220 GEOLOGY OF THE HIGH PLATEAUS. 



or plains, their velocity is at once checked by the diminished slope and the 

 coarser debris comes to rest. These streams lie (within the mountains) in 

 ravines usually profound, with steep flaring sides, and opening upon the 

 valley bottoms or plains through magnificent gateways, and every long 

 range or ridge has usually many such gateways opening at intervals of a 

 very few miles along its flank. At the gateway the stream begins to 

 surrender a part of its freight and to build up its channel. The check 

 given to the velocity of the stream here is marked, indeed, but less incisive 

 than might at first be supposed. The profile of the bed of the stream does 

 not have an angle at this point, but is curved very gently, and is concave 

 upward. Indeed, it is so throughout the entire course of the stream out- 

 side the gate and generally for a considerable distance inside the gate. 

 Thus the velocity of the stream slows down gradually and not suddenly. 

 As the velocity gradually diminishes so the stream gives up more and 

 more of its load. But the stuff which it drops along any small part of its 

 course is by no means of the same size; that is to say, there is no rigorous 

 sifting of the material in such a manner that the stones or particles at any 

 given place are of uniform size, while finer ones are carried on to be scrupu- 

 lously selected where the slope and velocity are less. On the contrary, all 

 sorts are deposited everywhere. Nevertheless there is a tendency to sort- 

 ing. Higher up the slope there is a greater proportion of coarser deposit; 

 lower down there is a larger proportion of finer deposit; but everywhere 

 the coarse and the fine are commingled. 



Where the stream is progressively building up its bed outside of the 

 gate, it is obvious that it cannot long occupy one position ; for if it persisted 

 in running for a very long time in one place it would begin to build an 

 embankment Its position soon becomes unstable, and the slightest cause 

 will divert it to a new bed which it builds up in turn, and which in turn 

 becomes unstable and is also abandoned. The frequent repetition of 

 these shiftings causes the course of the stream to vibrate radially around 

 the gate as a center, and in the lapse of ages it builds up a half-cone, the 

 apex of which is at the gate. The vibration is not regular, but vacillat- 

 ing, like a needle in a magnetic storm; but in the long run, and after very 



