic. 

 i. Epidotic. j. Garnetiferous. k. Andalusitic, or containing andalusite in disseminated 

 crystals. 1. C?/aniYic; contains cyanite. va. Fibrolitic. n. §«art.Jose ; the quartz largely 

 in excess, o. Quartzytic ; consists largely of quartz in grains, and intermediate between 

 quartzyte and gneiss, p. Porphyritic. q. Spherophyric. r. Quartzophyric ; containing 

 quartz in defined crystals in a fine-grained base. 



Some gneiss is very little schistose, being in thick, heavy beds, granite-like, while 

 other kinds, especially those containing much mica, are thin-bedded, and very schistose ; 

 the latter graduate into mica schist. 



Greisen (Hyalomicte). — A micaceous quartz- rock, at Zinnwald, where it sometimes 

 contains tin ore. 



Protogine, Protogine Gneiss. — Granite or gneiss-like, but containing some hydro- 

 mica, or chlorite, or both. 



MiNETTE, Ortholtte. — A fiue-graiucd rock consisting of mica and orthoclase with- 

 out quartz (mica-syenyte) . The Vosges, France. 



Mica Schist. — Metamorphic. Mica, with usually much quartz, some feldspar. On 

 account of the mica usually thin schistose. Either or both muscovite and biotite present, 

 and the latter (black mica) commonly much the most abundant. Colors silvery to black, 

 according to the mica present. Often crumbles easily, and roadsides sometimes spangled 

 with the scales. 



Varieties. — a. Ordinary. b. Gneissoid ; between mica schist and gneiss, and 

 containing much feldspar, the two rocks shading into one another, c. Hornblendic. 

 d. Garnetiferous. e. Staurolitic. i. Gyanitic. g. Andalusitic. h. Fibrolitic ; cont&ming 

 fibrolite. i. Tourmalinic. j. Ottrelitic. k. Calcareous, limestone occurring in it in occa- 

 sional beds or masses. 1. Graphitic or Plumbaginous ; the gi-aphite being either in scales, 

 or impregnating generally the schist, m. Quartzose ; contains much quartz, n. Quartzytic ; 

 a quartzyte with more or less mica, rendering it schistose, o. Specular, or Itabyryte ; con- 

 taining much hematite or specular iron in bright metallic lamellae or scales. 



