60 LITHOLOGICAL GEOLOGY. 



green prisms and fibrous masses, when it is called actinolite; or in 

 grayish and brownish-green fibrous or acicular forms, when it is 

 called anthophyllitc ; also in long white prisms and fibrous masses, 

 when it is called tremolite; also in short, thick, white prisms, called 

 white hornblende; also in very delicately-fibrous masses, the fibres 

 flexible like flax, when it is called asbestus; also in rock-masses 

 made of coarse cleavable grains. Hardness nearly or quite that 

 of feldspar. Hornblende rocks, very tough. Before the blowpipe, 

 somewhat fusible. Composition: Silica, with magnesia, lime, prot- 

 oxyd of iron (and sometimes protoxyd of manganese), with occa- 

 sionally a little alumina. A lime-and-magnesia variety contains — 

 Silica, 60.7, lime, 12.5, magnesia, 26.8 ; an iron-a,nd-magnesia variety — 

 Silica, 58.6, magnesia, 25.9, protoxyd of iron, 15.5. 



(14.) Pyroxene (often called Augite). — In oblique prismatic crys- 

 tals (monoclinic) of 87° 5', and therefore nearly square, with cleav- 

 age parallel to the faces, often having all four edges replaced so as 

 to make an eight-sided prism. In colors, hardness, specific gravity, 

 and composition, like hornblende. Cleavable, massive kinds of a 

 dingy grayish-green color are called sahlite. In 

 volcanic rocks the crystals are black, rather 

 small, and like figs. 50 and 51 in form ; in meta- 

 morphic rocks they are usually grayish green, 

 and often large and coarse. In serpentine dial- 

 lage is common, and with labradorite liyper- 

 sthene frequently occurs. Before the blowpipe, 

 like hornblende ; some foliated varieties infusible. Composition : 

 The pale-green varieties contain — Silica, 55.7, magnesia, 18.5, lime, 

 25.8. Others of dark-green and black colors, Silica, 53.7, magnesia, 

 13.4, lime, 24.9, protoxyd of iron, 8.0; or, Silica, 48.6, lime, 22.5, 

 protoxyd of iron, 28.9. 



If £/tm-foliated, the brittle folia often partly separable, called 

 hypersthene when brownish green or bronze-like, and diallage when 

 grass-green. 



(15.) Chrysolite. — In small grains disseminated through basaltic 

 rocks and many lavas, also in imbedded masses and rectangular 

 crystals. Appearance glassy, and often dark green like bottle-glass, 

 (a variety called olivine) ; also pale green. Hardness nearly that of 

 quartz. Gr. = 3.3-3.5. Infusible, excepting some kinds contain- 

 ing much iron. Composition : essentially a silicate of magnesia, or 

 magnesia and iron ; a common kind contains — Silica, 41, magnesia, 

 50, protoxyd of iron, 9. 



(16.) Chondrodite. — In large and small grains or masses imbedded usually 



