CONTINENTAL SHELF 207 



From the data at present available the depths over 

 the total floor of the ocean are now estimated as 

 follows : 



Between the shore line and the 100-fath. line, 7 per cent. 



100-fath. „ 1,000 ,, 9 



1,000 ,, ,, 2,000 ,, 19 



2,000 ,, ,, 3,000 ,, 58 



Over 3,000 fathoms 7 



It will be seen from the above that the extent of 

 the sea-bottom lying between the shore line and a 

 depth of 100 fathoms (600 feet) is very nearly equal 

 to the area lying between 100 fathoms (600 feet) and 

 1,000 fathoms (6,000 feet). The first area is known as 

 the continental shelf, and is regarded as belonging to 

 the continental areas. Many portions of it were 

 evidently dry land at no very remote geological 

 period . Deep ravines frequently cross the continental 

 shelf, and are believed to be submerged river valleys, 

 which in some cases can be associated with still exist- 

 ing rivers — for instance, the Rhine and the Congo. 

 The materials which cover the continental shelf are 

 to a large extent under the influence of waves, tides, 

 and currents, and are constantly subjected to trans- 

 port by these agencies. It is believed that waves or 

 currents do not transport other than the very finest 

 particles at depths greater than 100 to 150 fathoms (600 

 to 900 feet). In cup-shaped depressions, or sheltered 

 bays within the 100-fathoms line, fine mud may be de- 

 posited in relatively shallow depths, but on coasts facing 

 the great ocean basins muddy deposits commence to 

 form at an average depth of 100 fathoms. Beyond 

 that depth the whole floor of the ocean may be re- 

 garded as an area of deposition. , \Ye have as yet no 

 evidence of transport by currents at these greater 



