138 W. J. McGee — Three Formations of 



are about equally represented in the entire section ; and the 

 transition is recorded not only in the commingling of types 

 but in a measure by the assumption of modern external forms 

 while the plants yet retained the archaic internal structure. 



It is significant that well preserved leaf impressions are not 

 found in the formation about Washington, in the lower mem- 

 ber at the head of Chesapeake ba}^ in the deposits near the 

 Schuylkill, in the " Yellow Rocks " at Trenton, nor in the im- 

 mediate vicinity of Richmond — the plant-bearing localities on 

 the James being several miles from the fall-line, — i. e., about 

 the mouths of the Mesozoic prototypes of the great rivers of 

 the region. 



Taxonomy. — The general facies of the wonderfully rich 

 flora most closely approaches that of the middle and lower 

 Neocomian of Greenland and Europe, and Fontaine is disposed 

 to refer the formation to the lower Cretaceous on this evi- 

 dence ; but since the flora is manifestly too nearly unique to 

 permit precise correlation, and since Marsh finds the verte- 

 brate remains to be distinctly upper Jurassic, the formation 

 must be at least provisionally assigned to the latter period. 



The Tuscaloosa formation of Alabama appears to be the 

 precise equivalent of the Potomac ;* and Hill has recently col- 

 lected data indicating that the Trinity formation (Dinosaur 

 Sand)f of Texas and Arkansas is coeval with the Potomac and 

 Tuscaloosa.;}: 



Sources of Materials. — Roughly classified, the materials of 

 the Potomac formation are (1) quartzite pebbles, (2) quartz 

 pebbles, (3) arkose, (4) quartzose sand, (5) plastic clay, and (6) 

 various combinations of these. 



1. The distribution of the quartzite pebbles is significant : 

 They are abundant and large on James river but diminish rap- 

 idly both in abundance and size about to the Appomattox on 

 the south and the South Anna on the north, where they finally 

 disappear — none occurring on the Nottoway or Roanoke, nor 

 on the North Anna, the Rappahannock, or the intermediate 

 smaller streams. They reappear near the Occoquan, and in- 

 crease rapidly in abundance and size to the Potomac, where 

 they form a predominant element in parts of the formation ; 

 but they again diminish in size and abundance northward, be- 

 coming inconspicuous north of Baltimore, to once more increase 

 in size and number about the Susquehanna, where the outliers 

 consist almost exclusively of well rounded quartzite pebbles 

 and bowlders. North of the Susquehanna like relations obtain, 

 so far as size is concerned, the pebbles gradually diminishing 

 in size over the Susquehanna-Schuylkill divide, enlarging 



- Bulletin U. S. Geol. Survey, No. 43. 



f American Naturalist, vol. xxi, 1887, 172. % Science, vol. xi, 1888. 



