1875.] 105 [Rogers, 



stumps of cycads, would seem to bring them into near relation with 

 the formation at Fredericksburg containing similar remains, and to 

 favor their being referred, at least in part, to the horizon of the 

 upper Jurassic rocks. Possibly we may find here a passage-group 

 analogous to the Wealden of British geology. Whatever may be the 

 result of farther discovery, it would seem to be premature at this 

 time to assume the whole of these deposits from the Potomac north- 

 ward, as belonging to the cretaceous series. 



Where the tertiary or the cretaceous rocks are present in this belt, 

 there is, of course, no danger of confounding the superficial gravel 

 and cobblestone deposit with the formation just described, but in 

 their absence, which is usual in the river valleys, this deposit rests 

 immediately on the broken and denuded surface of the secondary, 

 and by the intermixture of materials makes it more difficult to dis- 

 criminate between them. 



Excellent opportunities for observing the contact of the superficial 

 deposit with the denuded and much older formation below, are pre- 

 sented in the neighborhood of Washington, among which may be 

 specially mentioned the vertical cut at the extremity of ICth Street, 

 at the base of the hill occupied by Columbian College, and also the 

 continuation of 14th Street, ascending the same hill. At the former 

 locality the crumbling felspathic sandstone, or slightly adhering sand, 

 is exposed to a height of about thirty-five feet, with a very gentle 

 eastern dip, and having the color, composition and diagonal bedding 

 characteristic of the Fredericksburg and Aquia Creek sandstone. 

 The gravel and cobblestone deposit lying upon it descends with the 

 slope of the hill to the general plain below, resting at a somewhat 

 steep angle against the denuded edges of the underlying beds. 1 

 From this and other localities, it becomes obvious that the latter 

 formation has been deeply and extensively denuded before and dur- 

 ing the deposition of the surface strata, which form the chief subject 

 of this communication. 



At Richmond this gravel and cobblestone deposit presents itself at 

 various heights from the river bank to the tops of the hills, mantling 

 the irregularly denuded surface of the underlying formations; resting 

 at one place on the Upper Miocene, at others, on the infusorial 

 stratum, which lies at the base of the Miocene, or on the Eocene, 

 or on the yet older deposit, referable probably to an upper secondary 



1 Since this was written (April, 1875), the excavation and grading have greatly 

 changed the exposure by covering up much of the lower deposit. 



