GEOLOGICAL POSITION OF THE LOWER POTOMAC BEDS. Gl 



marked irregularly with colors due to oxide of iron. No regular lieddiii"- 

 or structure is found, but we have a series of interlocking lenticular la3'er.s, 

 showing extreme variablene.ss in composition, texture, and structure. The 

 arrangement shows mostl\- current-licdding, and not unconnnonly beach- 

 structure. Dispersed pebbles and clay balls, also nests and pockets of 

 pebbles and cobbles and disturbed clay, are frequent. Lenticular sheets, 

 occasionally jjartially cut away and sometimes containing plant-remains, 

 are often met with. These are intercalated in the sand in the most irreg- 

 ular manner, and occur at no fixed horizon, lying- sometimes inclined to 

 one another, as if deposited on a shifting- surface. 



These features mark the lower Potomac at all its exposures from the 

 Nottoway River to ]3altimore, and, so far as they can be shown in borings, 

 they appear to be present as far east as Fortress Monroe." Such charac- 

 ters could be produced only in deposits accumulating in unrpiiet and 

 comparatively shallow shore waters, or in the estuar\^ of a great river. 



The entire absence of all marine life shows that the waters must have 

 been fresh, or at most brackish. 



That the deposits accumulated near land, and probaljly in part at 

 least in estuary waters, is strongly indicated by the nature and distribu- 

 tion of the plant-remains. The great quantity of dispersed lignite in the 

 form of isolated logs and limbs seems to show that the amount of drift 

 timber must have been large. These trees were probably floated down in 

 the larger streams. 



The areas showing thickly placed logs of lignite, appearing to be 

 formed out of trees which fell where tliey grew, seem to point to portions 

 of the shore or to islands in the estuary which were suddenly submerged. 

 The delicate ferns and other plants, often preserved in great perfection, 

 could not have been drifted far or floated long in the agitated waters with- 

 out being destroyed. They probal)l\- grew on islands. 



It seems probable that the James and Potomac Kivers had their 

 representatives in lower Potomac times in two streams, which iu their 

 course may not have differed essentially from the present rivers. 



If we are permitted to assume that at the time of the accumulation of 

 the lower Potomac sediment two such rivers existed, and, further, that 



