SILICA. 287 



Coccolitc — Granular Pyroxene. This presents various sliados of green and bluisli green, 

 and occurs in small translucent masses or grains of irregular shapes, which cohere verj- 

 slightly. They are, Iiowcver, sufficiently hard to scratch glass. The structure is lamellar, 

 and the lustre vitreous. 



Jcffersonite. This was described as a distinct species by Keating. It occurs in lamellar 

 masses of a dark olive-green colour, passing into brown. Lustre on the planes of cleavage, 

 semi-metallic. It fuses readily before the blowpipe, into a black globule ; not acted upon by 

 the magnet. 



Amianlhus. This includes certain fibrous varieties ; but it is rather a form belonging to 

 several different minerals. 



Composition. The different varieties differ somewhat in the proportions of their consti- 

 tuents. 



White varieties — Silica 51.83, lime 21.76, magnesia 18.55, alumina 0.28, protoxide of 

 iron 0.99 {Bonsdorf). 



Green varieties — Silica 51.08, lime 23.47, magnesia 11.49, protoxide of iron 10.02, 

 manganese 0.51 {Rose). 



Black varieties — Silica 53.36, lime 22.19, magnesia 1.19, protoxide of iron 17.38, oxide 

 of manganese 0.09 (Ruse). 



Diopside — Silica 57.00, lime 16.50, magnesia 18.25, oxides of iron and manganese 6.00 

 (Luugier). 



Coccolite — Silica 50.00, lime 24.00, magnesia 10.00, alumina 1.50, oxide of iron 7.00, 

 oxide of manganese 3.00 {Vauiiiiclin). 



Jeffersonitc — Silica 56.00, lime 15.10, alumina 2.00, protoxide of manganese 13.50, per- 

 oxide of iron 10.00, oxide of zinc 1.00 {Keating). 



The varieties of this mineral may be considered as composed of bisilicates of lime, mag- 

 nesia, protoxide of iron or of manganese ; but these are mixed in such variable proportions, 

 that it is difficult to determine the exact formula. 



Geological Situation. The white and light green varieties are usually found, in this 

 State, associated with white limestone and dolomite ; while the dark green ones occur in gra- 

 nite and gneiss, and often accompany the magnetic oxide oi iron. The former are usually 

 associateil wilh, and often pass into, treinolite. This is espcciallv ihe case where they occur 

 in dolomitic limestone. 



LOCALITIES. 



Dutchess Countv. The white variety of pyroxene is found in the beds of doloniilic lime- 

 stone which traverse the eastern part of this county. It has not, however, been iound in such 

 abundance, nor so well characterized, as in the counties of Putnam, Westchester and New- 

 York, which see. 



Essex County. Pyroxene, in almost all its varieties, is found in this county. 



