658 GEOLOGY. 



rial from the land lightened the continental sectors and tended to disturb the 

 balance between them and the oceanic sectors. The superior shrinkage of the 

 oceanic sectors thus caused was probably attended by some crowding upon the 

 weaker continental sectors between them, which appears to sufficiently account 

 for the slightly increased protuberance of the continental platforms, together 

 with the gentle warpings which they manifest, while the increase in the basin 

 capacity resulting from the subsidence of the ocean bottoms accounts for the 

 withdrawal of the epicontinental waters. The outer shell of the crust, being 

 rigid and not under a load equal to its rigidity, appears to have accommodated 

 itself to the shrinking interior by lateral shear and folding concentrated into 

 tracts, instead of by a more distributive crumpling, or crushing. In this lateral 

 thrust and folding is found the explanation of the post-Carboniferous mountains of 

 eastern North America and western Europe, and such similar foldings as may 

 perhaps have taken place in other regions at this time. 



//. The Immediate Sequences of the Deformation. 



Interference with circulation. — It is obvious that the withdrawal of the exten- 

 sive sheets of water which had submerged the continents to the extent of per- 

 haps 20,000,000 or 30,000,000 square miles, resulted in a serious restriction of 

 the oceanic circulation, and correspondingly reduced the effects which such a 

 general circulation had upon the climate of the times. These effects were prob- 

 ably profound. The surface temperature of the oceanic waters of the Mis- 

 sissippian and Carboniferous periods may, without any pretension to accu- 

 racy, be assumed to have been 25° C, or some such figure, as against the present 

 15° C. The submerged remnants of the continental platforms, while they must 

 have interfered somewhat with the general oceanic circulation, probably did 

 not so far interrupt it as to prevent the polar regions being warmed by a most 

 effective water-heating system. No small factor in this, perhaps, was the deep 

 circulation of the ocean, a matter of vital moment in all climatic studies. 



In the discussion of the Pleistocene glaciation the possibility of the reversal 

 of the deep-sea circulation by means of a change in the relative influence of the 

 concentration of salinity by evaporation in the low latitudes on the one hand, and 

 of increased density due to the low temperature in the high latitudes on the 

 other, is entertained and its bearings discussed. 



The deep oceanic circulation is now dominated by the polar temperatures, 

 for it is the cold waters of the polar regions that, descending and flowing toward 

 the equator, control the temperature of the deep sea. This is now low because 

 of the low temperature of the polar regions; but if the circulation of the Sub- 

 carboniferous, and Carboniferous periods, whatever its nature, kept the polar 

 regions at a mild temperature, the great body of the deep sea must have had 

 temperatures correspondingly higher, and the water must have had so much the 

 less depressing influence on the temperature when it rose to the surface. This 

 must have contributed to the widespread warmth which the Subcarboniferous, 

 and perhaps the Carboniferous, period enjoyed. The mildness of the polar 



