44 'I'lIK CHICAGO ACADEMY Ol SCIKNCES. 



% 



angle with the cleavage cracks is from 12 to 15, hence this 

 is a in>od /one for distinguishing between hornblende and 

 augite. The behavior of hornblende before the blowpipe re- 

 sembles that of pyroxene. Hornblende readily changes to 

 biotite. It has been made from an aqueous solution, but not 

 yet successfully by smelting. It is found in furnace- slags as 

 an artificial product. It occurs in granites, diorites, and 

 gneisses. The rock mentioned above as being one of the 

 the sources of magnetite is made up chiefly of hornblende. 



Garnet. (FeMg) :5 Al,(SiO 4 ) 3 Regular. 



In the clay pit of a brick yard at South Evanston are 



found fresh granite boulders and fragments of a black mica 



gneiss which contain large numbers of garnets. They are es- 

 pecially abundant in the gneiss. 



Fig. 16. Fig. 17. 



The grains showdodekahedral (no) and trapezohedral (21 1) 

 planes (Figs. 16-17). They are usually irregular. No cleavage 

 is seen, but there is a parting parallel to the dodekahedron 

 (no). The fracture is subconchoiclal to uneven. They are 

 brittle and easily ground to powder. Their hardness is 6.5 to 

 7.5; specific gravity 3.15 to 4.3; luster resinous to vitreous. Of 

 the various colors exhibited by garnet (red, brown, yellow, 

 green, white and black) these specimens show only red and 

 brown. The streak is white. Translucent. Under the micro- 

 scope the high index of refraction (1.776) causes them to stand 

 out from the neighboring minerals. Specimens examined 

 showed normal isotropic qualities. These individuals fused 

 easily to a brown glass, which is slightly magnetic. They de- 

 compose upon fusion with alkaline carbonates, and are then 

 soluble in hot hydrochloric acid. To the same test for magne- 

 sium which was employed in the investigation of dolomite, 

 after previous ignition, they gave very slight white precipitate 

 of magnesium. A large amount of iron is present, and conse- 



