THE PERMIAN PERIOD. 659 



climates in these and several other periods, before and since, is one of the 

 most remarkable climatic features of geologic histor}^. The crucial feature is 

 the maintenance of a mild temperature throughout the long polar night. 

 Almost the only approach to a satisfactory solution seems to lie in a warm 

 oceanic circulation, blanketed by a heat -retaining atmosphere, in which vapor 

 of water and carbon dioxide, abetted by a prevalent mantle of clouds gen- 

 erated by the warm ocean, conjoined their equalizing and conservative in- 

 fluences. 



The transition from the Subcarboniferous to the Permian involved a change 

 from an effective circulation of warm water to an interrupted circulation which 

 limited the previous equalizing and conservative influences. This was one 

 step toward a lowering of the polar temperatures, which perhaps ultimately 

 reached the amount of 20° C. or more, and toward a lowering of the tempera- 

 ture of the great body of the ocean through the deep-sea circulation initiated 

 and controlled by the polar temperatures. This was a step in the differentiation 

 of regional temperatures, and this, as we shall proceed to note, was abetted by 

 several other agencies arising from the same fundamental source, deformation. 



Topographic sequences. — As already indicated, the deformation resulted 

 directly in a pronounced change in the topography of the continents, in that 

 it gave marked relief to the portions that were sharply folded, and a less declared 

 but appreciable relief to the portions that were bowed upwards more gently. 

 Both resulted in an increase of the general gradients of the surface, and hence 

 in a rejuvenation of the streams. The increased area of the land obviously 

 increased the number and the length of the trunk streams, and added to the 

 volume of water that flowed through them, qualified, to be sure, by the aridity 

 that followed. An inevitable result of these changes was a more rapid removal 

 of the residual earth that had doubtless accumulated during the previous low- 

 lying stages, and, following this removal, a dissection of the underlying for- 

 mations and a lowering of the ground-water. An inevitable attendant of these 

 sequences was an increased contact of the atmosphere with unleached rocks, 

 combining at once the effects of the increased land area, the removal of the 

 residual mantle, the dissection of the surface, and the lowering of the ground- 

 water. The increase was probably several fold. 



A complementary result of the increase of the land was obviously a reduc- 

 tion of the oceanic area and, particularly, a reduction of the shallow-water por- 

 tion which had been the chief habitat of the limestone-forming life. From an 

 area of perhaps 20,000,000 or 30,000,000 square miles in the early Carboniferous, 

 the epicontinental seas were probably reduced to 10,000,000 or 15,000,000 square 

 miles in the Permian, perhaps even to 5,000,000 square miles at the extreme 

 stage of the Permian emergence. So great a reduction seriously curtailed the 

 deposition of epicontinental organic beds, and markedly increased the deposi- 

 tion of clastic beds, and this was attended by other consequences now to be noted. 



