72 LITHOLOGICAL GEOLOGY. 



may be (a) small-porphyritic, or (/?) large-porphyritic, and have the base (y) coarse granu- 

 lar, or (6) line, and even subaphanitic. c. Albitic granite; contains some albite, which is 

 usually white; rarely, albite is the predominating feldspar, and then the rock is albite- 

 granite. d. Oligoclase granite (Miarolite) ; contains oligoclase. e. Microcline granite ; 

 contains the potash triclinic feldspar, microcline. f. Hornblendic granite ; contains 

 black or greenish-black hornblende, along with the other constituents of granite, g. 

 Black micaceous granite ; consists largely of mica, with defined crystals of feldspar (por- 

 phyritic), and but little quartz, h. Globuliferous granite; contains concretions which 

 consist of mica, or of feldspar and mica. i. Gneissoid granite ; a granite in which there 

 are traces of stratification; graduates into gneiss, j. Pegmatyte, or Graphic granite; 

 consists mainly of orthoclase and quartz, with but little whitish mica; but the quartz is 

 distributed through the feldspar in forms looking like Oriental characters (Fig. 59). 



2. Granulyte (Leptynyte). — Like granite, but containing no mica, or only traces. 

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

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



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



e. Magnetitic ; containing disseminated grains of magnetite. 



3. Gneiss. — Like granite, but with the mica and other ingredients more or less dis- 

 tinctly 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. Porphyritic. b. 

 Albitic. c. Oligoclase-bearing. d. Hornblendic. e. Micaceous, f. Globu lifer ous. g. 

 Epidotic. h. Garnetiferous. i. Andalusitic, or containing andalusite in disseminated 

 crystals, j. Cyanitic ; contains cyanite. k. Graphitic; contains graphite disseminated 

 through it. 1. Quartzose; the quartz largely in excess, m. Quartzytic; consists largely 

 of quartz in grains,Vand intermediate between quartzyte and gneiss. 



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. 



4. Mica Schist. — Consists largely of mica, with usually much quartz, some feldspar, 

 and, on account of the mica, divides easily into slabs, that is, is very schistose. Usually 

 both of the potash micas, muscovite and biotite, are present, and the latter (black mica) 

 is commonly much the most abundant. The colors vary from silvery to black, accord- 

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

 with the mica scales. 



Varieties. — a. Gneissoid; between mica schist and gneiss, and containing much 

 feldspar, the two rocks shading into one another, b. Hornblendic. c. Garnetiferous. 

 d. Staurolitic. e. Cyanitic. f. Andalusitic. g. Fibrolitic; containing fibrolite. h. 

 Tourmalinic. i. Calcareous, limestone occurring in it in occasional beds or masses., 

 j. Graphitic, or Plumbaginous ; the graphite being either in scales or impregnating 

 generally the schist, k. Quartzose; contains much quartz. 1. Quartzytic ; a quartzyte 

 with more or less mica, rendering it schistose ; m. Specular, or Jtabyrite ; containing 

 much hematite or specular iron in bright metallic lamellae or scales, n. Mica slate ; or 

 fine grained mica schist, the scales of mica scarcely visible without a lens, the rock 

 being between a mica schist and argillyte. This variety of the rock is often garnetifer- 

 ous, and sometimes staurolitic Sometimes through mica schist or slate small mica crys- 

 tals are disseminated, set transversely to the bedding. 



5. Hydromiea Schist or Slate. — A thin schistose rock, consisting either chiefly 

 of hydrous mica, or of this mica with more or less quartz; having the surface nearly 

 smooth, and feeling greasy to the fingers; pearly to faintly glistening in lustre; whitish, 

 grayish, pale greenish in color, and also of darker shades. This rock used to be called 

 talcose slate, but, as first shown by Dr. C. Dewey, it contains no talc. It includes paro- 

 phite schist, damourite slate, and sericite slate (glanz-schiefer, sericit-sckiefer, and part 

 of the glhnmer-schiefer of the Germans). 



Varieties. — a. Ordinary; more or less silvery in lustre, b. Chloritic ; contains 

 chlorite, or is mixed with chlorite slate, and has therefore spots of olive-green color ; 



