654 THE ANCIENT LAKES OF WESTERN AMERICA : 
of weakness where the changes of level experienced by the 
continent have been most sensibly felt. 
It is a somewhat remarkable fact that the collections of 
fossil plants made by Dr. Hayden from different localities 
differ so much among themselves. In every newly discov- 
ered plant-bed he has obtained more or less species of which 
we before had no knowledge, and it is even true that between 
some of his collections there are no connecting links. It is 
also true that much of the material he has collected has not 
yet received the study it needs. From these facts it will be 
seen that much yet remains to be done before the great inter- 
val of time during which this series of fresh-water Tertiaries 
accumulated can be divided into definite periods, and before 
we can venture to affirm that a flora of any epoch had such 
or such a botanical character and, therefore, this or that 
average annual temperature. Some interesting facts came 
out, however, at once in the examination of these materials ; 
to these I will briefly refer. 
In the beginning of the Cretaceous age, North America, 
as we know, presented a broad land surface, having a climate 
similar to the present, and covered with forests consisting, 
for the most part, of trees belonging to the same genera with 
those that now flourish upon it. In the progress of the Cre- 
taceous age, the greater part of the continent west of the 
Mississippi sank beneath the ocean, and the deposits made 
during the later portions of the Cretaceous age contain a 
vegetation more tropical in character than that which had 
preceded it. It seems probable that at this time the lands 
which existed as such, west of the Mississippi, were islands 
of limited extent, washed by the Gulf Stream, which appar- 
ently had then a course north and west from the Gulf of 
Mexico to the Arctic Sea. 
The earlier Tertiary epochs were, however, marked by an 
emergence of the continent and a gradual approach to previ- 
ous and present conditions. This is indicated by the fact 
that the oldest Tertiary deposits (Eocene?) contain a flora 
