FLORA OF OLDER POTOMAC FORMATION. 381 



Creek the coarse sands appear to rest on the granite. Nowhere could I 

 find the basal clay forming a bed below the sands. 



The high bluffs on the left bank of the Appomattox at Point of 

 Rocks show the sands more thorougly lithified than at any other point 

 in the Potomac formation. In fact they are very hard and resemble 

 quartzite, and also resemble the white rocks for which Professor Uhler 

 named his Albirupean formation, but, unUke these, and agreeing in this 

 respect with the Rappahannock freestone, they contain small clay inclu- 

 sions. No signs of the Potomac could be found above Richmond or 

 Petersburg, but the Tertiary beds extend far up the James and rest on 

 the crystalline rocks. 



On October 16, 1892, I discovered chocolate-colored clays below 

 Mount Vernon yielding fine impressions of plants, of which I made a 

 small collection. These lie upon the freestone and underlie the Aquia 

 Creek beds. I named them the Mount Vernon clays. On November 

 6 a much larger collection was made from the same locahty. This 

 entire region, including all the bluffs of the Potomac below Mount 

 Vernon as far as Masons Neck, was further investigated during the 

 autumn of 1892, the last excursion being made on the 18th of Decem- 

 ber. In the work-I was voluntarily assisted by Mr. Victor Mason. The 

 Mount Vernon clays were found in White House Bluff, and plants were 

 collected from them there. They were traced up Doag Creek and as 

 far west as the railroad near Pohick Creek. 



Active operations were also carried on in the environs of Washington 

 on both sides of the Potomac and Anacostia rivers. Owing to extensive 

 excavations at nearly all points, the conditions were favorable for studying 

 the relations of the several members of the formation, and making sections 

 at points which were undergoing rapid change, so that most of them 

 could never be seen again to the same advantage. I availed myself of 

 these opportunities, visiting and taking careful notes on all the new 

 exposures. The most important results were obtained in the northwest 

 section of Washington City. One of the most instructive of these ex- 

 posures was that on Ontario avenue, on the south side of Lanier Heights. 

 At the west end the decomposed crystalline (micaceous schistose) rocks 

 are seen underlying the white Rappahannock sands. The crystallines 

 are strongly tilted to the east, and the sands lie on their inclined surface 



