THE OZARKIAN AND ITS SIGNIFICANCE : 535 
III. SIGNIFICANCE OF THE OZARKIAN. 
In my papers on Critical Periods tn the History of the Earth, in 
1877 and in 1898" I try to show that there are periods of great 
and widespread changes in the earth’s crust, in climate, and in 
organic forms. These I call ‘‘critical periods’”’ and insist that 
they separate the primary divisions of geological, time, viz., the 
eras. Being periods of great elevation and enlargement of con- 
tinents, they are to a great extent “lost intervals,” z. ¢., periods in 
which the usual record of stratified rocks and contained fossils 
is interrupted. The last of these was the Quaternary. The 
Ozarkian was largely a lost interval so far as stratified record is 
concerned. 
Great cycles in the evolution of the earth—Now an era, 7. é., 
the time from one critical period to another, must be regarded 
as one great cycle of widespread changes; during which, how- 
ever, there may have been and indeed undoubtedly were, sub- 
ordinate and more local cycles. Such a great cycle was the 
Paleozoic, very regular in its course in this country. Such another 
great cycle was the Mesozoic, but affected in this country with 
a well marked subordinate division at the end of the Jurassic. 
Still another such great cycle and again very regular in this 
country was the Cenozoic, and such another, I am convinced, is 
even now commencing. I have called it the Psychozoic. 
I have supposed these to be cycles in the evolution of the 
earth. They are,’ therefore, such in every department alike. 
They are cycles in the evolution of earth-forms, constructively, 
2. ¢., by tntertor forces in continental elevation and mountain for- 
mation. They are cycles in the evolution of earth-forms, destruc- 
tively, t.e., by extertor forces and erosive sculpturing. They are 
cycles of evolution in climatic conditions. And, finally, as the 
result of all these, they are also cycles in the evolution of organic 
forms and their geographical distribution. Most of these various 
forms of cyclical movement I have discussed in my previous 
Papers, especially “thatmin’ 1895. These, therefore, 1 merely 
* Am. Jour., Vol. XIV, 99, 1877. Bull. Geol. Dept. Univ. Cal., Vol. I, 314, 1895. 
