410 DESCRIPTIONS OF ROCKS. 



black pyroxene ; also the green hornblende or actinolite; the 

 green foliated hornblende called smaragdite, and the foli- 

 ated pyroxene sometimes wrongly called hypersthene, and 

 another variety called diallage; also occasionally the species 

 hypersthene and cnstatite. 



• 5. The Feldspar-like minerals, nephelite (p. 269) and leu- 

 cite (p. 271), which are related in constituents and quan- 

 tivalent ratios to the feldspars, alumina being the only ses- 

 quioxide base, and lime, potash, and soda the protoxide bases 

 afforded in analyses ; the atomic ratios for the protoxides, 

 sesqnioxide, and silica being in nepheiite, 1 : 3 : 4, as in 

 anorthite ; and in leucite 1 : 3 : 8, as in andesite. Also, less 

 abundantly, Socialite (p. 270), which has essentially the 

 ratio of anorthite and nephelite. 



6. Minerals of the Saussurite group. These jade-like 

 species differ from the feldspars — (1) in being always fine- 

 granular in texture ; (2) in haying a high density, G. = 

 2 *9-3 4 ; in varying from the feldspar type chemically. 

 They are near some soda-lime feldspars in constituents, but 

 not always in the atomic relations of the constituents, nor 

 in the absence uniformly of magnesia. There are two 

 prominent kinds. One is between anorthite and zoisite in 

 composition (see p. 263) ; yet, unlike these minerals, its 

 analyses afford seyeral per cent, of soda and some magnesia. 

 The second approaches labradorite ; Delesse obtained for a 

 specimen from Mt. Genevre (Alps), Silica 49*73, alumina 

 29'65, iron protoxide 0*85, magnesia 0*56, lime 11-18, soda 

 4;04, potash 024, water (with a little C0 2 ) 3*75 ; and a 

 Silesian specimen afforded Torn Rath nearly the same result. 

 A third kind from Corsica, according to Boulanger's analy- 

 sis, has nearly the same composition as zoisite. A fourth 

 is jadeite (p. 263), a stone occurring in the Swiss lake-dwell- 

 ings — but not yet found in the saussurite rocks of Switzer- 

 land. 



The saussurite of Siberia and the Alps has been observed 

 to have sometimes the form of twins of a triclinic feldspar. 

 This, and the texture, density, and composition, show that 

 saussurite is, in part at least, pseudomorphous, and. in some 

 regions, after labradorite. By some peculiar conditions in 

 the process of metamorphism — perhaps long-continued heat 

 with an unusual amount of moisture — the feldspar crystal- 

 lizations that formed in the incipient stages of the process 

 were afterward changed to a species of higher density and 



