34 STUDIES, SCIENTIFIC AND SOCIAL chap. 



happen ; for we see from the diagram that they might one 

 after the other disappear with a corresponding rise of the 

 adjacent portion of the ocean bed and still leave the 

 ocean over the whole earth almost as deep as it is now. 

 But, as will be shown further on, the geological record, 

 imperfect as it is, teaches us that no such general sub- 

 mergences have ever taken place. 



Contour of the Ocean Floor as indicating Permanence. — 

 Before extensive soundings revealed the depth of the 

 ocean and the form of its floor, it was supposed that it 

 would exhibit irregularities corresponding to those of the 

 land, such as mountain-ranges, great valleys, escarpments, 

 ravines, etc. But we now know that the main char- 

 acteristic of the ocean floor is, that it is a vast slightly 

 undulating plain^ the slopes rarely exceeding a hundred 

 feet in a mile except near the margins of the continental 

 areas, while usually the gradients are so slight that they 

 would be hardly perceptible to the eye. Contrast this 

 with the forms of all mountain ranges whose general rise 

 for long distances is often several hundred feet in a mile, 

 while slopes at angles of from 20° to 60° are by no means 

 uncommon. 



Now if we suppose that considerable portions of the 

 ocean depths have been formed by the subsidence of con- 

 tinents, we should certainly expect to find some indication 

 of those surface features which characterise all the con- 

 tinents, but which appear to be absent from all the deep 

 oceans. In order to account for the actual contours of 

 the ocean on this theory, we must suppose that, during 

 subsidence, all the mountain ranges, peaks, valleys, and 

 precipices were reduced to an almost uniform level surface 

 by marine denudation, which, unless the process of subsi- 

 dence were incredibly slow, seems most improbable. Mr. 

 Jukes-Browne, however, does not hold the view that they 

 have been thus denuded, for he approvingly quotes Mr. 

 Crosby as saying that — " the oceanic islands are, of course, 

 merely the tops of submerged mountains, and it is only 

 with the highest points of continents that they can 

 properly be compared." If this is correct, then we ought 

 to find in the vicinity of such islands all the chief features 



