J. LeConte—Critical Periods in the History of the Earth. 109 
now another, as their resistance is overcome. Finally, in times 
of revolution nearly all forms yield to the pressure of external 
conditions and change rapidly; only the very exceptionally rigid 
being able to pass over the interval to the next period of re-arjusted 
equilibrium. 
ous 
revolution, was the death-sentence of the long-continuing and 
therefore rigid Paleozoic types. But the sentence was not 
immediately executed. The Permian represents the time 
between the sentence and the execution—the time during 
which the more rigid Paleozoic forms continued to linger out a 
painful existence in spite of changed and still changing condi- 
tions. But the most critical time—the time of most rapid 
change—the time of actual execution—was the lost interval. 
Only a very few most rigid forms pass over this interval iuto 
the Trias, 
‘ow, the Quaternary is also such a critical or transition 
Period, marking the boundary between two great Eras. The 
lying unconformably on the eroded edges of an older ‘series— 
"iver sediments in old river valleys, marine sediments in fiords 
{2 other words, we have unconformity on a grand scale. 
— In connection with these oscillations = _— great 
ehanges in physical geography, and corresponding and very 
wide-spread changes in climate, and therefore corresponding 
