280 DESCRIPTIONS OF MINERALS. 



Felsite is compact, uncleavable orthoclase, having the 

 texture of jasper or flint, which it much resembles. It often 

 contains some disseminated silica. It occurs of various col- 

 ors, as white, gray, brown, red, brownish red and black, and 

 is sometimes banded. It is distinguished from flint or jas- 

 per by its fusibility. 



Felsite is the material of beds or strata in some rock for- 

 mations, and also of dikes or masses of eruptive rocks. It is 

 the base of much red porphyry. The vicinity of Marblehead, 

 Mass., is one of its localities. 



The name feldspar is from the German word Feld, mean- 

 ing field. It is; therefore, wrong to write it felspar. 



Orthoclase is used extensively in the manufacture of por- 

 celain. The large granite veins of Middletown and Port- 

 land, Conn., are quarried in several places for both orthoclase 

 and quartz for this purpose ; the places are often called 

 China-stone quarries. 



Kaolin. This name is applied to the clay that results 

 from the decomposition of feldspar. See Kaolinite, p. 310. 



m. SUBSILICATES. 



In the Subsilicates, as stated on page 242, the combin- 

 ing or quantivalent ratio between the bases and silica is 1 

 to less than 1. In Chondrodite, the first of the following 

 species, the ratio is 4 : 3 ; in Tourmaline, Andalusite, Cya- 

 nite, and Fibrolite, 3:2. Analyses of Andalusite obtain 1 

 of alumina, Al 3 , to 1 of silica, Si 2 , giving the oxygen 

 ratio for bases and silica 3 : 2. This is the composition also 

 of cyanite and fibrolite ; so that the three species, anda- 

 lusite, cyanite, and fibrolite are the same in constituents 

 and atomic ratio while differing in crystalline form, exem- 

 plifying a case of trimorphism among minerals. 



The ratio 3 : 2 exists also in Topaz, Euclase and Da- 

 tolite in Titanite or sphene, and in Keilhauite. In Stau- 

 rolite, the ratio was formerly regarded as 2:1; but the 

 most recent analyses, those of Rammelsberg, give 11 : 6, or 

 1§ : 1. In datolite and tourmaline the basic constituents 

 include boron ; in titanite and keilhauite, titanium ; in da- 



