FLORA OF OLDER POTOMAC FORMATION. 357 



On January 7, 1883, Prof. P. R. Uhler delivered a lecture before 

 the Naturalists' Field Club of Baltimore, an abstract of which was pub- 

 lished the same year," in which he discussed the "Geology 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 gives the forma- 

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

 consisting only of the Astrodon Johnstoni of Leidy) and flora and offers 

 the following description of the beds: 



The whole series of the beds having been derived from the comminuted or 

 chemically altered elements of the Archean rocks, we find accordingljr at the very 

 bottom of the formation a stratum of micaceous sand mixed with finely ground 

 mica and aluminous matter. This is arranged in superposed layers, the fine white 

 clay alternating with the white sand until a thickness of 140 feet has been accumu- 

 lated. Next above this is a layer of pale clay, 20 feet in thickness, followed by 6 

 feet or more of fine white sand. And so sands, clays, gravel layers, and three differ- 

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

 red and pale claj^s, complete the series up to near the surface. Above these the 

 gravel beds of the Glacial period, with perhaps still others from the Cliamplain 

 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 reached this area, charged with bowlders of fossil-bearing rocks 

 torn from the mountains more than 70 miles distant. 



The Wealden formations were built in comparative quiet as sediments at the 

 bottom of shallow water, and near the upper part 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 River. 



The scattered remnants of this great sheet of 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 end of this stratum 

 passes across Magothy River, outcrops on the Severn, and reappears in a ravine 

 near Collinwood, on the Popes Creek Railroad. 



Professor Fontaine had now (1883) commenced making extensive 

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

 correspondence with him on the subject began with the beginning of 

 1883, and he kept me well informed as to his results from that time. 

 On June 16 he visited Washington and brought a few of the archaic 



a .John Hopkins University Circulars, February, 1SS3, Baltimore, 188:3, pp. .52-.5.3. 



