192 Holmes's Notes on the Geology of Charleston. 



amination of this deposit, and assisted him in taking its level- 

 As there was much water in the well, this point could not be 

 accurately determined ; but it was estimated to be at least three 



feet above high water mark. 



The low country of South Carolina is well watered by a num- 

 ber of bold streams, which for the most part, are connected a few 

 miles above their mouths by creeks passing between the sea 

 islands of the coast and the main land, providing a safe and con- 

 tinuous inland passage for steamboats and small craft, from 

 Georgetown to Savannah. In one of these passages, the most 

 extensive exposure of the Post-pliocene and its superimposed 

 diluvium or alluvium may be seen at the plantation of William 

 Simmons, Esq., on Yonge's Island, about twenty-five miles S. W. 

 of Charleston ; it is separated from Wadmalaw Island by the 

 river of the same name, (sometimes called " the Sound,") which 

 also unites the waters of the Edisto and Stono rivers. The rapid 

 current which sets in upon the bluff of Yonge's Island, sweeps 

 off with every southeasterly gale, portions of this noble bluff, 

 carrying with it the cotton plants that grow in the field above, 

 the level of which is about twenty-two feet in its highest part 

 from the surface of the shell bed forming the beach at its base. 



Most of the shells in this bed are in a perfect state of preserva- 

 tion, and when denuded as they are for a great extent, of their 

 covering of sand and clay, are seen in clusters of a dozen or more 

 each species forming a little colony or family, occupying the 

 positions in which they lived and died. The adaptation of the 

 mud in which they are imbedded, and which is so suitable to the 

 habits of the testacea, is beautifully exemplified by the fact of 

 recent species taking up their abode in it ; and hence the diffi- 

 culty sometimes, of determining the recent from the fossil of the 

 same species; the dead shells of the former being often found in 

 the same patches with the latter. 



The first specimen of Pholas costata discovered, was put down 

 in our catalogue of fossil species, and it was not until many visits 

 had been made, and the bed dug up one or two feet, that the 

 living ones were found. Two species of Pholas, (P. costata and 

 P. truncata,) with Petricola pholadiformis, are abundant. Occa- 

 sional specimens of Pholas oblongata are also to be had. Thus 

 we were obliged to discard many fine specimens as recent, which 

 had been preserved as fossil. The living Ranella caudata, Venus 

 mercenaria, and Cultellus cariboeus, have also been found at this 

 locality. 



The characteristics of the Post-pliocene testacea, are supposed 

 by Professor Agassiz, who examined cursorily a few of the most 

 prominent specimens in my collection, to be indicative of specific 

 differences from those of similar forms now inhabiting the waters 

 of the coast. I acknowledge my obligations to this gentleman 



7 



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