CLIMATES OF GEOLOGIC TIME—SCHUCHERT. 307 
the fourth (late Proterozoic) apparently also followed a period of 
elevation. The Table Mountain tillites of South Africa, if correctly 
correlated, fall in with the time of the making of the great Caledonian 
Mountains in the Northern Hemisphere. On the other hand, the 
very marked and world-wide mountain-making period, with decided 
volcanic activity, during late Mesozoic and earliest Eocene times, 
was not accompanied by a glacial climate, but only by a cooled one. 
The cooled period of the Liassic also followed a mountain-making 
period, that of late Triassic time. We'may therefore state that 
cooled and cold climates, as a rule, occur during or immediately 
follow periods of marked mountain making—a conclusion. also 
arrived at independently by Ramsay (1910). 
Geologists are beginning to see clearly that the lands have been 
periodically flooded by the oceans, and the times of maximum sub- 
mergence and emergence of the continents since earliest Paleozoic 
time are fairly well known. The two marked glacial periods since 
Cambric time (Permic and Pleistocene) and the three other more 
or less cooled climates (late Siluric, Liassic, and late Cretacic) all 
fall in with the times when the continents were more or less exten- 
sively and highly emergent. There were no cold climates when the 
continents were flooded by the oceans, and it may be added that the 
periods of widespread limestone-making preceded and followed, 
but did not accompany, the reduced climates. On the other hand, 
the periods of greatest coal making (Upper Carbonic and Upper 
Cretacic) accompanied the time of greatest continental flooding and 
preceded the appearance of cooled climates. 
The more or less coarse red sediments seen at many horizons of 
the geologic column are interpreted as the deposits of variably arid 
climates, or those that are alternately wet and dry. In the Paleozoic 
they are seen more often at the close of the periods when the seas 
were temporarily withdrawn and the lands were most extensive. 
These red deposits alternate with formations that are either wholly 
marine or of brackish-water origin, and in the latter case of gray, 
green, blue, or black color. 
Humphreys has shown that volcanic dust in the isothermal region 
of the earth’s atmosphere does appreciably reduce the temperature 
at the surface of the globe. It is thought that if explosive volcanoes 
continued active through a more or less long geologic time, this 
factor alone would bring on, or largely assist in bringing on, a more 
reduced temperature or even a glacial climate. If then, we may 
further postulate that volcanic activity is most marked during 
times of mountain making, i. e., during the “critical periods” at 
the close of the eras and the less violent movements at the close of 
the periods, we should expect ice ages, or at least considerably cooled 
climates, occurring here also. Let us see how the facts agree with 
this hypothesis. 
