128 



CLASSIFICATION 03!' THE PUBLIC LANDS. 



surveys. These facts and relations are brought out in the following 

 table: 



Approximate area of phosphate lands, in square miles. 



CLASSIFICATION OF PHOSPHATE LANDS. 



FACTOBS nrVOLVKD. 



As the phosphate deposits on the public domain exist as stratifonn 

 sedimentary beds or as residual deposits of the placer type, they are, 

 as a rule, readily found and their extent may be determined by the 

 ordinary methods of areal geologic mapping. In examining deposits 

 of this type the geologic problems involved are chiefly structural. The 

 purely economic considerations of accessibility, means of transpor- 

 tation, and nearness to market are highly important in the problem 

 of establishing a commercial mine but are not involved in the classi- 

 fication of the land as phosphate or nonphosphate land. 



The classification of a given tract as phosphate land is governed 

 by the facts observed in the course of a field examination. A 

 knowledge of the general geology of the region tells the examiner 

 in what geologic environment the deposits may be expected, and a 

 knowledge of the local succession of the beds tells him at what hori- 

 zons and at what depths such deposits may be found in the tract 

 examined. Before making a mineral or nonmineral classification of 

 withdrawn phosphate lands it is essential to know exactly the distri- 

 bution of the valuable deposits and to ascertain their relation to 

 the legal subdivisions, so that the lands may be correctly described 

 and classified. In making a classification it is also necessary to know 

 the thickness and the number of the phosphate beds ; the proportion 

 of phosphoric acid (P2O5), or its equivalent expressed as tricalcium 

 phosphate, that they contain; and the depths below the surface at 

 which they occur. 



The facts determined and recorded in the course of a field examina- 

 tion are shown on the township maps submitted by the field geologists 

 to the land-classification board. If any of the data collected can not 

 be represented on the maps they are supplied in the form of descrip- 

 tive notes. One of the most essential factors to consider in the classi- 



