ORIGIN OR THE HARD ROCK PHOSPHATES. 
49 
of these sources is regarded by Eldridge (1893) as a difficult 
question. Alteration of the limestone and precipitation of phos¬ 
phate from solution are both regarded as having been active in the 
formation of the primary phosphates. Phosphate boulders, 
Eldridge suggests, may have been formed by chemical precipita¬ 
tion of layer upon layer of phosphate, either on a surface exposed 
to the air or within a cavity in the limestone. By continued growth 
in the latter case the cavity would become filled with laminated or 
massive rock which upon the solution of the surrounding materials 
or the complete breaking down of the formation, as in later times, 
would result in a rounded body of phosphate of lime resembling 
a sea rolled boulder. 
Referring to phosphate of lime in sedimentary rocks Eldridge 
says (p. 18), “Its presence in sea-water; its broad distribution in 
both plant and animal life; its occurrence in rocks of all ages, 
even to the extent of economic value; and its special presence in 
limestones, more particularly in Cretaceous and Tertiary lime¬ 
stones, are facts long recognized. Its occurrence in recent time in 
the form of leached and soluble guanos on many of the oceanic 
islands, and the phosphatization of the underlying strata, have also 
been noted by many authorities; the last is by actual observation 
a tangible source, but the features first detailed point t» some other 
and more general origin of phosphate of lime than localized bird- 
deposits, or the but little more widely distributed accumulations 
of animal remains. Its presence in sea-water, after the manner 
of carbonate of lime, though in far smaller amount, is well 
established; both materials are of general occurrence, and each 
play a prominent part in sea-life. The transfer of a consider¬ 
able percentage of phosphate of lime to localities having condi¬ 
tions favorable for its deposition, either in sediments, then 
settling, or on surfaces of rocks already laid down, has doubtless 
been accomplished in many cases through the instrumentality 
of animals secreting it. Oceanic currents may have assisted this 
accumulation. Again, southern waters, swamps, and lands give 
evidence of the presence in them of abundant life, secreting 
phosphate of lime and afterwards returning it to the beds on 
which this life rests.” 
