LESSONS IN GEOLOGY. 321 



and sodium, while Anorthite is a silicate of aluminium and cal- 

 cium. Several other feldspars are thought to be only mixtures 

 of the ones just mentioned. In general they are very much like 

 orthoclase in appearance, but on a fresh surface their crystals seem 

 like strips of glass on which fine lines may often be found that 

 do not occur in the orthoclase. These are usually called triclinic 

 feldspars. Broken down weathered feldspar is the basis of clays, 

 shales and slate rocks. 



Mica is a silicate of aluminium and potassium or magnesium, 

 with iron and sometimes a little lime. It splits easily into thin 

 flexible leaves, which are nearly transparent, and are frequently 

 used in windows. It often occurs with silica and feldspar as an 

 ingredient of some kinds of granite rock. Sometimes from their 

 glistening appearance .scales of mica are mistaken for gold. 



Hornblende and Pyroxene are silicates of magnesium and cal- 

 cium, containing oxides of iron and manganese. Some varieties 

 contain in addition a little aluminium. They vary in color from 

 white to black, but in general they are greenish-black or black. 

 In general the darker varieties are the ones containing alumin- 

 ium. The fibrous mineral asbestos is a variety of hornblende. 

 The darker varieties are more common, and are easily distin- 

 guished from mica by their hardness and crystalline form, and 

 are not liable to be mistaken for any other mineral. These are 

 common minerals in basalt and lava, in trap and syenite, and 

 sometimes the light-colored varieties occur in limestone. 



Talc is a hydrated silicate of magnesium with a little oxide of 

 iron. It is the softest mineral known, separating easily into 

 thin scales. It has a light green color, a pearly luster and a 

 greasy feel. It is best known as steatite or soapstone. 



Serpentine is much like talc in composition and qualities but 

 is a little harder and has a rich greenish color. 



Chlorite contains more aluminium than talc and is harder 

 than that mineral. It has an olive green color and often a gran- 

 ular structure. 



Calcite, carbonate of calcium, is a mineral that crystallizes in 

 several different forms. When pure it has a pearly luster and is 

 L. s. 21 



