HANDBOOK FOR THE DEPARTMENT OF GEOLOGY. 



STRATIFIED ROCKS. 



521 



Kind. 



Specific 

 gravity. 



Composition. 



Calcareous : 



2. 6 to 2. 8 

 ^2. f< to 2. 95 



2. 6 to 2. 7 



2.6 



2.6 to 2.8 



2.5 







Carbonate of lime. 







Siliceous : 



Carbonate of lime and magnesia. 



Same as granite. 



Mainly silica. 



60 to 80 per cent, silica. 



Mainly silicate of alumina. 



Schist 



Argillaceous : 





ERUPTIVE ROCKS. 



Acidic group : 



Granite 



Liparite 



Obsidian 



Obsidian pumice 



Intermediate group : 



Syenite 



Trachyte 



Hyalotrachy te 



Andesite 



Basic group : 



Diabase 



Basalt 



Peridotite 



Peridotite (iron rich) 

 Peridotite (meteorite) 



Specific Grav- 

 ity. 

 2. 58 to 2. 73 



2. 53 to 2 



70 



2. 26 to 2. 41 



Floats 

 water. 



on 



2. 73 to 2 



86 



2. 70 to 2 



80 



2. 4 to 2. 5 



2. 54 to 2 



79 



2. 66 to 2 



88 



2. 90 to 3 



10 



3. 22 to 3 



29 



3.86 





3.51 





Per cent, silica. 

 77. 65 to 62. 90 

 76. 00 to 67. 61 

 82. 80 to 71. 19 

 82. 80 to 71. 19 



72. 20 to 54. 65 

 64. 00 to 60. 00 

 64. 00 to 60. 00 

 66. 75 to 54. 73 



50. 00 to 48. 00 

 50. 59 to 40. 74 

 42. 65 to 33. 73 

 23.00 

 37.70 



IV. — THE COLOR OF ROCKS. 



The color of a rock is dependent upon a variety of circumstances, but 

 which may here all be generalized under the heads of mineral and chem- 

 ical composition and physical condition. Iron and carbon, in some of 

 their forms, are the common coloring substances and the only ones that 

 need be considered here. The yellow, brown, and red colors, common 

 to fragmental rocks, are due almost wholly to free oxides of iron. The 

 gray, green, dull brown, and even black colors of crystalline rocks are 

 due to the presence of free iron oxides or to the prevalence of silicate 

 minerals rich in iron, as augite, hornblende, or black mica. Rarely cop- 

 per and other metallic oxides than those of iron are present in sufficient 

 abundance to impart their characteristic hues. As a rule, a white or 

 light-gray color denotes an absence of an appreciable amount of iron in 

 any of its forms. The bluish and black colors of many rocks, particu- 

 larly the limestones and slates, is due to the prevalence of carbonaceous 

 matter. 



