REPORT ON THE GEOLOGY OP NORTH AMERICA. 35 



colonized a tract in the bed of the ocean, much deeper than 

 would be compatible with the known habits of the oyster, until 

 the occurrence of an alteration of the level, from what may be 

 termed deep sea, to a shallow estuary; the clay enveloping 

 the lower shells, indicating perhaps the ooze peculiar to the 

 one, the oysters above lying in a sediment equally characteristic 

 of the other, namely, sand. The supposed change of circum- 

 stances would be just such as would tend to banish the pelagian 

 shells and to introduce in abundance a race like the oyster, de- 

 lighting in protected coves and shoals. A second elevation of 

 the region must have taken place to bring both beds to their 

 present position above high tide, and to expose them to receive 

 their covering of diluvial pebbles, which is said to be thick and 

 well characterized. The deposit at the mouth of the Potomac 

 is the only one of its exact period at present discovered, though, 

 from the appearance of successive small upheaves at various 

 times, along nearly the whole Atlantic plain, it seems reason- 

 able to look for the occurrence of beds of nearly similar age in 

 other sections of the coast; if, indeed, we have not already 

 found one at Charleston, and on the adjoining coast of South 

 Carolina, 



Formations of the older Pleiocene and Meiocene periods. — 

 These deposits which, before the appearance of Mr. Lyell's no- 

 menclature, went under the name of the 'upper marine' formation, 

 from a supposed identity with the beds of that name in certain 

 basins in Europe, constitute by far the most extensively distri- 

 buted portion of our tertiary beds yet explored. There is even, 

 I think, reason to believe that we have deposits of a wide range 

 w^hich may be separately classed, some in the meiocene, others 

 in the older pleiocene period, though in many cases it is not 

 possible, from the present limited catalogue of their fossils, al- 

 ways to infer with precision their exact comparative age : on this 

 account, and also from the circumstance that their mutal geo- 

 graphical connexion has never been yet properly examined, I 

 prefer, for temporary convenience, to treat them for the present 

 under one head, as the deposits of one great middle tertiary 

 group. These are clearly separated by a well-marked and per- 

 haps wide interval from the more recent newer pleiocene on 

 the one hand, and the more ancient eocene on the other, though 

 there is some ground to believe that they will be found to blend 

 into each other by various shades of approximation, — unless, 

 indeed, future researches may point out among them an addi- 

 tional number of distinguishing fossils. It is very probable, 

 from present indications, that when we have investigated the 

 deposits of these two periods we shall find it requisite to inter- 



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