ORIGIN OF THE WHITE PHOSPHATES 37 



phate of lime. When water which has slowly percolated through 

 the rocks at some depth emerges at the surface or into a cavity 

 in which it is no longer subjected to pressure, the excess of car- 

 bonic acid escapes, and the substances which had been held in 

 solution by means of that acid may be redeposited. Thus many 

 springs are now forming about their exits extensive deposits of 

 materials which they have dissolved in the course of their under- 

 ground passage. The most common spring deposits are calcare- 

 ous, although siliceous and aluminous deposits are not uncom- 

 mon, particularly in the case of thermal waters. When several sub- 

 stances are held in the same solution the least soluble will gen- 

 erally be the first to separate, and hence will form deposits nearer 

 the exits. Also, when a solution of a difficultly soluble substance, 

 as lime phosphate, comes in contact with one which is more easily 

 soluble, as lime carbonate, there is generally an exchange effected 

 the more soluble substance is taken up and the less soluble one 

 is deposited in its place. 



A simple application of these principles suggests the probable 

 mode of formation of these deposits. The altitude at which they 

 are found indicates that they were formed when the valleys of the 

 region had about two-thirds of their present depth. The region was 

 probably heavily forested, the decay of vegetation furnishing an 

 abundant supply of organic acids to the percolating surface 

 waters. It was also a region of. sluggish streams, the valleys of 

 which may have been to some extent occupied by swamps. The 

 waters, thus highly charged with organic acids, descending 

 through the more or less porous formations which occupy the 

 higher portions of the country, dissolved calcium carbonate, and, 

 in less quantity, calcium phosphate. The former, by reason of its 

 greater solubility, was carried into the streams and thence to the 

 sea. The phosphate, however, was deposited, the form of the 

 deposits being modified by local conditions. In some cases the 

 waters containing these substances in solution found an outlet in 

 a mass of fragmental chert derived from the decay of the over- 

 lying formations. Under such conditions the breccia was formed, 

 the phosphate merely cementing the fragmental material. In 

 other cases the waters flowed through open cavities of consid- 



