440 DEBCBIFHONS OF ROCKS. 



natural size, a small piece of the porphyritic gneiss of Birmingham, 

 Conn. 



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. The so-called granite of Monson, Mass., is a granitoid 

 gneiss. Its gneissoid structure facilitates greatly the quarrying. 



4. Protogine. Protogine-giieiss. — Coarse to fine granular, 

 granite-like or gneissoid in structure, and mostly the lat- 

 ter : of a grayish-white to greenish-gray color ; consists of 

 quartz, white or grayish-white,, rarely flesh-red, orthoclase, 

 a dark green mica and often chlorite, with some greenish- 

 white talc, and white oligoclase. Metaniorphic. 



The dark green mica approaches chlorite, as shown by Delesse, in 

 its very large percentage of iron oxide (Fe 2 3 21 31, FeO 5 03), but it 

 gave him only 90 of water, with 6 05 of potash. Among accessory 

 minerals are hornblende, titanite, garnet, serpentine, magnetite. In 

 an analysis of the protogine as a whole, Delesse obtained Silica 74-25, 

 alumina 1158, iron oxide 241, lime 1 OS, water 0-97, leaving 10*01 for 

 potash, soda and magnesia. From the region of Mont Blanc and 

 other parts of the Swiss Alps. 



5. 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, according 

 to the mica present. Often crumbles easily; and roadsides 

 are sometimes spangled with the mica scales. The dissemi- 

 nated scales or crystals of biotite are sometimes set trans- 

 versely to the bedding. Metamorphic. 



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

 containing much feldspar, the two rocks shading into one another, 

 b. Hornblendk. c. Garnetiferous. d. Staurolitk. e. Cyanitk. f. An- 

 dalusitic. g. Fibr otitic ; containing fibrolite. h. Tourmalink. i. Cal- 

 careous; 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; consisting largely of 

 quartz. 1. Quartzytk ; a quartzyte with more or less mica, rendering 

 it schistose, m. Specular, or ltabyrite ; containing much hematite or 

 specular iron in bright metallic lamellae or scales. In fine-grained 

 mica schist, the scales of mica are sometimes scarcely visible without 

 a lens. 



6. Hydromica Schist. — 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 



