210 DESCRIPTION OF YBRTEBRATE REMAINS, CHIEFLY 



Coosaw, Asheepo, and other rivers. According to Prof. Holmes, from "fifteen to 

 eighteen inches may be considered the average thickness of the stratum of the 

 phosphate rocks."* 



The exact stratigraphical relations of the beds and the relative age of these 

 and contiguovis strata have not been as thoroughly investigated as is desirable, and 

 in many cases the particular horizon to which belong the fossils that have been 

 discovered has not been positively determined. According to Prof. Holmes the 

 phosphate beds are of the post-pliocene period and overlie strata pertaining to the 

 pliocene period, and these are again succeeded by a soft marl rock of eocene age, 

 the whole being covered by modern alluvium. 



The phosphatic rocks, or nodular masses of the phosphate beds, said to contain 

 as high as 60, or even more, per centum of calcium phosphate, are of irregular 

 shape, and range in size from small pieces up to masses of a thousand pounds or 

 more.f They contain many casts of molluscous shells, which appear to be of the 

 same forms as those which occur in the eocene or miocene marl rock beneath. 

 They also frequently contain imbedded bones and teeth, mainly those of marine 

 Fishes and Cetaceans. 



The phosphatic nodules are supposed to have been derived from the tertiary 

 marl bed beneath, and are considered to be detached and altered fragments from the 

 surface of that bed. The irregular, eroded, and porous masses have the appearance 

 of being detached and water-rolled fragments of the tertiary marl rock after it had 

 been tunnelled by various boring mollusks. It is, indeed, not improbable, as has 

 been suggested, that in the later part of the eocene or miocene period and subse- 

 quently the easily penetrated rock was bored and rendered spongy by the incessant 

 labors of multitudes of Gastrochcena, Petricola, Pholas, etc. At the time or later, 

 neighboring and superficial islets, the resorts of myriads of sea fowl, may have 

 furnished the material which, when washed with the ocean and mingled together 

 with the decomposing remains of marine animals, supplied the element for con- 

 version of the porous marl rock into the more valuable phosphatic compound. 



Besides the phosphatic nodules, the Ashley beds present a remarkable inter- 

 mixture of the remains of marine and terrestrial animals, consisting of bones, 

 teeth, coprolites, shells, etc., derived from the contiguous formations of various 

 ages from the early tertiary to those of a comparatively recent period. 



Of remains of Vertebrates, those of Fishes and Cetaceans prevail, especially the 

 teeth of Sharks and the vertebrae of Whales. Less frequently there occur the ver- 



* The Phosphate Rocks of South Carolina. By Francis S. Holmes, A.M. Charleston, 1870, p. 70. 



t A nodular mass, on exhibition in the Government building, from Charleston, S. C, weighs 1150 pounds. 



