THE CAUSE OF THE GLACIAL PERIOD. 3^9 



crust which have produced the mountain-chains, but by 

 the gentler swells of larger continental areas whose strain 

 was at last relieved by the folding and mashing together 

 of the strata along the lines of weakness now occupied by 

 the mountain systems. The formation of the mountains 

 seems to have relieved the accumulating strain connected 

 with the continental elevations, and to have brought about 

 a subsequent subsidence. 



Doubtless, also, correlated subsidences and elevations 

 of the earth's crust have been aided by the transfer of the 

 sediment from continental to oceanic areas, and, as already 

 suggested, during the Glacial period by the transfer of 

 water evaporated from the surface of the ocean to the ice- 

 fields of the glaciated area. For example, present erosive 

 agencies are lowering the level of the whole Mississippi 

 basin from the Alleghanies to the Rocky Mountains at the 

 rate of a foot in five thousand years. All this sediment 

 removed is being transferred to the ocean-bed. Present 

 agencies, therefore, if not counteracted, would remove the 

 whole continent of America (whose average elevation 

 above the sea is only 748 feet) in less than four million 

 years ; while the great rivers which descend in all direc- 

 tions from the central plateau of Asia are transferring 

 sediment to the ocean from two to four times as fast as 

 the Mississippi is, and the Po is transferring it from the 

 Alps to the Adriatic fully seven times as fast as the Mis- 

 sissippi is from its basin to the Gulf of Mexico. This 

 rapid transfer of sediment from the continents to the 

 ocean is producing effects in disturbing the present equi- 

 librium of the earth's crust, which are too complicated 

 for us fully to calculate ; but it is by no means improb- 

 able that when accumulating for a considerable length of 

 time, the ultimate results may be very marked and per- 

 haps sudden in their appearance. 



The same may also be said of the accumulation of ice 

 during the Glacial period. The glaciated areas of North 



