g6 STUDIES IN INSECT LIFE, ETC. 



earth were removed and filled into all the hollows 

 we should have a world smooth as a billiard-ball 

 and completely surrounded by an ocean 1,450 

 fathoms deep. This is called the " mean sphere 

 level/' 



The Atlantic has an average depth differing 

 but little from the " mean sphere level." Com- 

 pared with the other great oceans it has an un- 

 usually large area of comparatively shallow water. 

 Of its total area 27-5 per cent, is covered by water 

 less than 1,000 fathoms deep : 18 per cent, lies 

 between 1,000 and 2,000 fathoms : and 47 per 

 cent, between 2,000 and 3,000 fathoms ; the 

 remaining 7-5 per cent, is still deeper. 



At the foot of the continental slope lies an 

 illimitable plain, of a uniform dull, greyish-buff 

 colour, flat and featureless as the desert, and only 

 diversified by an occasional as yet uncovered rock 

 or wreck or the straight line of a recently laid 

 cable. This plain continues with hardly a change 

 in scenery or in level until we approach the great 

 mid-Atlantic ridge. As Bruce has shown, this 

 ridge, which roughly bisects the Atlantic, extends 

 from Iceland as far south as 53 of south latitude, 

 with a slight and quite inexplicable break just 

 under the equator. The ridge runs almost parallel 



