298 Formation of Valleys. paet n. 



successive levels, in the same manner as we see now 

 taking place at the heads of all those many, deep, 

 winding fiords intersecting the southern coasts. To my 

 mind, this has been one of the most important con- 

 clusions to which my observations on the geology of 

 South America have led me ; for we thus learn that one 

 of the grandest and most symmetrical mountain-chains 

 in the world, with its several parallel lines, 1 have been 

 together uplifted in mass between 7,000 and 9,000 

 feet, in the same gradual manner as have the eastern 

 and western coasts within the recent period. 



Formation of Valleys. 



The bulk of solid rock which has been removed in 

 the lower parts of the valleys of the Cordillera has been 

 enormous ; it is only by reflecting on such cases as that 

 of the gravel beds of Patagonia, covering so many thou- 

 sand square leagues of surface, and which if heaped into 

 a ridge, would form a mountain-range, almost equal to 

 the Cordillera, that the amount of denudation becomes 

 credible. The valleys within this range, often follow 



1 I do not wish to affirm that all the lines have been uplifted 

 quite equally ; slight differences in the elevation would leave no 

 perceptible effect on the terraces. It may, however, be inferred, 

 perhaps with one exception, that since the period when the sea 

 occupied these valleys, the several ranges have not been dislocated 

 by great and abrupt faults or upheavals ; for if such had occurred, 

 the terraces of gravel at these points would not have been conti- 

 nuous. The one exception is at the lower end of a plain in the Yalle 

 del Teso (a branch of the Maypu), where, at a great height, the 

 terraces and valley appear to have been broken through by a line of 

 upheaval, of which the evidence is plain in the adjoining mountains; 

 this dislocation, perhaps, occurred after the elevation of this part of 

 the valley above the level of the sea. The valley here is almost 

 blocked up by a pile above 1,000 feet in thickness, formed, as far as 

 I could judge, from three sides, entirely, or at least in chief part, of 

 gravel and detritus. On the south side, the river has cut quite 

 through this mass ; on the northern side, and on the very summit 

 deep ravines, parallel to the line of the valley, are worn, as if the 

 drainage from the valley above had passed by the.-.e two lines before 

 following its present course. 



