THE POTOMAC OR YOUNGER MESOZOIC FLORA. 



By W. M, Fontaine. 



INTRODUCTION. 



The plants described in this memoir, with very few exceptions, were 

 collected personally by me. 



The formation from which the plants were obtained was for a longtime 

 and by most writers inclnded iu the so-called Trias of the Atlantic slope. 

 Prof. William B. Rogers, however, in his earliest publications on the geol- 

 ogy of Virginia, recognized the diflference between this group of strata and 

 most of the Mesozoic of that State. In his first publications he called the 

 beds Upper Secondary, and expressed the opinion that they are of about 

 the age of the English Purbeck. Later' he asserts the possibility that they 

 may form a group of passage beds, comparable to the Wealden. In his 

 more recent writings he calls the formation Jurasso-Cretaceous. 



Mr. W J McGee, of the U. S. Geological Survey, has proposed the name 

 "Potomac" for the group, and this name will be used by me. 



Nearly all the plants described in this work come from the Potomac 

 formation in Virginia. The few exceptions were found in Marj'land, and 

 these embrace all the plants not collected by myself. They will be more 

 particularly noticed further on. 



With the exception of the Maryland fossils, all the plants which form 

 the subject of this work were not only collected bv me, but were packed, 



' Geology of the Virgiuias, p. 712. 



