chap. x. in the Valleys of the Cordillera. 291 



inclined towards its centre in such a manner as to 

 show that the whole bottom must once have been filled 

 up with a smooth and slightly concave mass, as still are 

 the dry unfurrowed valleys of northern Chile. Where 

 two valleys unite into one, these terraces are particu- 

 larly well exhibited, as is represented in the following 

 diagram. The thickness of the gravel forming these 



No. 28. 



{Bed oh 

 \Jtl\cr! 



Ground-plan of a bifurcating valley in the Cordillera, bordered by smooth, sloping 

 gravel-fringes (A A A), worn along the course of the river into cliffs. 



fringes, on a rude average, may be said to vary from 

 thirty to sixty or eighty feet ; but near the mouths of 

 the valleys it was in several places from 200 to 300 

 feet. The amount of matter removed by the torrents 

 has been immense; yet in the lower parts of the 

 valleys the terraces have seldom been entirely worn 

 away on either side, nor has the solid underlying rock 

 been reached : higher up the valleys, the terraces have 

 frequently been removed on one or the other side, and 

 sometimes on both sides ; but in this latter case they 

 re-appear after a short interval on the line, which they 

 would have held had they been unbroken. Where the 



