18 PREFACE 



in the form of impressions, and these, while abundant and diverse, are 

 with some notable exceptions poorly preserved as well as fragmentary, 

 much more so than the diagrammatic figures built up from various frag- 

 ments by previous students would lead one to suspect. The silicified 

 wood and lignite, while abundant, has for the most part undergone so 

 much decay before fossilization that the bulk of it is worthless. In 

 addition to the sections made by Dr. F. H. Knowlton and forming the 

 basis of his paper on the Fossil Wood and Lignite of the Potomac, a 

 large number of sections have been studied by Mr. Berry, most of which 

 proved unidentifiable, because of the extreme stage of decay before 

 fossilization. The most perfectly preserved show only the comparatively 

 unimportant features of the secondary wood. The petrified Cycadeoidea 

 trunks were also found to be poorly preserved, constituting in tliis 

 respect a remarkable contrast with those from the Black Hills area and 

 elsewhere. 



The necessity of some sort of systematic treatment of the maze of 

 described forms in the literature of the Potomac which would enable 

 the geologist or the botanist to obtain some idea of the flora has long 

 been felt. The pre-existing multiplicity of species has made it necessary 

 to retain a number of extremely doubtful forms. Many have, however, 

 disappeared by reduction to synonymy, and some basis for the correlation 

 of a number of genera with their living representatives has become 

 apparent during the progress of the work. 



Certain important forms known only from the continuation of the 

 Maryland deposits in the Virginia area have been included, while others 

 upon which no new light could be shed have been omitted. These latter 

 will be discussed on a subsequent occasion in a work devoted to the 

 Virginia area and in course of preparation for the Geological Survey 

 of that state. Mr. Berry is indebted to various friends and colleagues 

 both at home and abroad for assistance during the progress of the work. 

 He is under especial obligations to the U. S. National Museum and Dr. 

 F. H. Knowlton for facilities in the study of the large Lower Cretaceous 

 collections of that institution as well as for many other courtesies. The 

 British Museum through Dr. A. Smith Woodward rendered invaluable 



