412 



MESOZOIC, FLORAS OF UMTED STATES. 



of Marvlaml, ami nearly all from that section of the Potoniae l)elt which 

 lies between the city of Baltimore and the District of Cohimhia. 



The width of that zone of the Potomac belt which inclndes the su[)- 

 posed original sources is about 10 miles, the vertical range being from 

 tide level up to 300 feet. 



The following table shows the taxonomy of the Potomac group, as well 

 as the relations of its deposits to the subjacent and superjacent terranes, 

 according to the usage of the Maryland Geological Siu'vey. The accom- 

 panying map and sections published by this Survey, PI. LXXX, shows the 

 distribution of the formations of the Potomac group in Maryland and their 

 stratigraphic relations. The principal localities for fossils have Ijeen 

 added. For a full discussion of the taxonomy of the Potomac beds 

 employed in this paper, together with its correlation with other taxono- 

 mies, the reader is referred to the paper cited on p. 520 above. 



Table showing the relations of the formatio'iUi. of the Potomac group to subjacent and superjacent terranc. 



Group. 



Formation. 



Age. 



Origin. 



Lithology. 



Columbia. 



Talbot Pleistocene. 



Chesapeake . 

 Pamunkey.. 

 Severn 



Wicomico or Sun- 

 derland. 



Lafa_vette. 



Calvert 



Aquia. . . . 

 Matawan. 



I Pleistocene . 

 IPleistorene . 



Potomac. 



Raritan.. 

 Pal apsco . 

 Arundel. . 

 Patuxent. 

 Newark". 



Pliocene 



Miocene 



Eocene 



Upper Cretaceous. 



Lower Cretaceous. 

 Lower Cretaceous. 



Upper Jurassic 



Upper Jurassic. .. 

 Tria-ssic 



Fluviatile Clay 



Estuarine and niarine Loam, peat, sand. 



Ciravel and conglomer- 

 ate. 



I 



Marine ' "Marlite,"etc. 



Marine Glauconitic marls. 



Marine ' Clay marls, glauconitic 



and carbonaceous. 



Estuarine ; Sands and clays. 



Estuarine J Clavs and sands. 



Estuarine? 



Estuarine >. 



Estuarine 

 neous. 



>s- 



Algonkian Pre-<^'aml)i 



Metamorpliic, sedi- 

 nu'titary, and ig- 

 neous. 



Clays. 



Sands and clays. 



Red sandstone, lime- 

 stone, breccia, shale, 

 and trap. 



Crystalline rocks. 



"The Newark is usually wuiiting in the section, and tlu- overlying formations frequently so. When the latter are 

 present only one or two commonly occur at the same time to the landward, while to the seaward there may be several. 



Only one of the Potomac cycad trunks (W. C, B., No. 1481) is posi- 

 tively claimed to have been seen in situ, its alleged bed being a compact 

 argillaceous sand near the summit of the Patuxent terrane. \\'ith a few 



