CONTINENTAL BORDER 233 



the Continental Shelf." This shelf or margin, sloping 

 gradually to a depth of 100 or 200 fathoms, and ex- 

 tending for a distance of from 20 to 200 miles from 

 the shore, is succeeded by a relatively steep declivity, 

 the Continental Slope, descending to the ocean floor ; 

 this latter then stretches, at a depth of between 1,000 

 and 3,000 fathoms, over an area measuring about one- 

 third of the whole surface of the globe, and is hollowed 

 out in places into depths of 4,000 or even 5,000 fathoms 

 (compare Chapter VI.). The edge of the Continental 

 Shelf may be taken as a dividing line between two f aunal 

 areas, the sloping descent of a continent to the ocean 

 floor being regarded as a sort of neutral ground which, 

 while having a few inhabitants that belong more 

 properly to the Continental Shelf and a few peculiar 

 to itself, is for the most part peopled by species which 

 may be regarded as belonging to the fauna of the deep 

 sea. 



It is to be remembered that this dividing line does 

 not mark any sudden change either in the physical 

 conditions or in the animals to be met with, and that 

 any limit which may be chosen must in the nature of 

 things be more or less an arbitrary one, and its position 

 will be different in different regions. 



The physical conditions prevailing over the sea 

 bottom are dealt with fully in other parts of this 

 volume (Chapters II. and VI.), and it is only necessary 

 to call attention to some of the chief differences between 

 the areas which we are considering. 



The Continental Shelf. 



Throughout the area of the Continental Shelf we find 

 that the variability and liability to sudden alterations 

 in physical conditions, which are so noticeable in the 

 shallow water, are very much reduced. The changes 



