WORLD-MAKING 3! 



uncertain proportions, by the mantle 'of bold and gratuitous 

 hypothesis. 



So soon as we find evidence of continents and oceans we 

 raise the question, Have these continents existed from the first 

 in their present position and form, or have the land and water 

 changed places in the course of geological time ? This ques- 

 tion also deserves a separate and more detailed consideration. 

 In reality both statements are true in a certain limited sense. 

 On the one hand, any geological map whatever suffices to show 

 that the general outline of the existing land began to be formed 

 in the first and oldest crumplings of the crust. On the other 

 hand, the greater part of the surface of the land consists of 

 marine sediments which must have been deposited when the 

 continents were in great part submerged, and whose materials 

 must have been derived from land that has perished in the 

 process, while all the continental surfaces, except, perhaps, some 

 high peaks and ridges, have been many times submerged. 

 Both of these apparently contradictory statements are true ; and 

 without assuming both, it is impossible to explain the existing 

 contours and reliefs of the surface. 



In exceptional cases even portions of deep sea have been 

 elevated, as in the case of the Polycistine deposits in the West 

 Indies ; but these exceptions are as yet scarcely sufficient to 

 prove the rule. 



In the case of North America, the form of the old nucleus of 

 Laurentian rock in the north already marks out that of the 

 finished continent, and the successive later formations have 

 been laid upon the edges of this, like the successive loads of 

 earth dumped over an embankment. But in order to give the 

 great thickness of the Palaeozoic sediments, the land must have 

 been again and again submerged, and for long periods of time. 

 Thus, in one sense, the continents have been fixed ; in another, 

 they have been constantly fluctuating. Hall and Dana have 

 well illustrated these points in so far as eastern North America 



s. E. 3 



