52 THE POTOMAC OR YOUNGER MESOZOIC FLORA. 



disturbed clay which is frequently associated with these deposits of stones. 

 The rich phint-lajer at Fredericksburg rested on a mass of cobbles. The 

 plants found on the Telegraph road near Potomac Run, and those on the 

 railroad near Brooke, as well as the richest deposit of clay at Dutch Gaj), 

 are all associated with pebbles. 



It is worthy of mention that Professor Rogers, noticing the abundance 

 and large size of the Potsdam cobbles at Point of Rocks, and knowing 

 that they must have been brought from the Blue Ridge, speaks of them as 

 marking a diluvial era of ancient times. At that period many geologists, 

 and Professor Rogers among them, were disposed to explain the phenom- 

 ena of the drift of the glacial period by assuming the existence of a 

 diluvial rush of waters. 



The clays found in the lower Potomac are, as has already been 

 stated, merely subordinate masses occurring occasionally in the sand. 

 They are important simply from the fact that tliey contain all the plant- 

 impressions and nearly all of the undisturbed lignite. Their char- 

 acter and mode of occurrence have already been given. The original 

 isolated pure clay layers are usually thin, being one or two feet thick, 

 but they may suddenly swell up to considerable dimensions, as at the 

 lower entrance of Trent's Reach. They may split up and be lost in the 

 sand, and they generally disappear by thinning out and dovetailing 

 between sand layers. 



The sandy mottled cla3'S formed by the graduation horizontally of a 

 sand into clay are more considerable in amount. This material may be 

 greenish or grayish in color, and when traced horizontally it may assume 

 reddish colors in large masses ; or, again, the colors may be intermixed so 

 as to resemble mottled castile soap. In the vicinit)- of Fredericksburg 

 this kind of material replaces large portions of the normal sands, and it 

 sometimes extends over considerable areas, but without any regularity of 

 occurrence. As illustrating its occurrence, we may take the condition 

 of things seen at Cockpit Point. Here the lowest exposed material is a 

 greenish, gray argillaceous sand. If we follow it, we see it in places be- 

 coming more nearly a pure clay, but it then soon becomes again sandy, 

 and is now marked in huge blotches with a brilliant red color. In places 



