GEOLOGY OF THE TOTOMAC BEDS. 55 



considered typical of this portion of the lower Potomac, and for this reason 

 the section at Baltimore will be given further on. We may, then, witli 

 some probability conclude that the original uppermost portion of the lower 

 Potomac in most of the areas was more argillaceous than the typical sand 

 now representing most commonly the formation. 



I am thus particular in speaking of this possible uppermost inonilx-r 

 for several reasons. It seems tliat, if we can assume its existence, it may 

 explain the difference seen in the facies of the flora found at certain puints 

 It has already been stated that the most noteworthy plants ftuind in the 

 embedded ])owlders at Deep Bottom, also those in the disturbed clay at 

 tlie 72(1 mile-post ne.ir Brooke, and in the bank near the railroad in the 

 vicinity of tlie last locality, are dicotyledons; and that these in many 

 cases have a younger aspect than the plants found elsewhere, as at Fred- 

 ericksburg and at Dutch Gap. 



At the same tin)e, in the places iirst named, the dicot\ledons out- 

 number the ferns, cycads, and conifers, which is not the case at most of tlie 

 Potomac plant localities. Tiie material at Deej) Bottom containing these 

 plants, as well as that at the 72d mile-post, looks as if it were the ruins of 

 this argillaceous upper memlier. The clay hncr in the Ijank near the 

 mile-post, but farther down tlie railroad, looks like an uiieroded remnant 

 of such a member, and this is true of the plant-layer at Baltimore. This 

 latter contains some dicotyledons similar to some of those found near 

 Brooke. It might, then, be supposed that the younger aspect of the 

 dicotyledonous plants at these places is due to the fact that they belong to 

 a flora distinct from that typical of the lower Potomac, and that this 

 occurs in an independent younger group of beds. 



But there are many difficulties in the way of this su))position. The 

 connecting links are too numerous. Too many plants found in the local 

 floras having an older facies occur with tliese. The stratum in White 

 House Bluff, which contains numerous dicotyledons like those occurring 

 near Brooke, is not found so placed that we can admit that it belongs to 

 a group of beds distinct from and younger tiian the normal lower Poto- 

 mac sand. It has twenty to tliirty feet of this coarse sandy matter above 

 it. and it is clearly a member of the typical lower Potomac. In all the 



