THE NATURAL HISTORY SURVEY. 43 



Augite. CaMgSi 3 O fi +(Mg,Fe) (AlFe) 2 Si 2 O G Monoclinic. 



These crystals occur in short, thick prisms formed by the 

 orthopinacoid (100), the prism (no), the clinopinacoid (oio), 

 and the pyramid (ill), and in addition to these the orthodome 

 (101), the base (ooi), the clinodome (021), and low pyramids. 

 The nearly square columns often have corners truncated till a 

 symmetrical eight-sided figure results. This eight-sided figure 

 and the cleavage habit of augite are of prime importance in 

 differentiating augite from hornblende. It occurs often coarsely 

 or finely granular. The cleavage is distinct, parallel to the 

 prism, and makes prism angles of 92 54'. Parting due to 

 the twinning transverse to the c axis is seen in lamellar masses. 

 The fracture is uneven to conchoidal. The hardness is 

 5 to 6 and specific gravity 3 to 3.5. Luster is vitreous to res- 

 inous on fresh surfaces; color is usually green, brown, or black, 

 and the streak a greenish grey, transparent to opaque. Under 

 the microscope it shows but weak pleochroism. It has high 

 relief, high interference colors, highest in sections parallel to 

 the clinopinacoid. These sections give no optical figure in 

 convergent light, show the cleavage marks parallel to the c 

 axis, and have an extinction angle of 35 to 56. Before the 

 blowpipe augite melts to a glass which is dark because of the 

 contained iron. It is not acted upon by acids. It is found in 

 the slags of lime kilns and cement furnaces. The rocks in 

 which it is found in this region are gabbro and diabase. 



Hornblende J Ca(Mg,Fe) 3 (Si O 3 ) 4 chiefly with 



\ Na 2 Al 2 (Si0 3 ) 4 and(Mg,Fe) 8 (Al, Fe) 4 Si 2 O 12 Monoclinic 



Crystals show but few planes. They are usually columnar 

 or long slender prisms that give blades and sometimes are 

 microscopic in fineness; forming flax-like fiber, rarely lamel- 

 lar granular masses. The prisms have six sides instead of 

 eight as augite has, and their cleavage, highly perfect, parallel 

 to the prism, forms the characteristic angle of 124. The frac- 

 ture is from conchoidal to uneven. They are brittle, have a 

 hardness of from 5 to 6 and specific gravity of 2.9 to 3.4, vary- 

 ing with the amount of iron present. The luster is vitreous to 

 pearly. The color, various shades of green or dark brown to 

 black. Streak is paler than color. Opaque to translucent. 

 Under the microscope, strongly pleochroitic. High relief, 

 high interference colors. In sections parallel to the clinopin- 

 acoid (oio) there is no axial figure in convergent polarized 

 light, the interference colors are highest and the extinction 



