FEATURES OF THE OCEAN BOTTOM 17 



The British telegraph cable steamers found extremely ilat 

 surfaces in the Atlantic. Between 46 and 38^ W. the 

 Britannia found in only one instance a slope which would be 

 appreciable to the naked eye (30' 44")> and for the whole of 

 this distance the slope was only 9' 42". Flat stretches of this 

 kind and extent are not met with on dry land. Agassiz found 

 in the South Pacific on a course of 3,200 miles the greatest 

 difference in depth to be only 400 fathoms that is, on the 

 average 9 inches to the mile. Similar results have been 

 obtained by American cable steamers in the North Pacific; 

 for 100 miles between 173 and 175 E. Long, the greatest 

 deviation from the mean depth of 5,938 metres was only +3 8 

 and -38 metres. Again, between 155 and 160 E. Long, 

 the greatest deviation from the mean of 5,790 metres was only 

 + 103 and - 112 metres. The great success of the submarine 

 telegraph cables is in part due to this flat surface of the ocean 

 bottom. Only near the land in the neighbourhood of the- 

 continental shelfs is there any steep and broken slope, and it 

 is precisely here that the breaks in the telegraph cable mainly 

 occur. In the North Sea west of Heligoland five-mile areas 

 show a difference in depth of about 2'8 metres, a flatness which 

 is nevef met with on land surfaces. 



Even when the flatness of the land surface approaches that 

 of the sea bottom, there are in reality striking differences. A 

 profile of the mid-European plain would show quite a number 

 of sharp folds, whereas a profile of a similar area in the bottom 

 of the North Atlantic would show at most a gentle swell or 

 ridge. Not only is the ocean bottom free from the denudation 

 caused on land by atmospheric influence, but it is also, apart 

 from a narrow zone in continental regions, free from those 

 dislocations of the earth's crust which have been and are respon- 

 sible for so great a diversity on the land surface of the globe. 



The steepest submarine slopes are those which support 

 oceanic islands. St. Helena has submarine slopes of from 

 38J to 40; Tristan D'Acunha, 33^; and St. Paul, for short 

 slopes, as much as 62. Many of these oceanic islands are due 



