SUBMARINE MOUNTAINS. 375 



not very far from three miles beneath the surface, is 

 about 100 millions of square miles. * Now when we 

 speak of the ocean, we must endeavour to regard it 

 as one grand and continuous liquid expanse which 

 extends around the whole terrestrial globe. 



Beneath its waves, it is known to possess its sub- 

 marine mountain ranges. When the tops of some of 

 them come near the surface, they are then known as 

 banks, or ledges of sunken rocks : in reality they form 

 plateaux of these submarine ranges. The tops of others, 

 which are higher, rise here and there above the sur- 

 face of the waters, and are known to us as islands: 

 these are merely the summits of submarine mountains 

 upon which weather, but especially frosts and rains, acting 

 during the course of ages, have levelled down many of 

 the inequalities, and converted the surface into soil upon 

 which vegetation has sprung up, leaving them in the 

 condition in which they are now seen to exist. Large 

 continental islands, on the other hand, such for instance 

 as Australia, are probably created by the subsidence 

 of areas of land which have sunk beneath the seas 

 leaving them remaining detached from the main land. 

 The shallow nature of the present sea, studded with 

 numerous islands, which lies to the northward of 

 Australia, would appear to give colour to this theory: 

 there are also many questions connected with the 

 peculiarities of the flora and fauna, some of which are 

 held to be evidence in support of it, whilst others 

 appear rather to tell against it; but whichever way 

 this may be, it is certain that this subsidence if it 

 occurred took place at a very remote period of time ; 



* Introduction to the Scientific Report of the Challenger Expedition 

 summary of general conclusions, p. 46. 



