THE HISTORY OF THE NORTH ATLANTIC 6 1 



but unless he had also enjoyed opportunities of studying the 

 rocks of the earth in detail and close at hand, or had been 

 favoured by some mundane friend with a perusal of " Lyell's 

 Elements," or "Dana's Manual," he would not be able to ap- 

 preciate as we can the changes which the Atlantic has seen in 

 geological time, and in which it has been a main factor. Nor 

 could he learn from such superficial observation certain secrets 

 of the deep sea, which have been unveiled by the sounding 

 lead, the inequalities of the ocean basin, its few profound depths, 

 like inverted mountains or table-lands, its vast nearly flat 

 abyssmal floor, and the sudden rise of this to the hundred 

 fathom line, forming a terrace or shelf around the sides of 

 the continents. These features, roughly represented in the 

 map prefixed, he would be unable to perceive. 



Before leaving this broad survey, we may make one further 

 remark. An observer, looking at the earth from without, 

 would notice that the margins of the Atlantic and the main 

 lines of direction of its mountain chains are north-east and 

 south-west, and north-west and south-east, as if some early 

 causes had determined the occurrence of elevations along 

 great circles of the earth's surface tangent to the polar circles. 



We are invited by the preceding general glance at the surface 

 of the earth to ask certain questions respecting the Atlantic, 

 (i) What has at first determined its position and form? (2) 

 What changes has it experienced in the lapse of geological 

 time? (3) What relations have these changes borne to the 

 development of life on the land and in the water ? (4) What 

 is its probable future ? 



Before attempting to answer these questions, which I shall 

 not take up formally in succession, but rather in connection 

 with each other, it is necessary to state, as briefly as possible, 

 certain general conclusions respecting the interior of the earth. 

 It is popularly supposed that we know nothing of this beyond 

 a superficial crust perhaps averaging 50,000 to 100,000 feet in 



