CONTINENTAL TERRACES AND SUBMARINE VALLEYS I57 



they demonstrated that there was a rhythmical waxing and 

 waning of the ice-caps dirough at least half a million years; 

 five million square miles of ice still remain. So vast were the 

 dimensions of the vanished ice-caps that the shape of the whole 

 planet was affected. Under the heavy masses of ice the earth's 

 crust was basined, slowly to recoil with each slow melting. 

 From observations on the recoil, as set forth in the first chapter, 

 we learned about the exceeding weakness of the earth-shell 

 immediately beneath the crust. In the second chapter we 

 studied a second consequence of the glaciations — the world-wide 

 lowering of sealevel, which had drastic effects on marine or- 

 ganisms, particularly the reef-building corals. In this final 

 chapter we have traced still another glacial control — the pro- 

 longed muddying of shore waters, with the development of 

 bottom currents in the ocean and the world-wide furrowing 

 of the submerged continental slopes. While man and animals 

 were driven to and fro by the climatic changes on land, while 

 in the struggle for life man was mentally quickened into fire- 

 maker, tool-maker, and artist, and while the sealevel was 

 swinging down and up over broad regions that were alternately 

 dry land and drowned land, the shallow-water organisms were 

 fighting with mud baths, and shoals and continental flanks 

 were remodelled on a grand scale. The biologists and geologists 

 of the future will be more and more impressed with the power 

 of glacial controls in the later stages of organic and planetary 

 evolution. 



