38 AGRICULTURAL ANALYSIS 



erable size. When these were the interstices among blocks of 

 chert, there resulted the coarse breccia. The cavities were 

 wholly or in part filled with compact phosphate, which shows, by 

 differences of texture and color, that it was deposited from solu- 

 tion in successive layers. In some cases it appears that the cavities 

 were in a pure limestone. After they had been to a greater or 

 less extent filled by the phosphate, by reason of some change in 

 conditions, the limestone was dissolved, leaving the phosphate dis- 

 seminated through the residual clay, which represents the original 

 insoluble constituents of the limestone. Finally, in some places, 

 instead of finding open cavities in which the phosphate might be 

 deposited, the solution, before emerging at the surface, came in 

 contact with a siliceous limestone under conditions such that a 

 transfer of materials was effected. The more soluble carbonate 

 was taken up and the less soluble phosphate was deposited in its 

 place. These conditions gave rise to the stony variety, in which 

 the phosphate is clearly seen occupying the place originally held 

 by the carbonate. 



If this explanation of the origin of these phosphates is the cor- 

 rect one, some important economic conclusions follow as to the 

 extent of the deposits. So long as the waters were percolating 

 slowly and at considerable depths they would take up rather than 

 deposit phosphate. They would find conditions favorable for the 

 latter process only comparatively near the surface, where the 

 excess of carbonic acid might readily escape. Hence the deposits 

 must not be expected to extend to any considerable depth. They 

 are essentially superficial pocket deposits, and in most cases their 

 depth will be limited by the depth of the residual mantle of chert 

 and clay with which they are so intimately associated. It seems 

 probable that the stony variety may extend to greater depths than 

 any of the others, since the process to which it is attributed is 

 one which does not depend directly on surface conditions the 

 escape of carbonic acid and the evaporation of the solution but 

 upon some conditions, not fully understood, favoring replace- 

 ment. 



The deposits were probably much more extensive than now. 

 The deepening of the valleys has removed the greater portion of 



