340 New Locality of the Infusorial Stratum in Virginia. 



our knowledge, it may be impossible to determine the precise 

 value of the characters to be derived from deposits of microscopic 

 fossils, yet whatever that value may be, it must increase with 

 every discovery tending to show the extension and cotempora- 

 neous character of such deposits. 



The existence of the " infusorial stratum" at Petersburg, taken 

 in connexion with the discovery mentioned above, proves that it 

 is no partial deposit, but one that may be looked for over our 

 widely extended tertiary. Within the precincts of Petersburg, 

 on Poplar Lawn, the western edge of the eocene is cut through 

 and exposed in a small stream which crosses Walnut Street. 

 For about the distance of one quarter of a mile southeast 

 from this point, the eocene is again hid by what are called the 

 "Heights," being immense diluvial beds which cap the high 

 grounds around Petersburg. These beds are again cut through 

 by a stream which forms the boundary of the corporation ; and 

 on the west side of the valley of this stream, the infusorial stra- 

 tum occurs. During the summer I frequently visited this inter- 

 esting locality, but was deterred from making a closer examina- 

 tion, by looking in vain for the very low specific gravity so 

 characteristic of specimens from the Richmond stratum, which 

 is found only in specimens taken from the middle of the stratum; 

 and in a similar manner, doubtless, others have been misled. A 

 recent visit to the Richmond deposit, satisfied me, as far as rela- 

 tive position and other external characters could, that the two 

 deposits were identical. I was now encouraged to attempt, for 

 the first time, a microscopic examination of the Petersburg stra- 

 tum, and had the satisfaction to detect a few of the forms figured 

 in this Journal, Yol. xlii, No. 1. To remove every doubt, a few 

 grains were enclosed in a letter to Prof. Bailey, West Point, who 

 politely writes, that he " found without difficulty, a number of 

 the most characteristic microscopic fossils of the Richmond bed." 



Prof. Rogers. I believe, has not referred the Richmond deposit 

 to either division of the tertiary. I think it will prove to be a 

 lower stratum of the miocene, at least such is the fact with re- 

 gard to the Petersburg deposit, as may be seen by the casts of 

 Pectens and other miocene fossils, which separate it from the 

 eocene, whilst at Richmond similar casts are found between the 

 top of the stratum and the overlying diluvium. The Petersburg 

 stratum is not so conspicuous an object as that of Richmond, 



