i'ROBLKMS OF THE PERMIAN 



483 



was the complexity of the combination, and the extent of glaciation 

 and aridity. 



The chronological setting of the combination lends some advan- 

 tages to its study. It lies in the midst of geologic history, with 

 periods of climatic uniformity and polar geniality both before and 

 after. No appeal can be taken to a supposed final cooling of the 

 earth, or to any senile condition. It was an episode in the midst 

 of a long history, and its problems must be faced with this setting 

 in mind. 



