66 



STRUCTURAL GEOLOGY. 



slender. Have H = 6-7. Are infusible. Andalusite occurs in gray, stoutish, nearly 

 square, prisms (90° 48') which are often tesselated inside with white (then called chias- 

 tolite). Cyanite is commonly in long, bluish, bladed crystals ; and fibrolite in rhombic 

 prisms and fibers, having a brilliant diagonal cleavage. 



Staukolitk. — In rhombic prisms of 129° 20', imbedded in slaty rocks. Usual colors, 

 brown to black. The crystals are often crossed as in Fig. 39, and hence the name, from 

 the Greek for cross. H = 1-1\. Composition: Silica 29-3, alumina 53-5, sesquioxide of 

 iron 17-2 = 100. Infusible. 



TouRMALiNK. — Usually lu three-sided or six-sided black crystals, having the luster 

 within, when black, like that of a black resin ; and it has no distinct cleavage, and thus 

 differs from hornblende. Figs. 40, 41 show two of the forms ; and Fig. 42, the appear- 



42. 



40. 



41. 



1'^ 



Tourmaline. 



Tourmaline. 



ance of the crystals in the rock, which is often quartz. Besides black, there are also brown^ 

 green, red, and white tourmalines. H = 7-7^. Constituents : Silica, alumina, magnesia, 

 with fluorine and some boracic acid. Fusible, but fusibility varying much in varieties. 



Garnet. — In crystals of the forms in Figs. 43, 44. H = 6|-7i. Colors usually red 

 to brown and black, rarely green and colorless ; sometimes chrome-green. H = 6-7. 

 Consists of silica and alumina, with either iron, or lime, or manganese, and varying in its- 

 characters according to composition. 



43. 



Epidote. — In yellowish green to hair-brown prismatic crystals and masses. A peculiar 

 yellowish green color is most common. It has nearly the composition of an iron garnet. 

 G = 3-25-3 -5. Zoisite is a I'elated mineral of ash-gray to whitish color, containing much 

 lime and little or no iron. It has high specific gi-avity, G = 3-1-3-4. Constituents as in 



45. 



^3iM 



"^"^^ 



46. 



a 



Idocrase. — In square prisms, of a brown to oil-green color. 

 H = 6|. Composition: One kind, silica 37-3, alumina 16-1, iron 

 sesquioxide 3-7, lime 35-4, magnesia 2-1, iron protoxide 2-9,, 

 water 2-1 = 99-6. Fusible. 



Topaz. — In rhombic prisms of 124° 17', remarkable for 

 cleaving with ease and brilliancy parallel to the base of the 

 prism. Colors, yellowish to white, also brown. Two of the 

 forms of its crystals are shown in Figs. 45, 46. H = 8. Con- 

 sists of silica 16-2, silicon fluoride 28-1, alumina 55-7 = 100. The amount of fluorinfr 

 present is a remarkable quality. Infusible. 



Topaz. 



