250 The Realm of Nature CHAP. 



coast and the port of Adria, which originally gave its name 

 to the sea. 



326. Submarine Canons. Mr. Buchanan points out that 

 along the margin of the Gulf of Guinea the soft mud 

 brought down by the Niger and the Congo builds up the 

 slope of the transitional area, diminishing its steepness ; but 

 that right under the broad, swift, and deep current of the 

 Congo there is a deep submarine gully or canon walled 

 by the soft mud, but kept clear from deposit by a strong 

 counter-current of sea-water setting along the bottom up 

 the estuary. This counter-current is due to the same cause 

 as that through the Bosphorus ( 238). Professor Forel 

 has pointed out a similar sub -lacustrine ravine under the 

 impetuous Rhone as it enters the Lake of Geneva laden 

 with glacier mud. 



327. River Work on Dry Plateaux. When a river 

 flows across an elevated plateau it wears out a channel for 

 itself, the form of which depends on the nature and arrange- 

 ment of the rocks and on the rainfall over the surface of 

 the region. The result of dip has already been referred to 

 (321). Lines of faults frequently mark out the sites of 

 valleys and affect their formation. For the sake of sim- 

 plicity and contrast, it will suffice to explain the extreme cases 

 of river action on arid and on rainy plateaux composed of 

 horizontally stratified rocks. In a dry plateau the river 

 flowing from a snow -topped mountain range, over the 

 steepest slope, receives few and small tributaries as it pro- 

 ceeds, and the action of the water loaded with wind-borne 

 sediment is to wear its channel down through the rocks. 

 Cutting now on one side, now on the other, it makes 

 rapid progress through the softer strata, forming banks of 

 comparatively gentle slope, and slower progress through the 

 harder which are cut into steeper cliffs. The walls of the 

 valley retain the original slope as the detritus, instead of 

 accumulating in a talus, is swept away as it is formed, and 

 weathering takes place very slowly in the dry atmosphere. 

 The valley becomes eroded in a somewhat V-shaped curve, 

 and forms a gorge narrow compared with its depth and 

 sunk far below the level of the plain. Such gorges occur 



