THE ADIRONDACK MOUNTAINS 87 



of lime and iron). All these have two fairly well-defined cleavages 

 meeting at nearly 90°. . Augite and hypersthene are dark green to 

 black and usually in good crystalline forms difficult to distinguish 

 (without the microscope) from hornblende except by ditferences in 

 cleavage directions. Augite is common in the anorthosite, syenite- 

 granite,, gabbro and diabase. Hypersthene is rare except in the 

 gabbro, anorthosite and diabase. Coccolite (or hedenbergite ) is a 

 grayish green pyroxene abundant in certain Grenville gneisses. 



Pyrrhotite. Commonly called " magnetic pyrites." It is a sul- 

 phide of iron somewhat like pyrite, but is softer than a knife, has 

 a bronze color, and it attracted by the magnet. 



It exists as small scattering grains in certain Grenville limestones 

 and gneisses, and in the anorthosite. 



Quartz. This very common mineral is an oxide of silicon which 

 often crystallizes in characteristic hexagonal prisms capped by six- 

 sided pyramids. It is notably harder than steel ; clear and glassy 

 looking when pure ; very brittle ; and without cleavage. 



It is very abundant as distinctly visible grains in all the granites 

 and most of the syenites and pegmatites of the Adirondacks. It also 

 largely constitutes the Grenville quartzites and is prominent in 

 many of the lighter colored Grenville gneisses and limestones. 



Serpentine. A dull green to yellowish green mineral usually in 

 irregular massive form. Never crystallized. It is a hydrous 

 silicate of magnesia with a greasy luster and is easily scratched by 

 a knife. A fibrous variety is known as serpentine asbestos which 

 is rarely found in small veins in Grenville limestone as near Thur- 

 man P. O. in Warren county. 



Serpentine at times occurs in considerable quantity in the Gren- 

 ville limestone, causing the rock to have a grayish green or mottled 

 green and white appearance. Such rock is variously called " ser- 

 pentine marble." " ophicalcite," or " verde antique." the serpentine 

 having resulted from the chemical alteration of pyroxene in the 

 limestone. Such serpentine marble has been quarried at several 

 localities in Warren and Essex counties. 



Tourmaline. The chemical composition of this mineral is very 

 complex, it being a silicate of baron and various metals. It often 

 appears in good prismatic crystals which are usually triangular or 

 wnth faces in multiples of three. It is transparent to opaque: 

 harder than quartz or flint ; without good cleavage ; and. in the 

 Adirondacks. is nearly always either black or brown. 



Black tourmaline crystals, from a fraction of an inch to several 

 inches long, are fairlv common in the coarse grained pegmatite 



