42 PHYSTCAIi AND CHEMICAL PROPERTIES OF ROCKS 



ing, on analysis, upwards of 60% silica as acidic, and those 

 showing less than 50%, but rich in iron, lime, and magnesian 

 constituents, as basic. The extremes, as will be noted, are rep- 

 resented by the rocks of the granite and peridotite groups. 



A series illustrating the above-mentioned properties may be 

 arranged as on p. 41. "With the eruptive rocks only the silica 

 percentages are here given. "The results of the complete chem- 

 ical analysis of each variety are given further on, in the pages 

 devoted to their description. 



4. THE COLOR OF ROCKS 



The color of a rock is dependent upon a variety of circum- 

 stances which may all be generalized under the heads of min- 

 eral and chemical 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 prevalence of silicate minerals rich in iron, as augite, 

 hornblende, or black mica. Earely copper, manganese, 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 amber, bluish and 

 black colors of many rocks, particularly the limestones and slates, 

 are due to the prevalence of carbonaceous matter. 



Among siliceous crystalline rocks the more basic are, as a rule, 

 of a darker color than the acid varieties, the color being due to 

 the fine grain and predominance of dark iron-magnesian sili- 

 cates, such as hornblende, augite, or black mica, or their chloritic 

 alteration produ.cts. The red or pink color sometimes occurring 

 in granitic rocks is due to the predominance of red or pink 

 feldspars, which in their turn owe their color to the presence 

 of iron. 



Many feldspar-bearing rocks owe their color to the physical 

 condition of this important constituent. Thus with rocks like 

 the norite of Keeseville, New York, and the Quincy, Massa- 

 chusetts, granite, the dark color is largely due to the fact that 

 the feldspar is clear and glassy, allowing the light rays to pene- 

 trate and become absorbed. The beautiful chatoyant play of 



