320 NATURE AND MAN. 



case of the North Atlantic, which 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 sinks 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 flatness (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 &quot;ocean basins&quot; 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 fiom 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 sca!e enormously in excess of the 

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



