594 Mansfield — Western Phosphates of United States. 



Carbon dioxide (C0 2 ) is retained most abundantly by 

 waters of low temperature and this gas is supplied not 

 only from atmospheric sources but also from organic sub- 

 tances that decompose in sea water or on the sea floor. 

 Conditions would thus be unfavorable for the growth of 

 coralline limestone or for the chemical precipitation of 

 lime. Moreover, in such waters calcareous objects would 

 tend to be dissolved and the formation of limestones com- 

 posed of shells and skeletons of marine organisms would 

 be hindered. But if the precipitation of phosphate was 

 not checked that material would accumulate in relatively 

 pure form. The great volume of the deposit (see tonnage 

 estimates) needs no further explanation than the con- 

 tinued or extensive application of the process that initi- 

 ated the formation of the phosphate. 



The Western phosphates are agreed by all who have 

 seen them in the field to be original marine deposits, 

 analogous to those of Tunis, Algeria, England, 10 and 

 to the blue phosphates of Tennessee. The physiographic 

 conditions of their deposition are little known but there 

 are at least six lines of evidence which throw light upon 

 the problem, and from which it may be possible to deduce 

 a working hypothesis. 



(1) The Fauna, according to Girty, 11 is quite different 

 from Carboniferous faunas of the Mississippi Valley and 

 even among western faunas has an extremely individual 

 and novel f acies. Thus the area of deposition, though of 

 great extent, must have been separated from the main 

 ocean or was more or less restricted. 



(2) Analyses of higher grade phosphate rock such as 

 constitutes the main bed show generally less than 12 per 

 cent Si0 2 , A1 2 3 , Fe 2 3 , and MgO, all added together. 12 

 Silica constitutes the greater part of this percentage and 

 some of this may be of organic origin. It thus appears 

 that detrital material from the land is largely absent 

 from the deposit. This condition may be explained in 

 several ways : (a) the deposit may have been laid in rela- 

 tively deep water, like some of the modern oozes ; or 

 (b) the water of deposition, though shallow, may have 

 been too far from land to receive much detritus from that 



10 Blackwelder, Eliot, op. cit., p. 294. 



11 Girty, G. H., The fauna of the phosphate beds of the Park City forma- 

 tion in Idaho, Wyoming, and Utah, U. S. Geol. Survey, Bull. 436, p. 8, 1910. 



12 Gale, H. S., and Bichards, K. W., op. cit., p. 465. 



