668 LC CLA MG TALON 
37° north latitude, and mantling also the plains of middle 
Europe and high altitudes quite generally, but it must assign 
agencies for the oscillations which attended it. 
General cause—The atmospheric hypothesis finds a genera] 
cause for the Pleistocene glaciation in that notable extension 
and elevation of the land which reached a climax near the close 
of the Pliocene period.” lt isi) not, however, through jtsmdinect 
topographic influence that this was accomplished, though this may 
have been incidentally tributary, but through its effects on the 
constitution of the atmosphere. The recently named Ozarkian 
or Sierrian period embraces the specially effective stage of this 
great land area, and it is with much pleasure that I support the 
emphasis laid upon the significance of this period by Professor 
Le Conte in his paper in the last number of the JouRNAL,” though 
I interpret that significance in different terms. The wide extent 
and high elevation of the land at that time are so strongly set 
forth by Dro We Conte as to leave no (need ror additional 
emphasis here. 
A rude estimate of the land area in Middle Tertiary times, 
when the climate was mild far to the north, gives about 
44 million square miles. A similar estimate for the Ozarkian 
or Sierrian period gives about 65 million square miles, while the 
received estimate of present land is about 54 million square 
miles. Taking the Middle Tertiary area as a basis of compar- 
ison, the land was increased in the Ozarkian period about 47 
per cent., and afterwards fell off to the present area, which is 23 
per cent. greater than that of the mid-Tertiary. It is probably 
conservative to estimate that the average elevation of the 
Ozarkian land was at least two or three times, perhaps three or 
four times, as great as that of the mid-Tertiary. Combined in 
the light of the suggestions previously made regarding elevation, 
these indicate a very great change in the effective contact of the 
atmosphere with the earth. If we measure the actual contact 
by the surfaces of the grains, pores, fissures, and minute crevices 
with which the air and the atmospheric waters come in contact 
* Pp. 525-544. 
