Geology of the Virginias. 201 



streams traversing it afford numerous exposures of Tertiary 

 strata, with their characteristic lithological features and fossil 

 remains. These are readily accessible on several lines of rail- 

 way, and along the rivers below Fredericksburg, Richmond and 

 Petersburg. 



We shall direct attention to only a few points of special interest 

 in connection with Prof. Rogers's work in this interesting field. 

 As early as 1835 he writes, "The existence of an extensive 

 Eocene formation in eastern Virginia is now for the first time 

 announced as furnishing an interesting step in the geological 

 inquiries which are now on foot," etc. 



The Miocene covers the greater part of the Tertiary of Vir- 

 ginia, but we shall note only two points of special geological 

 interest, as illustrating the appreciation the scientific world has 

 placed upon the work of Rogers in this portion of his great 

 field. His description of the cliffs near Yorktown, and the de- 

 tailed account he gives of the contents of their fossil beds, have 

 made that region classic ground for the modern geologist, and 

 have given the Miocene the title of " Yorktown Group " — a 

 title found in almost every extended discussion of the Tertiary 

 age among American writers. 



A second point relates to the history of the first discovery of 

 the famous infusorial bed, which crops out conspicuously along 

 the slopes of the hills on which the city of Richmond stands, 

 and at several other places in Virginia, as well as on the Mary- 

 land side of the Potomac. Although this interesting feature 

 of our geology has for years commanded the attention and ad- 

 miration of the scientific world, and the beautiful picture of its 

 diatoms developed by Ehrenberg's microscope, become familiar 

 to the eye of every geologist, we doubt whether many of our 

 younger co-workers know much about the history of its first 

 discovery. We deem it proper, therefore, to say that, after 

 giving a general account of his first discovery and microscopic 

 examination of the contents of this wonderful deposit of what 

 was then regarded as "infusorial animals," Rogers says, "In 

 view of these interesting facts, the discovery of the infusory 

 Stratum, as one of the members of our series of Tertiary de- 

 posits, cannot fail to be regarded as an important addition to 

 our knowledge of the Tertiary of this country, and has the 

 greater interest at present, as being the first example yet ob- 

 served in the United States of the occurrence of infusorial 

 remains in any but the most recent geological formations." His 

 latest view of the geological position of this formation is, that 

 it is near the base but still within the Miocene group. We are 

 ready, from personal observations, to accept this conclusion. 



Virginia and West Virginia together present a vast field, em- 

 bracing within itself an almost complete system of geological 



