324 C. Schuchert — Evolution of Geologic Climates. 



Wlien zoo-paleontologists have more clearly evaluated 

 the climatic significance of the varied geographic marine 

 fannas known to them, and geologists have learned the 

 temperature import of the great variability in marine 

 sediments, the conclusion will all the more certainly be 

 drawn that the earth throughout the geologic ages has 

 been subject to climatic variation. These variations will 

 be seen to be slightest, indeed very slight, during the mid- 

 dle parts of the geologic periods when the world has 

 almost no temperature belts ; and variably greatest during 

 their earliest and latest parts, when more or less marked 

 climatic zones and even glacial climates were developed. 

 To-day the variation on land between the tropics and the 

 poles is roughly between 110° and — 60° F., in the oceans 

 between 85° and 31° F. In the geologic past the tempera- 

 ture for the greater parts of the periods of the oceans 

 probably was most often between 85° and 55° F., while on 

 land it may have varied between 90° and 0° F. At rare 

 intervals the extremes were undoubtedly as great as they 

 are to-day. The conclusion is therefore attained that 

 throughout its history the earth has had temperature 

 zones, varying from an intensity as marked as that of to- 

 day to almost complete absence so that the greater part of 

 the earth had an almost uniformly mild climate, without 

 winters. 



