320 NATURE AND MAN. 



case of the North Atlantic, Avhich had been carefully sounded 

 along certain lines with a view to the laying of telegraph cables. 

 The first systematic survey of this kind brought out a set of facts 

 which were then supposed to be exceptional, but which the sound- 

 ings of the Challenger, taken in connection with those of the 

 United States ship Tuscarora and the German Gazelle, have shown 

 to be general ; viz. (i) that the bottom smks very gradually from 

 the coast of Ireland, westward, for a hundred miles or more ; (2) 

 that then, not far beyond the hundred-fathom line, it falls so 

 rapidly that depths of from 1200 to 1500 fathoms are met 

 with at only a short distance further west; (3) that after a 

 further descent to a depth of more than 2000 fathoms, the 

 bottom becomes a slightly undulating plain, whose gradients 

 are so low as to show scarcely any perceptible alteration of 

 depth in a section in which the same scales are used for vertical 

 heights and horizontal distances ; * and (4) that on the American 

 side as on the British this plain is bordered by a very steep slope, 

 leading up quickly to a bottom not much exceeding 100 

 fathoms in depth, which shallows gradually to the coast-line of 

 America. Nothing seems to have struck the Challenger surveyors 

 more than the extraordinary yfrt-Zz/i^ji- (except in the neighbourhood 

 of land) of that depressed portion of the earth's crust which forms 

 the floor of the great oceanic area ; the result of one day's 

 sounding enabling a tolerably safe guess to be formed as to the 

 depth to be encountered on the following day ; and thus, if the 

 bottom of the mid-ocean were laid dry, an observer standing on 

 any spot of it would find himself surrounded by a plain only com- 

 parable to that of the North American prairies or the South 

 American pampas. 



Thus our notions of the so-called " ocean basins " are found 

 to require considerable modification ; and it becomes obvious 

 that, putting aside the oceanic islands which rise from the bottom 

 of the sea, as mountain-peaks and ridges rise ficm the general 

 surface of the land, the proper oceanic area is a portion of the 

 crust of the earth which is depressed with tolerable uniformity 

 some thousands of feet below the land area, whilst the bands of 



* Sections drawn (as usual) with a vertical scale enormously in excess of the 

 horizontal altogether misrepresent the real character of the oceanic sea-bed. 



