60 



of Infusorial earth from the Tertiary strata of Virginia and Mary- 

 land, and gave a description of the geological and other conditions 

 in which this and the associated deposits exhibit themselves in 

 and near Richmond in the former of these States. 



The Tertiary formations which underlie the wide plain extend- 

 ing from the seaboard to the eastern margin of the granitic and 

 gneissoid rocks, approach their termination along this meridian in 

 a series of strata which are separated by only a short interval 

 from the irregular granitic floor. A little further toward the 

 west they reach their boundary, partly by a rapid thinning away 

 and in part by abutting along the hill-sides against the indented 

 shore of these ancient rocks, here rising to the level of the gen- 

 eral upland surface. 



In the deep ravines leading into the valley of Shockoe Creek, 

 especially on its w^estern side, we meet with several extensive 

 exposures of the Tertiary strata, one of which embraces nearly 

 the whole thickness of both the Eocene and Meiocene formations 

 as locally developed in this neighborhood. In all these localities 

 the infusorial deposit is found occupying a position immediately 

 above the upper limit of the Eocene strata or separated from it 

 by a thin layer of whitish or of more or less ferruginous clay. 

 Like the associated beds, it fluctuates in thickness as traced from 

 one neighboring exposure to another, varying from twenty to 

 upwards of thirty feet at the diflferent localities on the north side 

 of the valley, and presenting, where measured some years ago, 

 on the opposite or Church-Hill side, a thickness of nearly fifty 

 feet. In addition to the microscopic fossils, which in a more or 

 less perfect condition make up so large a portion of the mass, this 

 deposit presents a few casts of shells of well known Meiocene 

 forms, of wdiich the Astarte undulata may be mentioned as of 

 most frequent occurrence. It also contains imperfectly preserved 

 remains of a slender creeping plant, as well as fragments of woody 

 stems and branches flattened and converted into lignite and in 

 some cases filled in all directions with the perforations of a 

 Teredo. 



The material of the Infusorial stratum is generally of a very ' 

 fine texture, admittinsi; of beins; bruised between the fins^ers into 

 an almost impalpable powder, singularly free from gritty par- 

 ticles. Although usually of a light gray, almost white color, 



