J. LeConte—Critical Periods in the History of the Earth. 103 
e 
re-adjustment of equilibrium and prosperous development of 
these forms, 
ike the previous lost interval, this was also a period of 
and culminated in the lost interval, which is in fact for that 
very reason lost. . 
Far less in length of time and perhaps in the sweeping char- 
acter of the change of organisms, but far more important and. 
interesting on account of the high position of the animals in- 
volved is the lost interval between the Mesozoic and Cenozoic. 
The length of time lost here is comparatively small. In Amer- 
‘ca in many parts of the west the uppermost Cretaceous seems 
to pass into the lowermost Tertiary without the slightest break 
of Continuity. There may be some break, some unconformity, 
some lost record, but certainly it cannot be large. Yet the 
change especially in the higher animals is immense. In Amer- 
1ca the break and the lost interval is much greater between the 
: urassic and Cretaceous than between the ( roms — Ter-— 
Niry, yet the organic change is far greater in the latter case. 
The reason is that the ehatges of physical geography and cli- 
most and which therefore produce the profoundest changes in 
Organic forms, 
