INTRODUCTION, 23 
lower than the European Chalk and higher than the upper member of the 
Trias. Such other evidence as can be gainéd in regard to their precise 
geological age must be derived from their abundant plant remains, among 
which are a number of species that are common to the Dakota sandstones 
of the interior of the continent, to the Atane and Patoot beds of Green- 
land—known to be Upper Cretaceous—to the Cretaceous clays of Aachen, 
Germany, and to the Upper Cretaceous rocks of Bohemia. 
The relation of the Amboy Clays to the Potomac formation of Virginia 
is not easily demonstrated, as the line of junction has not been fully traced, 
but we may say that the Potomac is the more ancient formation, and. that. 
probably a somewhat long interval of time separated the epoch of the 
Potomac group from that of the Amboy Clays. This is indicated by 
the almost entire distinctness of the floras of the two formations, which 
shows that a great change took place during that interval in the character 
of the vegetation which clothed the eastern shore of. North America. 
Professor Fontaine has described, from the Potomac group of Virginia and 
Maryland, 365 species of plants, of which not one is certainly found in the 
Amboy Clays; and the difference in the character of the vegetation is 
shown by the fact that in the long list furnished by Professor Fontaine 
there are but 75 angiosperms (about one-fifth of all), whereas in the New 
Jersey clays, throwing out fragmentary and doubtful remains, of 156 
described species all but 10 are dicotyledonous plants. 
The relation of the Amboy Clays to the Dakota group can be much 
more definitely determined, for the proportion between the angiosperms 
and lower plants in the Dakota group is about the same as in the Amboy 
Clays, showing a similar stage of progress in the development of plant? 
life. We have already obtained 12 species common to the two formations, 
a number that will undoubtedly be considerably augmented with the 
further exploitation of the Amboy flora. The Dakota group is known to 
occupy about the middle of the Cretaceous system. Until recently it was 
supposed to be the basal member of that system as developed on the North 
American continent, and it was believed that until about the middle of the 
‘This was written previous to the publication of Lesquereux’s Flora of the Dakota Group, edited 
by F. H. Knowlton, and my discoveries in the Cretaceous of Staten Island and Long Island. We are 
now enabled to identify at least 40 species as common to the two formations.—A. H. 
