Association of American Geologists and Naturalists. 257 



animals of the North American ocean ; for probably the greater num- 

 ber of species are still living there, as they have already been found 

 abundantly on the German coast of the North Sea. The geological 

 position of the strata must be determined by the order of superposi- 

 tion, the larger included organic remains, &c., as it cannot be decided 

 by means of the infusoria." 



In the same memoir, Ehrenberg acknowledges in terms of just 

 praise, the value of the careful researches of Prof. Bailey, gives lists 

 of the fossil infusoria from two deposits discovered by Dr. Charles T. 

 Jackson in Maine, and states that the knowledge of the microscopic 

 organisms of Massachusetts has been much extended by Prof. Hitch- 

 cock through the discovery of several deposits there, during his geo- 

 logical survey. All of these deposits are referred, I believe, by their 

 discoverers, to the most modern epoch. 



POST PLEIOCENE PERIOD. 



The later tertiaiy strata of this country, though existing in but cir- 

 cumscribed patches, possess much interest on account of the questions 

 suggested by their organic remains, concerning the changes which this 

 portion of the globe has undergone in the level of its surface, and in 

 its temperature during the epochs next antecedent to the introduction 

 of the human race. Of these post pleiocene, or pleistocene deposits 

 as they have been called, several small areas have been described by 

 Mr. Conrad. The principal ones are in St. Mary's County, Maryland, 

 and on the Neuse Eiver below Newbern in North Carolina. To the 

 same period he also refers the numerous small beds of Ostrea Virgin- 

 iana, which skirt the low margins of the islets and rivers in Delaware, 

 Maryland, and Virginia, and by many people attributed to the agency 

 of the aborigines. The deposit on the Potomac in St. Mary's County, 

 is especially interesting for containing several southern species, one 

 in particular, an estuary shell, the Gnathodon cuneatus, now restricted 

 to the warmer waters of the Gulf of Mexico. Mr. Conrad infers that 

 the association of the Gnathodon, Myiilus hamatus, and Area ponde- 

 rosa, with species now inhabiting our coast as far north as Massachu- 

 setts, indicates a climate at the period of the formation, equivalent to 

 that of Florida ; perhaps we should say, an aquatic climate. The 

 cause of the change of temperature which banished these shells from 

 the waters of our middle and southern Atlantic bays, connected as it 

 is with some of the widest questions in our science, will, I doubt not, 

 receive hereafter from American geologists and naturalists, the atten- 



