:)74 MESOZOIC FLORAS OF UNITED STATES. 



OENKRAL REMARKS AND COXCU-SIONS. 



The localities deserihed in the preceding pages are all that have 

 furnished fossil plants from horizons l)elow the Raritan. The size of 

 the collections coming from the. various localities can not be judged 

 fi-om the number of forms given as fovmd at them, for when determina- 

 ble forms exist no mention is made of the number of specimens that 

 are not determinable. The proportion of these, in collections that have 

 afforded a considerable munber of identifial)le .species, varies much. 

 For example, many specimens from the Arlington localities can not be 

 determined, while hardly a rock fragment from Vinegar Hill or from 

 Covington and Clement streets is without some identifiable imprint. 



The study of the fossils in the collections of the Maryland Survey 

 and the Woman's College of Baltimore makes it plain that the same 

 flora existed in Maryland and Virginia in Lower Potomac times and 

 that it undei-w^ent the same changes with the lapse of time. It appears 

 that there is no important difference between the plants that existed 

 in the times of the deposition of the Patuxent, Arundel, and Patapsco 

 members. The flora is very poorly represented in the Patuxent, prob- 

 ably from the conditions of entoml^ment and the unfitness of the rock 

 to preserve plant remains. Fossil plants are much more abundant in 

 the Arundel and Patapsco, but they. give simply a continuation, and 

 perhaps an amplification, of the Patuxent elements. An important 

 change does not take place until the Raritan is reached. The plants 

 show that nearly all the localities Ijelong to the Rappahannock or James 

 River meml^er of the Potomac of Virginia. The Mount Vernon mem- 

 ber is not shown. This, perhaps, was to be expected, for the Mount 

 Vernon flora in Virginia seems to exist at l)ut few spots, and to be pre- 

 served in local clay lenses only a few feet below the Aquia Creek group. 



The Aquia Creek member of the Potomac seems to be generally 

 absent from the Maryland localities whose fossil plants have Ijeen 

 described in this paper. The plants collected by Professor Ward at 

 Fort Foote, on the Potomac River below Washington, show that it is 

 found there. 



The comparison of the Maryland species with those of ^'irginia 

 shows the unexpected fact that the large Maryland collections add 

 ver\- few new forms to the I^ower Potomac flora, as made known in 

 Monograph XV. 



