28 THE POTOMAC OR YOUNGER MESOZOIC FLORA. 



pressure and embedded in clay. It is evident that these logs represent 

 trunks of trees that settled to the bottom in eddies or reaches of water that 

 remained quiescent just long enough to permit the deposition of a small 

 amount of clay around the trunks. One of these logs was seen surrounded 

 by a crust of marcasite about an inch thick, which was partially changed 

 to limonite. This was interesting as suggesting one way in \vliich the form 

 of portions of plants may be retained in limonite. The wood from which 

 the lignite was produced seems to have been coniferous. Two kinds of 

 wood may be detected here even by a hast}^ inspection. One kind is that 

 which has given rise to nearly all the lignite of the Virginia Potomac ; 

 the other was seen here for the first time. This has a coarse grain and 

 open texture. The rings of annual growth are thick and dense; they stand 

 out prominently and are more durable than the material between them. 

 The lignite from this kind of wood is the most common form at Hanover. 



A recent visit to Baltimore enabled me to examine the Potomac forma- 

 tion there under favorable conditions. The gray shale of the lower Poto- 

 mac, which at Federal Hill shows a thickness of two or three feet resting on 

 the sand, was found to contain a number of plant-impressions. These in 

 some cases are specifically identical with plants from the Virginia Potomac; 

 in others they are nearly allied to Virginia plants, and in general character 

 this flora seems to be similar to that of the lower Potomac of Virginia. 



Professor Uhler assisted me in collecting plants from this locality, and 

 he informed me that several of them are specifically identical with some 

 of those formerly collected by him at Baltimore which were sent to New 

 Orleans. He informed me also that among the fossils sent to the last-named 

 city were a number that he had collected at Federal Hill, near the locality 

 now in question, but at a higher level, and that they all differed from the 

 species obtained by us jointly. This indicates that there are in the vicinity 

 of Federal Hill at least two fossiliferous horizons. 



Unfortunately m}- investigations in Baltimore do not fix the horizon of 

 the plants collected by Mr. Meek. None of the recently obtained species 

 are identical with his, with probably one exception. The rock material of 

 Meek's specimens differs so much from the shale which furnishes the speci- 

 mens on Belt and Covington streets as to indicate a difference in horizon. 



