EROSION AND ITS SIGNIFICANCE. 127 



1,500 feet. Exceptions to this rule occur near the heads 

 of the gorges, where canons 1,500 feet deep are not much 

 more than 200 or 300 yards across at their brinks. At a 

 considerable distance downstream the canons may become 

 much wider, sometimes attaining a width of 10 miles, and 

 nevertheless retaining their marked canon characters. 



Sapping and the action of storm waters have been credited 

 with most of the work of canon widening. The examination 

 of an ordinary side thalweg in such a canon, and which 

 receives only the drainage of the canon side, is interesting 

 however in this connection. In areas of dense geological 

 complexes acted upon by a considerable rainfall, the floor 

 of the channel will be found to consist of steep rocky ledges 

 and waterfalls, here and there cumbered with heavy rock 

 fragments. The whole stream course thus bears signs of 

 extreme youth. To leave the thalweg involves a scramble 

 or a difficult climb up an excessively steep spur to the sides 

 of which cling ferns, shrubs, jungle growths or even great 

 trees. The rocks are much shattered, granite blocks may 

 be easily detached ; they appear to have been wedged 

 apart; slates creep; and fresh scars of land and rock slides 

 are common, the latter showing the influence of great 

 master joints. Notwithstanding all this the spurs are 

 frequently heavily grassed, and their crests are often con- 

 vex to the sky. At the headwaters of the thalweg, that 

 is, at the angle where two spurs meet at the lip of the 

 canon (or at a point down the canon side) an amphithea- 

 trical enclosure may be seen often of appreciable size. 

 Here there is no catchment for rain waters, the rocks are 

 seen to be rotten, slates creep when present ; the material 

 is weathered, and by sapping from corrasion at a point 

 some distance down the thalweg the gravitative or amphi- 

 theatrical head is formed by the action of material falling 

 freely towards a common point. This amphitheatrical 



