LOCATION OF POTOMAC BEDS. 33 



These are given simply as restorations of the prol)al)U^ original character 

 of the leaf 



Mail}- of the ferns of the Potomac formation have features that cause 

 them to i-esemble one another, and yet there are differences not easy to 

 give in words. These ditferences are perhaps too slight to justif}- their 

 sejiaration into distinct species, and yet they are constant and give a facies 

 to the plants tliat appears to Ije worthy of delineation. I have for this 

 reason thought it best to give figures of these, although it increases the 

 number of illustrations. The somewhat i)eculiar grouping of the plants of 

 some localities, if the connecting links were neglected, would cause certain 

 local floras to appear to be different from the normal Potomac vegetation 

 It seems best in such cases to give figures of some at least of the widely 

 diffused plants which occur with the peculiar ones, even when they ma\- 

 have been figured from other places. 



My thanks are due to Prof. Lester F. Ward for valuable assistance in 

 collecting the fossils, in the identification of them, and in facilitating the 

 preparation of this memoir. I am under obligations to Dr. J. S. Newberry 

 for the loan of the fossils collected by Mr. Meek at Baltimore, and for per- 

 mission to describe them. Piof P. R. Uhler has aided me much, facilitat- 

 ing my examinations in Maryland, and affording me the opportunity to 

 describe Tyson's cycad truidis. To him I am indebted for the discovery 

 of the plant locality at Fredericksburg, which is the most important one 

 in the Potomac area. 



LOCATION AND GEOLOGY OF THE POTOMAC BEDS. 



In order to make clearer the occurrence and geologic relations of the 

 plants described in this work, it is necessary to give some account of the 

 location and the general geology of the Potomac beds. In this place only 

 so much will be said as is recjuired for this purpose. A detailed account 

 of the geology of the formation will be found in a monograph of the U. S. 

 Geological Survey, shortly to be published. 



LOCATION OP THE POTOMAC BEDS. 



In Virginia, the Potomac or Younger Mesozoic forms the greater por- 

 tion of those beds called by Prof. William B. Rogei's, in his annual re- 



