138 GEOLOGY OF THE BLACK HILLS. 



The large percentage of peroxide of iron in the Red Beds, to which they 

 owe their bright red color, bears an interesting relation to the absence of 

 fossils. The material of which sediments are formed is derived by the various 

 processes of denudation from the rocks of older land surfaces. Whatever iron 

 they contain is dissolved from the land and transported in the condition of 

 protoxide or some proto-salt, such as the carbonate, and the process is facili- 

 tated by the presence of carbonic acid in the water. Now iron occurs in 

 these older rocks as protoxide and peroxide, the former of which is soluble 

 and the latter insoluble in water. The peroxide, however, by the action of 

 organic matter, such as is held in solution by boggy waters, may be de- 

 prived of a portion of its oxygen and converted into protoxide and thus 

 rendered soluble. If the iron-bearing water is confined as in a shallow 

 basin and exposed long to the action of the atmosphere the protoxide of 

 iron absorbs oxygen, and is precipitated as the insoluble red peroxide of 

 iron. If, however, plant or animal life be present in sufficient quantity this 

 oxidation is prevented. . In case but little foreign material — clay or sand — 

 has been brought by the waters, the deposit will be an iron ore; in the 

 absence of organic matter the peroxide or brown hematite, or in its pres- 

 ence the proto-carbonate, such as the iron-ore beds of the Coal Measures. 

 But in case large quantities of foreign material are deposited from the waters 

 at the same time there will be produced in the absence of life a brown or 

 red clay or sandstone, and in its presence a white or light-colored forma- 

 tion containing the iron as a protoxide. This question of the removal and 

 deposition of iron has been ably elucidated by Dr. Newberry and others, 

 and Dr. Hunt "can hardly conceive of an accumulation of iron, copper, 

 lead, silver, or gold in the production of which animal or vegetable life has 

 not either directly or indirectly been necessary."* The point to be estab- 

 lished by this seeming digression is this ; that reasoning from the condition 

 in which the iron is found in the Red Beds we must conclude there could 

 have been little or no life, animal or vegetable, in the water from which 

 they were deposited. The conclusion is strengthened by the fact of the 

 presence of large quantities of gypsum, which has been derived from the 



* Chemical and Geological Essays, p. 226. 



