232 THE WASTE AND CONSERVATION OF PLANT FOOD. 



phosphate of lime is easily uuderstood wiieu the character of this lime- 

 stone is considered. 



Shaler, as quoted by Eldridge in the work already referred to, refers 

 to this characteristic of the limestone and says that the best conditions 

 for the accumnlatiou of valuable depos.its of lime phosphate in residual 

 debris appear to occur where the i)hosphatic lime marls are of a rather 

 soft character; the separate beds having no such solidity as will resist 

 the percolation of water through innumerable incipient joints such as 

 commonly pervade stratified materials, even when they are of a very 

 soft nature. 



Eldridge is also of the opinion that the remains of birds are not 

 sufficient to account for the whole of the phosphatic deposits in Florida. 

 He ascribes them to the joint action of the remains of birds, of land 

 and marine animals, and to the deposition of the phosphatic materials 

 in the waters in the successive subsidences of the surface below the 

 water line.' 



POTASH DEPOSITS. 



In the foregoing pages I have tried to set clearly before you the dif- 

 ferent ways in which the waste of nitrogen and phosi)horic acid has 

 been recovered by nature in a form suitable for restoration to arable 

 fields. In the case of potash, however, we have seen that this element 

 is not restored by the processes already mentioned, in amounts pro- 

 j)ortionate to nitrogen and i)hos])horus. Potash salts, being extremely 

 soluble, are likely to be held longest in solution. Some of them, of 

 course, are recovered in the animal and vegetable life of which we have 

 spoken, but the great mass of potash carried into the sea still remains 

 unaccounted for. The recovery of the waste of potash is chiefly secured 

 by tbe isolation of sea waters containing large quantities of this salt 

 and their subsequent evaporation. Such isolation of sea waters takes 

 place by means of geologic changes in the level of the land and sea. 

 In the raising of an area above the sea level there is almost certain to 

 be an inclosure, of greater or less extent, of the sea- water in the form 

 of a lake. This inclosure may be complete or only partial, the inclosed 

 water area being still in communication with the main body of the sea 

 by means of small estuaries. If this body of water be exposed to rapid 

 evaporation, as was doubtless the case in past geologic ages, there will 

 be a continual influx of additional sea water through these estuaries to 

 take the place of that evaporated. The waters may thus become more 

 and more charged with saline constituents. Finally a point is reached 

 in the evaporation when the less soluble of the saline constituents 

 begin to be deposited. In this way the various formations of mineral 

 matter, produced by the drying up of inclosed waters, take place. 



'For .au elaborate discussion of pliosiihate deposits consult Gltes Mineraux, par 

 Fuclis et DeLauuy, page 309 et seq. 



