46 THE POTOMAC OR YOUNGEE MESOZOIG FLORA. 



a very small space it lias stripped tlie floor of gneiss of the covering which 

 conceals it. In contact with the gneiss a layer a few inches thick is found, 

 consisting of coarse sand and gravel, showing some of the cliaracteristic 

 features of the Potomac. It looks like a mere remnant of what was a 

 formation of some extent, but which in this place has been almost entirely 

 removed by erosion, and it suggests the idea that the Potomac maybe 

 found as far south as this place. The exposure, however, is too slight to be 

 taken as anything more than suggestive of probabilities. 



If we take the lower Potomac as ending in the vicinity of the Notto- 

 way, and extending east only as far as Fortress Moiu'oe, it will still raidc 

 among the more extensive formations of the Atlantic coast region. Stretch- 

 ing from Baltimore to the Nottoway, its length would be aljout two hun- 

 dred miles. This, with an average width of sixty miles, would give twelve 

 thousand square miles as the approximate space occupied by the Potomac. 



There is good reason to think that the present westernmost exposures 

 of the Potomac do not determine the oi'iginal extension of the formation 

 in that direction. It seems at one time to have been present considerabl}^ 

 farther west than any of the outcrops now visible. The formation at 

 various horizons contains deposits of pebbles and cobble-stones. These are 

 chiefly composed of quartz, but in some portions rounded stones of various 

 kinds of crystalline Azoic rocks occur. In the vicinity of Washington 

 some cobbles and pebbles of Potsdam quartzite may be seen. In the 

 Petersburg area, especially along the James and Appomattox Rivers, the 

 Potsdam material is noteworthy for its abundance and the large size of the 

 stones. 



On the weathered surface of the lower Potomac and on that of the 

 formations adjacent to its outcrops a large amount of gravel and cobble- 

 stones occurs of tlie same nature as the material found in the Potomac. 

 It would seem that the original home of this surface material was the 

 lower Potomac, and that the stones were liberated on the erosion of that 

 formation. Except along the streams which cross from the Potomac into 

 the Tertiary terrane, these gravels do not pass far into the region occupied 

 by the Tertiary and later formations. On the rivers they are found 

 forming the basal portion of the Quaternary, and as this often rests 



