2*70 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1916. 



by many geologists, notably Dana among the older ones, and seems 

 reasonable ; but there are other geologists, especially paleontologists, 

 as well as zoologists and botanists, who display great recklessness in 

 rearranging land and sea. The trend of a mountain range, or the 

 convenience of a running bird, or of a marsupial afraid to wet its 

 feet seems sufficient warrant for hoisting up any sea bottom to con- 

 nect continent with continent. A Gondwana Land arises in place of 

 an Indian Ocean and sweeps across to South America, so that a 

 spore-bearing plant can follow up an ice age ; or an Atlantis ties New 

 England to Old England to help out the migrations of a shallow- 

 water fauna; or a "Lost Land of Agulhas" joins South Africa and 

 India. 



It is curious to find these revolutionary suggestions made at a time 

 when geodesists are demonstrating that the earth's crust over large 

 areas, and perhaps everywhere, approaches a state of isostatic equilib- 

 rium, and that isostatic compensation is probably complete at a depth 

 of only 76 miles. Hayford's results have been ably supported and 

 applied by my predecessor. Dr. Becker, in his address last year, but 

 some geologists hesitate to accept them. Barrell, after an elaborate 

 discussion of the whole question, thinks the equilibrium much less 

 complete than Hayford's results would suggest, but his arguments 

 do not seem entirely convincing.^ Great stress is laid on the sub- 

 marine deltas of the Nile and 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, however, 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 

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

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



1 Articles on the strength of the earth's crust. Jour. Geol., vols. 22 and 23. 



