394 HISTORICAL GEOLOGY 



are especially well developed in the interior regions of large 

 continents, for example, in central Asia. Wide extension of 

 lands, as in the Permian, therefore favors the making of deserts 

 in appropriate places. 



Glacial conditions in the tropics. While considering the 

 Permian climate we must not fail to note what is easily the 

 most remarkable fact now known about the period. Asso- 

 ciated with rocks of Permian age layers of glacial till have been 

 found lying upon scratched and grooved surfaces of older 

 rocks. These glacial beds have not been discovered in the 

 polar regions, as we might confidently expect, but in India, 

 South Africa, South America, and Australia ; that is to say, 

 near and even within the tropics. Nor are we to suppose that 

 glaciers were confined to lofty mountains. The limestones 

 and shales with which some of the layers of till are associated 

 show that they were deposited near or even below sea level. 



The existence of glaciers over so wide an expanse of the 

 earth's surface and even within the tropics themselves points 

 to most unusual climatic conditions. We may well believe 

 that they indicate a colder climate than now over much of the 

 globe ; but mere cold does not account for the strange dis- 

 tribution of the glaciers, and a satisfactory explanation of all 

 the facts is still lacking. 



SUMMARY OF THE PALEOZOIC ERA 



Geographic conditions. From the Cambrian to the 

 Permian, the more persistent lands were hi eastern Canada 

 and southeastern United States. The central and western 

 parts of the continent were repeatedly submerged by a rela- 

 tively shallow sea. As stated on a previous page, the copious 

 supply of detritus from the Appalachian land built up the 

 thickest Paleozoic formations along the eastern border of the 

 interior sea, while in the middle of the continent sedimenta- 

 tion went on more slowly. 



In the far West, the Colorado region and parts of the Pacific 



