574 MESOZOIC FLORAS OF UNITED STATES. 



GENERAL REMARKS AND CONCLUSIONS. 



The localities described in the preceding pages are all that have 

 furnished fossil plants from horizons below the Raritan. The size of 

 the collections coming from the various localities can not be judged 

 from the number of forms given as found 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 number of identifiable 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 Siu'vey 

 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 underwent 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 entombment and the unfitness of the rock 

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

 the Arundel and Patapsco, but they give simpty 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 belong to the Rappahannock or James 

 River member 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 but 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 Mar3dand localities whose fossil plants have been 

 described in this paper. The plants collected bj^' 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 Virginia 

 shows the unexpected fact that the large Maryland collections add 

 very few new forms to the Lower Potomac flora, as made known in 

 Monograph XV. 



