42 THE rOTOMAO OR YOUNGER MESOZOIC FLORA. 



showing' four or five feet uucler the Eocene. Here, as is commonly the 

 case in Virginia where the Eocene marine strata rest immediately on the 

 Potomac sand, a layer of large cobbles is found in the top of the latter. 

 The area of Potomac here shown may be called the Hanover area. 



Between the South Anna River and Richmond is another gap in the 

 exposures, none being found until Ricluuond is reached. 



Near Ashland, however, on the surface, there is a large quantity of 

 cobblestone, which seems to be the debris from eroded Potomac. A re- 

 cent well-boring in Ashland disclosed under the Appomattox, and over the 

 older Mesozoic, fifty-six feet of material that may belong to the Potomac. 

 It is highly probable that the formation underlies this interval, but too 

 deeply buried to be reached by the slight excavating power of the 

 streams. 



At Richmond, owing to the deep erosion of James River, the Potomac 

 is shown again. It is, however, barely exposed away from the imme- 

 diate bank of the river, being covered by the Tertiary, both Eocene and 

 Miocene lying above it. Immediately on the river all the Tertiary has 

 been cut away as far down as Deep Bottom, where it makes its first 

 appearance in the river bank. At Richmond, and down the James to 

 Deep Bottom, Quaternary alone is found in the river banks above the 

 Potomac. The same is true along the Appomattox to near Petersburg, 

 where the Appomattox formation, with the Tertiary underlying it, comes 

 above the Potomac. Away from these rivers, in the triangular area 

 between them and between Richmond and Petersburg, where we may 

 expect to find the Potomac, the surface is mostly composed of the Appo- 

 mattox formation, showing in a few places small patches of Tertiary 

 exposed beneath it. This area of Potomac may be called the Petersburg 

 area. Its west margin may be taken as approximately determined by the 

 Atlantic Coast Line Railroad, which runs between Richmond and Peters- 

 burg. Along this margin the Potomac is mostly hidden under its own 

 ruins and the thick covering of Appomattox material. It is to be seen 

 exposed only along the James and Appomattox Rivers, and from what 

 may be seen on these streams the formation is continuous at least as fiir 

 south as Petersburg, twenty miles from Richmond. 



