120 TRANSACTIONS OF THE [ Fes. 23 
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 
explain the filling of the fossils with carbonate of lime unless 
for some reason this crystalline carbonate was not attacked by 
phosphoric acid. The impermeable layer 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, perhaps, 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 beem 
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 suppositions 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 deposits of economic importance 
are chiefly lime phosphate. 
Further study of these beds will probably bring to ight a 
large and interesting fauna, and will also, it is to be hoped, 
throw more light on this rather obscure problem. 
