104 J. LeConte—Critical Periods in the History of the Earth. 
and were a principal agent in the extermination of the great 
reptiles. The wave of reptilian evolution had just risen to its 
crest and perhaps was ready to break, when it was met and 
overwhelmed by the rising wave of mammalian evolution. 
We have dwelt only on the great change in the higher 
classes, but the change really extended to all classes. This was 
therefore a time of exceptionally general and rapid changes 1n 
all departments alike. In other words it was a critical period 
in organic evolution. 2 
it was also a time of very great changes in Physical 
geography, here in America as well as elsewhere, is well known. 
The Cretaceous sea which extended from the Gulf of Mexico 
to the Arctic Ocean, covering the whole western plains an 
plateau region, and thus divided the American continent into 
two, an eastern Appalachian continent and a western or Basin 
region continent, was abolished at the end of the Cretaceous, 
and replaced by great fresh water lakes in the same region, 
and the continent became one. Moreover it is probable that 1t 
was a period of wide-spread oscillation, i. e, of upheaval and 
in subsidence to the condition of things found at the begin- 
ning of the Tertiary. It is probable that the upheaval which 
abolished the Cretaceous sea went much beyond the condition 
of things afterwards—-that just at the interval the land was 
higher and larger than in the Tertiary—that, in short, this was 
again a continental period and probably a period of greater cold 
than the subsequent Tertiary. 
The change in physical geography, then, was immense, but 
in most places by bodily upheaval, not by crumpling of the 
strata, and therefore the usual signs of such change, viz: uD- 
conformity is often wanting. The change of climate all over 
the American continent was no doubt very great and the 
