III. 

 The Evolution of Oceans and Continents. 



NATURAL Science is indebted to Dr. Wallace for clearing away 

 some misconceptions which have surrounded the question of the 

 permanence of Oceans, and for imparting some freshness to the con- 

 troversy by adducing several new arguments in its favour. Hitherto 

 the question has been discussed rather as if it were necessary to hold 

 one of two extreme views ; the one view being that the great con- 

 tinental plateaux and oceanic depressions were formed at a very early 

 period of the earth's history, and have occupied their present positions 

 ever since ; the other view being that the movements of the earth's 

 crust have been so frequent and so great that no part of it has 

 remained either in the state of sea or land through many periods of 

 geological time. 



I will not stop to enquire whether Lyell believed in the possibility 

 of such frequent interchange when he wrote that " continents, although 

 permanent for whole geological epochs, shift their positions entirely 

 in the course of ages." The extreme view may have been held at one 

 time when little was known regarding the depths of the ocean, but 

 few geologists of any repute would maintain it at the present time. 

 On the other hand, the idea of the fixity of the oceanic and continental 

 areas is supposed to have received strong confirmation from the 

 investigations of the " Challenger " Expedition. It was argued that 

 modern oceanic deposits differ so much from any rocks which form 

 part of the continental plateaux that none of these rocks could be of 

 oceanic origin, and consequently that oceans and continents could 

 never have changed places. It was confidently stated that all ancient 

 marine formations had been formed in comparatively shallow water, 

 and it was even predicted " that representatives of the abysmal 

 deposits of the central oceans are not likely to be met with among 

 the geological formations of past times." ' 



The supposed absence of oceanic deposits from continental areas 

 was regarded as proving that the interchange of sea and land was 

 confined to areas within 200 or 300 miles of the continental plateaux 

 where the water was less than 1,000 fathoms in depth. In this way 

 92 per cent, of the whole oceanic area was excluded from the regions 

 of possible interchange. Against this view I have protested as being 



^ Sir A. Geikie. " Text Book of Geology," ist edition, 1882, p. 608. 



