XI 



The Bed of the Oceans 



197 



from the summit of Mount Everest to sea-level is very little 

 steeper than the slope from sea-level to the bottom of the 

 Tuscarora Deep ; about i in i 5 or nearly 4. The steep- 

 ness of sloping land almost always appears greater to 

 the eye than it actually is. Only precipices of bare 

 rock have an angle of slope greater than 45 or a 

 gradient of i in i, and the steep slope of the lower part 

 of the world ridges probably rarely exceeds 35, which 

 on land would be felt a very steep hill to climb, a 

 gradient of i in ij. The steepest hill on a well-made 

 road is i in 20 or an angle of 3. Mr. J. Y. Buchanan 

 found that in some cases where the slope was comparatively 

 slight the original rocky wall had been covered by a 

 mound of sediment brought down from the neighbouring 

 land by great rivers ( 325, 326). In nearly all cases at 

 the top of the acclivity, usually at the point where the 

 depth of water is about 100 fathoms, the slope suddenly 

 becomes much more gentle, and continues very gradual up 

 to the coast line. This gentle slope has been termed the 

 Shore Flat, or the Continental Shelf. The typical profile of the 

 transitional area is given in 

 Fig. 38, which represents 

 the slope of part of the 

 Gulf of Guinea. The outer 

 curve shows the slope at 

 a part of the coast where 

 a pile of river -mud has 

 been thrown down like an 

 embankment in front of the 

 ridge face, thereby reduc- YlG 38 .-sio P es of the Gulf of Guinea, 

 ing its gradient. These The X^tV^V 5 , 40 times the 1 i ori " 



zontal. Solid black shows average slope 

 Slopes are represented forty of the coast edge ; the shaded part 



times steeper than they are SgSt*? 6 modified by river - borne 



in order to bring out the 



change of gradient, the vertical scale being forty times the 



horizontal. 



264. The Continental Shelf. The world ridges form- 

 ing the walls of the ocean-basins are flattened at the top 

 like the rim of a pudding-dish, and beyond the flat edge 



