Rogers.] 102 [May 19, 



cover tliem at any of the excavations where the coarser materials are 

 exposed, as well as in the piles of cobblestones in the neighborhood. 



In the specimens exhibited to illustrate this paper, collected chiefly 

 at Washington and Richmond, it will be seen that the casts of Scoli- 

 thus are very distinct and abundant. These masses are from two to 

 six inches in diameter, but in some of the localities much larger spec- 

 imens may be seen crowded with the fossil. Along with them are 

 occasionally found rounded masses or cobbles of fossiliferous sandstone 

 and of conglomerate, referable to higher positions in the Appalachian 

 series, ranging probably to the carboniferous rocks. The absence from 

 these deposits of fragments derived from the limestones, shales and 

 argillaceous slates of the Appalachian belt, is readily accounted for 

 by the comparative ease with which such materials would be disinte- 

 grated by the mechanical and chemical actions concerned in their 

 transportation and deposition, and the same explanation accounts for 

 the fact that so few fragments of the granites, schists and gneissoid 

 and hornblendic rocks of the wide intervening belt have been pre- 

 served in this formation, and that it retains little distinctly represent- 

 ing these rocks, except an abundance of quartz gravel and cobbles, 

 derived from them. 



The deposit in question extends at Washington over the entire 

 plain on which the city is built, having an average of seventy-five 

 feet, and rising on the north to about one hundred feet above mean 

 tide. Thence it spreads over the adjoining slopes, covering the high 

 ground on which Columbian College is situated, and the still higher 

 hill of the Soldier's Home, which is more than two hundred feet above 

 tide. At the latter locality the rolled fragments have a less average 

 size than at the lower level, though still often several inches in diam- 

 eter. In the neighborhood of the Capitol, and in the railroad cutting 

 near the Navy Yard, they are often as much as a foot in diameter, 

 and a recent excavation near Georgetown, some forty feet above the 

 creek, has brought to light masses of these transported rounded rocks 

 of still greater dimensions, some of them large enough to be called 

 boulders. 



Although the surface formation in question shows itself in, and 

 adjoining, the valleys of all the principal streams in the Middle 

 States, the fragments of paleozoic rocks have thus far been observed 

 only in the deposit as exposed in those river valleys which penetrate 

 westward and northwestward as far as, or into, the Appalachian belt. 

 It is reserved for further observation to ascertain whether they are 



