﻿Mr. Hall on Microscopic Shells found in Marl Slate. 293 



of a beach bordering a coral reef, and abounding in shells and 

 fragments of corals cemented together by calcareous and argilla- 

 ceous mud. It is by washing this marly clay, and examining it 

 by the aid of a magnifier, that Mr. Carly separates the fossils 

 under consideration, and also, as he informs me, others of a still 

 smaller size. 



The age of these strata has been settled beyond a question, 

 and they are universally regarded by geologists as synchronous 

 with the lower Silurian of England and Wales, and as correspond- 

 ing to the rocks of the Mohawk and Hudson valleys in New York. 

 This identity is manifest from a comparison of numerous organic 

 remains, and from stratigraphical succession. The principal dif- 

 ference, and that which gave rise to doubts of the correctness of 

 the inference, is the great proportion of calcareous matter existing 

 in the rocks of the West, while those of the Hudson valley are 

 nearly destitute of that material. This fact however is perfectly 

 reconcilable with all the other appearances, and we find the fossil 

 testacea of these western rocks much more numerous and better 

 developed than the same species in New York. So great indeed 

 is the disparity in size and form of the same species, that several 

 of them are not recognized as the same, and have been described 

 under different names. 



Among the fossils known to appear in the same association as 

 the minute species before us, are the Isotelus gigas, I. rnegistos, 





Ti 



r 



Delthy 



pa, Strophomena alternata, S. sericea, and other species ; Orthis 

 testudinaria, O. callactis, and other species ; Cypricardia rnodio- 

 laris, C. angusta, Cyrtolites ornatus, Bellerophon bilobatus, and 

 numerous other forms. 



Several of these forms are common both to the limestones of 

 the Mohawk valley and to the shales of the Hudson River group ; 

 but at the West, the great augmentation of calcareous matter has 

 rendered it impossible to define with the same exactness the sub- 

 divisions of the groups as they appear in New York. Without 

 dwelling upon this part of the subject at the present time, I will 

 briefly notice the fossils in question, presuming that, from their 

 great numbers, uniform size, and apparent perfection and matu- 

 rity of growth, they are adult individuals. No species of larger 

 size which can be identical with these, are known in the same 



Vol. xlviii, No. 2.— Jan.-Marcb, 1845. 38 



