214 ENVIRONMENT OF VERTEBRATE LIFE, ETC. 



In the northern portion of the province the deposits must have come 

 from the adjacent elevations of the Rockies and the Bighorn Mountains. 

 The conditions which produced red beds in the south must have been much 

 the same as those which produced the similar beds of the Plains Province 

 and, as has been shown, the mountain barrier was probably low at the 

 southern end. The reason for the replacement of red beds by phosphate- 

 bearing shales and limestones and of semiarid terrestrial conditions by 

 marine conditions farther north is less easily understood. 



Blackwelder,^ in discussing the deposits of phosphate rock in the northern 

 portion of the Basin Province and of phosphate rock in general, says, after 

 reviewing other theories : 



"Nevertheless, we find among the rocks derived from oceanic sediments in 

 many parts of the world, beds several feet thick which are rich in lime-phosphate 

 and extend rather uniformly over thousands of square miles. They contain 

 marine fossils which indicate that they have accumulated upon the sea bottom. 

 It is therefore evident that locally there must be conditions which cause the 

 fixation of the phosphoric acid among the bottom sediments. Some students of 

 these deposits have ascribed them to direct deposition of phosphatic shells, bones, 

 and teeth, and others have made appeal to the agency of mineral springs. Gen- 

 erally they have sought an explanation for the abundance of the phosphorus. 

 As the writer has already shown, however, the quantity of phosphorus dissolved 

 in sea-water is always sufficient to produce in a few thousand years even the 

 thickest known phosphate beds; and hence we need only to account for the 

 special conditions which cause it to be precipitated on the sea floor. There is 

 excellent reason to think that the immediately controlling conditions are chemical 

 or biochemical, but these chemical conditions in turn depend upon physiographic 

 and climatic factors difficult to analyze and estimate. The study of the latter 

 is a task for the geologist. 



"In its simpler aspects, the chemistry of the marine deposition of phosphates 

 has been plausibly interpreted by a number of European students of the question, 

 even as far back as 1870. The following is a modification of their views, based 

 on modern information: The process and results of bacterial decomposition of 

 organic matter vary according to the conditions as well as the particular class of 

 bacteria that are at work. In air and aerated water, decay is generally complete, 

 resulting in the production of carbon dioxide, water, soluble nitrates, sulphates, 

 phosphates, etc. In the absence of oxygen, however, the anaerobic bacteria 

 somewhat more slowly break down the organic compounds and produce a dif- 

 ferent series of end-products, of which the most important are various hydro- 

 carbons, nitrogen, ammonia, and hydrogen sulphide, with only so much of the 

 carbonic oxides as the available oxygen in combinations permits. In so far as 

 free oxygen is present in only small quantities, there should be a compromise 

 between the two processes. 



"Some of the most obvious characteristics of our marine phosphatic rocks 

 show that they have been associated in origin with the anaerobic phase of bacterial 

 action. Almost invariably they are black in color and, owing to the fact that they 

 contain noteworthy quantities of hydrocarbon oils, tars, and gases, they are 



^ Blackwelder, Eliot, The Geologic Role of Phosphorus, Amer. Jour. Sci., vol. xui, p. 291, 1916. 



