Mansfield — Western Phosphates of United States. 591 



Art. XXVII. — Origin of the Western Phosphates of the 

 United States; by George R. Mansfield. 1 



The Western phosphate field occupies extensive areas 

 in northeastern Utah, southeastern Idaho, southwestern 

 Montana, and western Wyoming. Adams and Dick 2 

 have reported the discovery in Alberta of phosphate 

 deposits similar to those in the states named. The phos- 

 phate rock occurs in mountainous districts where the 

 stratified rocks are folded and faulted on both a large 

 and a small scale and are greatly eroded. The phosphate 

 beds may be regarded as having been formerly more or 

 less continuous throughout the territory mentioned but 

 the agencies of mountain building and erosion have 

 separated the region into large and small phosphate- 

 bearing areas of generally synclinal structure, between 

 which the phosphate has either been removed or carried 

 so far below the surface that it cannot be considered 

 workable. 



Detailed studies by members of the IT. S. Geological 

 Survey, Department of the Interior, in parts of the West- 

 ern field have led to the establishment of great phosphate 

 reserves aggregating more than 2,600,000 acres of public 

 land which are estimated to contain more than 5,290,000,- 

 000 long tons of relatively high-grade phosphate rock. 

 When these studies, which are only partial, have been 

 completed it is probable that both acreage and tonnage 

 figures will be considerably increased. 



Phosphate deposits have been identified at two geo- 

 logical horizons, of upper Mississippian and Permian age 

 respectively, but the upper Mississippian deposits, so far 

 as known, are inferior in quality to the Permian deposits 

 and far less extensive. The remarks which follow on the 

 origin of the deposits have been prepared with special 

 reference to the Permian deposits but it is thought that 

 with some modifications they will apply also to the upper 

 Mississippian deposits. 



The origin of the Western phosphate deposits has an 

 important commercial bearing, for if they were residual 

 like those of the brown rock of Tennessee, or of second- 



1 Published by permission of the Director of the U. S. Geological Survey. 



2 Adams, F. D., and Dick, W. J., Discovery of phosphate of lime in the 

 Rocky Mountains, Commission of Conservation, Canada, Ottawa, 1915. 



