LETTER OF TRANSMITTAL. 



Department of the Interior, 



U. S. Geological Survey, 

 UniversUif of Virfjiniu, Virginia, Jaiimiri/ 10, 1881). 



Sir: T transmit witli tliis the manuscri[)t and plates describing- and 

 illustrating the flora of the Potomac or Younger Mesozoic formation of Vir- 

 ginia. 



Almost all of the material described in this memoir was obtained in the 

 State of Virginia. A small portion was collected in Maryland, at Baltimore. 



The work of collection occupied several years. A large number of 

 localities, extending over the belt between Baltimore, Md., and Petersburg, 

 Va., were carefully examined, and all significant specimens were selected 

 from a very large amount. of material. A number of the localities were 

 visited several times and new collections made. 



The extent of ground over which the fossiliferous exposures occur 

 and the thorough examination made entitle us to believe that the fossils 

 described in this monograph give a fair idea of the unique and interesting 

 vegetation existing in Potomac times. This flora is unique, for it ajjpar- 

 ently contains the oldest known angiosperms, and exhibits them in con- 

 siderable numbers associated with numerous Jurassic types, a conjunction 

 not hitherto known. 



Accompanying my manuscript is a series of tables, comparing the 

 Potomac plants with previously described fossil floras. These tables were 



