FLORA OF OLPKR I'O'IO.MAC FOKMATIOX. 'uu 



Oil .Inimary 7. 188.3. Prof. P. P. Plilcf (Iclivcrcd a locturo before 

 the Naturalists' Field Chili of Baltimore, an al)stract of whieh was puh- 

 lished the same year," in whicli he discussed [ho "(ieolo<iy of the Surface 

 Features of the Baltimore Area." A considerable part of this abstract 

 is devoted to what is now called the Potomac formation and which 

 he characterizes as Upper Jurassic or Wealden. He ,<iiv(>s (lie forma- 

 tion a thickness of 500 feet, refers in a sivneral way to its fauna (then 

 consistiiig- only of tlie Aslrodoii Johnstoni of Leidy) and flora and offers 

 the following description of tlie beds: 



The whole series ot tlie beds havin>; i)een derived from the comminuted or 

 chemieally altered elements of the Arehean rocks, we find aceonlin<,dy at iho very 

 l)ottom of the formation a stratinu of iiiicaceoiis sand mixed willi linely <,n-ouiid 

 mira and aluminous matter. This is arranj^ed in superposed layers, the line white 

 clay alternalins with the white sand until a thickness of 1-10 feet has been accunm- 

 lated. Xe.xl above this is a layer of pale clay. L'l) feet in thickness, followed by (i 

 feet or more of line white sand. And so saniis, clays, ,uravel layers, and three dilfer- 

 ent strata of cobblestone drift, overlain by other coarse drift and bowlders set in 

 red and pale clays, complete the series up to near the surface. Above these the 

 gravel beds of the Glacial period, with perha|)S .still others from tin- Cbamplain 

 epoch, rise in hills, or spread over the Wealden domes in deposits of varying thick- 

 ness. To the Susquehanna River we must look for the broad avenue through which 

 the general drift r(>ached this area, charged with bowlders of fossil-bearing rocks 

 torn from tlie mountains more than 70 miles distant. 



The Wealden formations were built in comparative quiet as sediin(>nts at the 

 l)ottom of sliallow water, and near the upper j)art of the .series a thick stratum of 

 white sandstone and conglomerate spread from the present shores of Chesapeake 

 Bay away back to the Belair road near the Gunpowder Riv(M-. 



The scattered remnants of this great sheet t)f stone may still be seen sticking 

 out of the water in Rock Creek, at the mouth of the Patapsco River, and also in 

 the soil of the region beyond White Marsh Run. The other vml of this stratum 

 passes across Magothy River, outcrops on the Severn, and reappi-ars in a ravine 

 near Collinwood, on the Popes Creek Railroad. 



Professor Font tune luid now (1883) commenced mtilving extensive 

 collections of fossil plants from the Older Potonuic of Virginia. My 

 correspondence with him on the subject began with the beginning of 

 1883, and he kept me well informed :is to his ivsults from that time. 

 On June K) he visited Washington and brought ;i few of the archaic 



"John Hopkins University Circulars, February^ 1SS3, Hiiltiinoie, 1S.S3, pp. .52-.5.S. 



