THE PATUXENT FORMATION. 65 



well shown along the James Eiver where just below Eichmond the Patuxent 

 deposits extend to tide, while about a mile and a half above Dutch Gap the 

 crystalline rock? reappear along a small stream which empties into the 

 James Eiver. The Patuxent beds where they have been differentiated at the 

 outcrop have a thickness of from 250 to 300 feet. 



Paleontologic character. — Although the Patuxent deposits are in general 

 unfossiliferous because of their coarse character, nevertheless a large flora 

 has been collected from clay balls and lenses and the more argillaceous 

 sands. This flora has been elaborated by Professors Ward and Fontaine 

 in various publications of the Y. S. Geological Survey" and is undergoing 

 revision by the writer at the present time. 



It includes a large element made up of survivors from the older Mesozoic 

 and is rich in species and individuals referred to the fern genera Clado- 

 phlehis and Ont/cJiiopsis. A variety of cycad fronds testify to the 

 abundance of this type of plant and is emphazised by the presence of 

 numerous silicified trunks in the Maryland area. Perhaps the most strik- 

 ing of the cycad remains are the splendid fronds referred to the genus 

 Dioonites which are extremely abundant in the Dutch Gap region. Other 

 forms of cycads present include species of Podozamites, Zamiopsis, Nilsonia, 

 Ctenis, Gtenopliyllum, most of which are confined to the older Potomac and 

 do not occur in the Patapsco deposits although some of them occur in the 

 Arundel formation in Maryland. 



Among the conifers are species of Baiera, Bracliy pliyllum , Sphenole- 

 pis, Frenclopsis, Nageiopsis, Artltrota.ropsis and Cephalotaa-opsis, repre- 

 sentative of various subfamilies which in the modern flora are largely 

 natives of other continents. Supposed Angiosperms, the most ancient 

 known, are represented by the archaic genera Rogersia, Proteaephyllum, 

 and Ficopliyllum, which may well represent the foliage of some Lower 

 Cretaceous member of the Gnetales and not angiospermous plants, a point 

 not yet definitely settled. 



Areal distribution. — The Patuxent formation has been recognized chiefly 

 in the Chesapeake Bay drainage basin in Maryland and Virginia, although 

 the Nottaway Eiver exposures in southern Virginia are very probably 

 referable to this formation. 



In the Virginia area the Patuxent formation is found near the head of 

 tide in the leading drainage basins lying directly upon the very uneven 

 floor of crystalline rocks which constitute the eastern margin of the Pied- 



aMonograph XV. 1889. 4°. XIV. 377 pp.. 180 pis. Fifteenth Ann. Report, 

 1893-94. Monograph XLVIII, 1906. 



