120 TRANSACTIONS OF THE [FeB. 2S: 



the finer particles in the water. "When there was not enough 

 'phosphate to form nodules, it merely coated the grains of sand, 

 or mingled with the mud which afterwards formed the cement 

 of the sandstone. 



The second hypothesis has been applied especially to the. 

 phosphate beds of South Carolina. The difficulty especially in 

 the way of its application here lies chiefly in the regular shape 

 and distinct boundaries of the nodules. These seem almost too. 

 regular to be due to the concretion of either carbonates or 

 phosphates. Moreover, the entire absence of the coarser grains 

 composing the rock around would seem to be against this 

 theory. A second infiltration would seem to be required to 

 exf)lain the filling of the fossils with carbonate of lime unless, 

 for some reason this crystalline carbonate was not attacked by 

 phosphoric acid. The impermeable laj'er of clay or mud in. 

 Zone 2 would, perhaps, serve to prolong the action of the phos- 

 phatic waters on the lime, and thus make a specially rich layer. 

 In Zone 3 there was, perhai)s, only enough phosphoric acid 

 leaching through, to convert into phosphate such carbonate of 

 lime as was in an amorphous and uncompacted condition, leaving 

 the crystalline grains but little changed. 



The replacement required by the third hypothesis has been, 

 shown to have very probably occurred in the Cretaceous phos- 

 phate beds of England, to which these deposits seem to be very 

 nearly related. The apparent objections to its adoption have 

 been already discussed. 



With our present knowledge of the nodules, it is not easy 

 to say which of these supj^ositions is the most probable one. 

 Altogether, it seems very likely that they originated as. 

 organic bodies, or by some form of concretionary action. The 

 large amount of iron is a somewhat unusual feature. I have 

 come across only one or two analyses showing so high a 

 percentage of iron, but iron phoshate is probably not uncom- 

 mon in this form, though the dejiosits of economic importance 

 are chiefly lime phosphate. 



Further study of these beds will probably bring to light a. 

 large and interesting fauna, and will also, it is to be hoiked,, 

 throw more light on this rather obscure problem. 



