GEOLOGY or THE POTOMAC BEDS. 53 



tlie entire mass is red. Still faitlifi- on it changes to a light gray sand, 

 which is totally unlike the material in other places. This sort of change 

 is conmion in the Fredericksburg area, but was not seen in the Peters- 

 burg district. 



A bciuitifuUy white, pure, plastic clay is sometimes found in the form 

 of balls, embedded in the sand. It was seen, for example, in the lower 

 part of White House Bluff. It has never been seen in the original or 

 undisturbed beds. 



The layers of isolated plastic clav in their original position some- 

 times show peculiar features in their arrangement, and illustrate the 

 iiregularitv of the lower Potomac formation. Sometimes, as- at Dutch 

 Gap, we may see in the same vertical exposure several of these layers. 

 Thev may be found in the sand inclined at different angles to one another, 

 and in one case several were seen diverging from a common point like 

 the outspread fingers of a hand. Of course such layers must be deposited 

 upon a shifting surface. These layers are sometimes cut off at one end 

 by coarse sand and gravel. When partially cut away they are frequently 

 associated with clay balls which have been torn from them. 



The isolated clay balls and masses often show peculiar feature.s. 

 They occasionally are seen where no trace is left of the bed which fur- 

 nished them. Some clay masses, four or five feet in diameter, embedded 

 in coarse sand, may be seen at the plant locality on the railroad near 

 Acquia Creek, the dark gray clay contrasting strongly with the light 

 colored sand. In Trent's Reach, where considerable cliffs of nearly white 

 sand occur, we may see a number of large balls of gray clay embedded 

 in the sand, and displayed on the same horizon for a considerable distance 

 in the face of the bluff. 



The embedding of clay balls in coarse sand is one of the most con- 

 stant and characteristic features of the lower Potomac. Clay balls thus 

 inclosed may be seen at all exposures of the formation from Baltimore 

 to the Nottoway River, and the clay is strikingly similar in all cases. 

 Clay balls and irregulai- deposits of disturbed clay are common in the top 

 of the lower Potomac sand, associated with the pebbles and cobbles of 

 that horizon. This clay .seems to point to the former existence of an 



