16 THE POTOMAC OR YOUNGER MESOZOIC FLORA. 



stones of the typical Potomac sand, here having the consistency of a sand- 

 stone. The cobble-stone bed itself is very local, and is nothing but a 

 pocket representing a local erosive horizon. Many of the stones are three 

 to five inches in diameter and some as much as eight to ten inches. Over 

 the fossiliferous stratum Quaternary deposits come, having large, partly 

 rounded Azoic masses in their base. On working the plant deposit and 

 following it in and along the outcrop it was found to thin out to the west 

 and north, while to the south the material continued without essential litho- 

 logic change, but lost entirely its fossils. 



I made repeated visits to this place and collected largely from it. I 

 was induced to do this because the study of each collection made known 

 to me fragments that were not otherwise identifiable, while they were clearly 

 different from the better-preserved imprints. Unfortunately the town au- 

 thorities availed themselves of the excavation I had made to get material 

 for mending roads, so that no doubt a large amount of good fossiliferous 

 matter was wasted. The quarrying operations carried on by myself and 

 the town finally exhausted the deposit, and the place now appears as a rec- 

 tangular recess, showing in the walls little besides the Quaternary and the 

 cobble-stone bed. From its present aspect no one would think that any 

 plant impressions had ever been found there. This locality will be referred 

 to in the descriptions of species as " Fredericksburg." 



I have made repeated and careful search for otlier plant localities in 

 the vicinity of this spot, but with no success. I found tliat on the same 

 horizon with these fossil impressions, in the immediate vicinity, the material 

 was totally different. The changes of rock material on the same horizon 

 were so many and so sudden, that it soon became evident that there was 

 nothing here like an extensive fossiliferous horizon. Here and there, very 

 locally and on different horizons, a few obscure impressions might be found, 

 but they in no case ever extended over more than a foot or so of space 

 taken horizontally, while in thickness the material would be a mere film. 



North of Fredericksburg, liowever, other localities yield plants. It 

 may be said here that in searching for plant-impressions in the Potomac 

 formation there is no guide except the presence of undisturbed clay. Any 

 layer of this kind may contain impressions, and hence before one can say 



