422 NATURAL SCIENCE. ^.^^^st. 



comparing the area of Africa with that of the whole of the land, and 

 the depth of the ocean with the mean height of the land, we shall 

 find that if all the land of the globe above sea-level could be trans- 

 ferred to mid-ocean, it would not be sufficient to form the new 

 continent, but would still leave it nearly 2,000 feet beneath the 

 surface. 



It thus appears that, if the elevation of the ocean floor, and the 

 corresponding sinking of whole continents, constitute a portion of the 

 regular change and development of the earth's surface, there would 

 be not only a chance but a great probability of entire continents 

 disappearing beneath the waters before new continents had risen to 

 take their place. Even the total disappearance of all the large land 

 masses might easily 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 submergences 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, 

 &c. But we now know that the main characteristic of the ocean 

 floor is, that it is a vast 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. Contrast this with the forms of all mountain 

 ranges whose general rise for long distances is often five 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 continents, we should 

 certainly expect to find some indication of those surface features 

 which characterise all continents, but which appear to be absent from 

 all 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 subsidence 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 of submerged mountain ranges — precipices, 

 deep valleys and ravines, arranged in diverging groups as they always 



