CHANGING OCEANS AND CONTINENTS 191 



the Congo as loads which should have depressed the floor on which they 

 were laid down, but have not done so. It should be remembered, how- 

 ever, that we know them only from soundings, and that assumptions 

 regarding them are more or less hypothetical. On the other hand, the 

 delta of the Mississippi seems to conform to the theory of isostasy, and 

 there are numerous examples of depression going hand in hand with the 

 formation of shallow-water deposits quite in accord with the isostatic 

 theory. The 14,000 feet of coal measures at the Joggins are an instance. 

 But more convincing still is Fairchild's demonstration that a wave of 

 elevation followed up the retreat of the ice-front during the closing stages 

 of the Glacial period. The thickness of ice near its margin could not 

 have been more than a few thousand feet, perhaps half a mile, which 

 would mean in weight of rock only 750 feet. If the stiff carapace of the 

 earth in the State of New York yielded to so slight a change of load, it is 

 hardly credible that 9,900 feet of sediments spread over 75,000 square 

 miles of sea-bottom off the coast of Africa could have no effect. 



If I understand Barrell's discussion aright, his differences from Hay- 

 ford's conclusions are rather of degree than of kind. He thinks the 

 earth's crust more rigid and considers adjustments to change of load 

 much less complete, and also that they are carried out by slow movements 

 in the "asthenosphere" much below Hayford's level of complete compen- 

 sation at 76 miles below the surface. 



He would probably agree that on the broad scale continents are buoyed 

 up because they are light, and ocean bottoms are depressed because the 

 matter beneath them is heavy. He would admit that to transform great 

 areas of sea-bottom into land it would be necessary either to expand the 

 rock beneath by several per cent or to replace heavy rock, such as basalt, 

 by lighter materials, such as granite. There is no obvious way in which 

 the rock beneath a sea-bottom can be expanded enough to lift it 20,000 

 feet, as would be necessary in parts of the Indian Ocean, to form a Gond- 

 wana Land; so one must assume that light rocks replace heavy ones be- 

 neath a million square miles of the ocean floor. Even with unlimited 

 time, it is hard to imagine a mechanism that could do the work, and no 

 convincing geological evidence can be brought forward to show that such 

 a thing ever took place. 



Discussing this question not long ago in the Journal of Geology, Pro- 

 fessor Chamberlin showed that the only typical case of deep-sea deposits 

 found on land, the well known one of the Barbados, occurs on one of the 

 great hinge lines around which motions of the earth's crust take place 

 and has no real bearing on the change of ocean bottoms to continents. 18 



1S .Tom-. Geol., vol. xxii, pp. 131, etc. 



XIV — Bull. Geol. Soc. Am., Vol. 27, 1915 



