THE LOWER PINE BELT, OR SAVANNA REGION. 49 



fossil teeth found far inland, imbedded in the Santee marls, and to those 

 of the fish now living in the vicinity. As to the origin of the phosphate 

 rock, the identity of the fossil shells it contains with those of the under- 

 lying marl make this much certain, that it consists of fragments broken 

 from the irregular surface of the marl, and that its rounded and nodular 

 form was imparted to it by the action of the waves and currents to whicli 

 it was subsequently subjected. The important question of how a marl 

 containing originally 60 per cent, of carbonate of lime and 2 to 4 per 

 cent, of phosphate of lime has been changed into one containing 50 to 60 

 per cent, of phosphate of lime and 5 to 10 per cent, of carbonate of lime 

 remains for consideration. * It is a noteworthy circumstance, that, while 

 the great body of the eocene marls in South Carolina have preserved their 

 constitution almost unchanged, a remarkable change is manifest at the 

 beginning and at the close of the series ; in the buhr-stone on the north- 

 ern border, and in the widely removed phosphate rock on the southern ; 

 in the buhr-stone the original carbonate of lime composing the shells has 

 been replaced b}^ silica, reiulering great masses of rock, that once might 

 have imparted valuable properties to the soils, valueless agriculturally ; 

 in the phosphate region masses of carbonate of lime have been converted 

 into the phosphate, rendering them still more valuable to the tiller of the 

 soil. Two theories have been offered to account for this substitution of 

 the phosphate for the carbonate of lime. 



One theory assumes that the fragments of marl were charged with the 

 sweepings from guano beds formed above them by the congregation there, 

 at some indefinite time in the past, of vast flocks of birds ; in this case, 

 bones of the birds should be among the fossils preserved in these beds. 

 No such remains having been found, but instead the remains of numerous 

 animals, such as the mastodon and elephant above mentioned, and it was 

 thought that immense herds of these animals had collected at one time 

 about the shallow salt lakes in which the nodules were left upon the re- 

 cession of the sea, just as animals now do about the salt licks of Kentucky, 

 and that the phosphoric acid derived from their excrements and remains 

 wrought the change in the marl. To this it is objected that the spots 

 where the most of these bones are found are not the richest in phos- 

 phates ; and while it is by no means probable that the nodules were in 

 all, or even in most instances, formed where they are at present found, it 

 is difficult to suppose that agencies of such local and restricted character 

 as salt licks could account for the conversion of so great a mass of material, 

 over an area so extensive, as that presented by the phosphate formation. 



The other explanation of the formation of these rocks is, that certain 

 mollusks possess the power of separating the phosphate of lime from sea 

 water, and that through their instrumentality the marl, and especially 

 4 



