54 



LITHOLOGICAL GEOLOGY. 



Fig. 34. 



Fig. 35. 



Feldspar and mica each include a number of distinct kinds or species. 

 Under feldspar, these species differ in the proportion of silica (the acid) to the other 



ingredients (bases), and in the particular alkali 

 (potash, soda or lime) predominant. The more im- 

 portant kinds are as follows, — (1) Orthoclase, or 

 common feldspar, a potask-ieldsipa.T; silica about 

 64 to 66 per cent, of the whole, the oxygen ratio of 

 the silica to the bases being 3 to 1 ; the cleavages 

 make a right angle with one another, whence the 

 name, signifying cleaving at a right angle. Fig- 

 ures 34, 35 represent crystals of this species. Cleavage takes place parallel to the faces 

 and ii. 



In the following kinds the cleavages make an oblique angle with one another, of 

 84° -87°, and hence they are sometimes called anorthic feldspars. 



(2.) Albite, a soda feldspar; 0. ratio of the silica to the bases 3 to 1, as in orthoclase. 

 (3.) Oligoclase, a soda-lime feldspar, the soda predominating ; 0. ratio of the silica to 

 the bases 2j to 1. (4.) Labradorite., a lime-soda feldspar, often iridescent; 0. ratio of the 

 silica to the bases 1J to 1. (5.) Anorthite, a lime feldspar; O. ratio of the silica to the 

 bases 1 : 1. Orthoclase and Albite are eminently acidic feldspars, and Labradorite and 

 Anorthite as eminently basic. Andesite is another feldspar, between oligoclase and lab- 

 radorite in composition. 



Under mica, the more common kinds are the following : (1.) Muscovite, or potash 

 mica (muscovy glass, of early mineralogy) usually whitish to brown in color. (2.) Biotite 

 (named after Biot, the French physician), a magnesia-iron mica, usually black. (3.) 

 Lepidomelane, an iron-mica, not elastic, of black color. (4.) Phlogopite, a magnesia- 

 mica of light brown to white color, common in connection with crystalline limestones. 

 (5, 6.) Margarodite and Damourite, micas like muscovite in composition, except the 

 presence of some water (whence called hydromicas) ; also like muscovite in color, but 

 more pearly in lustre, and less elastic; often look and feel like talc, and the slaty rocks 

 consisting largelv of them have been often called talcose slates, because soapy to the 

 touch, when really hydromica slates. (7.) Paragonite is a hydrous soda mica. 



Hornblende (often called Amphibole). — The most common 

 kind in rocks is an iron-bearing variety, in black cleavable grains or 

 oblong black prisms, cleaving longitudinally in two directions inclined 

 to one another 124° 30'. It occurs, also, in distinct prisms of this 

 angle, and of all colors from black to green and white. Figures 36, 

 37, and 38 represent these common forms, and 39 tufts of crystals as 

 they often appear in some rocks. The green kind is called actinolite, 

 — a common form of its crystals is shown in Fig. 38 ; the white (a kind 

 common in crystalline limestones, and containing much lime), tremolite. 



Fig. 36. Fig. 37. Fig. 39. 



1<_2S/ 



Fig. 38. 



The mineral is common in fibrous masses ; and, when the fibres are as 



