THE PERMIAN PERIOD 395 



slope were occasionally out of water, although there is 

 little evidence that they were as mountainous as now. Where 

 the Great Basin now is, the sea was especially long-lived, 

 and thousands of feet of varying marine sediments were 

 there laid down. 



On the whole, the seas overlapped the continent more in the 

 early than in the later part of the era. 



Throughout the Paleozoic periods volcanic activity was 

 confined almost entirely to the Pacific side of North America, 

 much as in recent times. In Europe, however, most of the 

 volcanoes were near the Atlantic. They were particularly 

 numerous at times in the British Isles. (How does this 

 compare with modern conditions?) 



Climatic conditions. The climates of the Paleozoic 

 periods have left only a scanty record. That there were 

 deserts in several countries at various times is proven by 

 such saline deposits as those of New York and Michigan; 

 and that moist conditions prevailed at other times is indicated 

 by the coal beds. It was formerly supposed that the earth 

 was hotter in Paleozoic times than now, but Cambrian and 

 Permian glacial deposits in regions which are now semi- 

 tropical seem to preclude this view. Of the distribution of the 

 climatic zones we know almost nothing. We can infer their 

 existence only because as long as the earth is round, and the 

 surface part receives its heat from the sun, such zones must 

 be present. 



Development of plants and animals. In reviewing 

 the progress of the living things we see many noteworthy 

 changes in the course of the Paleozoic era. The early periods 

 were dominated by the invertebrated animals. One by one, 

 new groups made their appearance ; and of these, some, like 

 the trilobites, passed their prime and slowly dwindled to extinc- 

 tion, while others merely retired to a subordinate position in 

 the scale of life. 



In the later periods, fishes and amphibians successively 

 came to the front, proved themselves more powerful than the 



