DEPTH OF THE OCEAN 29 



graphical miles — will ever be recorded in the 

 ocean. The extreme variation in the irre- 

 gularities of the external surface of the earth's 

 crust, so far as is known at the present day, is 

 61,091 feet, or over eleven and a half English 

 miles — that is between the top of Mount 

 Everest and the bottom of the Swire Deep. 



Submarine Elevations. — With the progress 

 of our knowledge regarding the depth of the 

 ocean the number of isolated submarine 

 cones made known from time to time has 

 been greatly increased. Some of these are 

 deeply submerged, while others rear their 

 summits so near to the surface as to become 

 dangerous to navigation. With the excep- 

 tion of a few situated near continental shores, 

 they are probably all of volcanic origin, 

 although some of them are now covered with a 

 white mantle of calcareous ooze or coral 

 growths. Many of the coral atolls and other 

 oceanic islands are merely the summits of such 

 volcanic cones. In the vicinity of these sub- 

 marine cones the sea-floor may be very irreg- 

 ular, and in one or two cases there is evidence 

 of precipitous cliffs on the submerged slopes, 

 but, generally speaking, what we know of 

 ocean soundings leads us to suppose that 

 the sub-oceanic slopes are extremely gentle. 

 The steepest gradients usually occur on the 

 continental slope in depths of 100 to 1700 



