THE PERMANENCE OF OCEANIC BASINS. 



421 



The accompanying diagram (taken from my book on " Dar- 

 winism ") will better enable the reader to appreciate these proportions, 

 which are of vital importance in the problem under discussion. The 

 lengths of the two parts of the diagram are in proportion to the 

 areas of land and ocean respectively, the vertical dimensions showing 

 the comparative mean height and depth. It follows that the areas 

 of the two shaded portions are proportional to the bulk of the 

 continents and oceans respectively. 



The mean depths of the several oceans and the mean heights of 

 the several continents do not differ enough from each other to render 

 this diagram a very inaccurate representation of the proportion between 

 any of the continents and their adjacent oceans ; and it will therefore 

 serve, roughly, to keep before the mind what must have taken place 

 if oceanic and continental areas had ever changed places. It will, I 

 presume, be admitted that, on any large scale, elevation and 

 subsidence must nearly balance each other, and, thus, in order that 

 any area of continental magnitude should rise from the ocean floor till 

 it formed fairly elevated dry land, some corresponding area must sink 



Diagram of proportionate meao height of Land and depth of Oceans. 



Land 



Area. -28 of area 

 ofGlobe. 



Ocean 



Area. 72 of area of Globe. 



I 



to a like extent. But if such subsiding area formed a part or the 

 whole of a continent, the land would entirely disappear beneath the 

 waters of the ocean (except a few mountain peaks) long before the 

 corresponding part of the ocean floor had approached the surface. 

 In order, therefore, to make any such interchange possible, without 

 the total disappearance of the greater portion of the subsiding continent 

 before the new one had appeared to take its place, we must make 

 some arbitrary assumptions. We must suppose either that when 

 one portion of the ocean floor rose, some other part of that floor sank 

 to greater depths till the new continent approached the surface, or, 

 that the sinking of a whole continent was balanced by the rising of a 

 comparatively small area of the ocean floor. Of course, either of 

 these assumed changes are conceivable and, perhaps, possible ; but 

 it seems to me that they are exceedingly improbable, and that to 

 assume that they have occurred again and again, as part of the regular 

 course of the earth's history, leads us into enormous difticulties. 

 Consider, for a moment, what would be implied by the building up of a 

 continent the size of Africa from the mean depth of the ocean. By 



