chap. x. Formation of Valleys. 299 



anticlinal but rarely synclinal lines ; that is, the strata 

 on the two sides more often dip from the line of valley 

 than towards it. On the flanks of the range, the valleys 

 most frequently run neither along anticlinal nor syn- 

 clinal axes, but along lines of flexure or faults ; that is, 

 the strata on both sides dip in the same direction, but 

 with different, though often only slightly different, 

 inclinations. As most of the nearly parallel ridges 

 which together form the Cordillera run approximately 

 north and south, the east and west valleys cross them 

 in zig-zag lines, bursting through the points where the 

 strata have been least inclined. No doubt the greater 

 part of the denudation was affected at the periods when 

 tidal creeks occupied the valleys, and when the outer 

 flanks of the mountains were exposed to the full force 

 of an open ocean. I have already alluded to the power 

 of the tidal action in the channels connecting great 

 bays ; and I may here mention that one of the surveying 

 vessels in a channel of this kind, though under sail, 

 was whirled round and round by the force of the current. 

 We shall hereafter see, that of the two main ridges 

 forming the Chilian Cordillera, the eastern and loftiest 

 one, owes the greater part of its angular upheaval to a 

 period subsequent to the elevation of the western ridge ; 

 and it is likewise probable that many of the other 

 parallel ridges have been angularly upheaved at different 

 periods ; consequently many parts of the surfaces of 

 these mountains must formerly have been exposed to 

 the full force of the waves, which, if the Cordillera 

 were now sunk into the sea, would be protected by 

 parallel chains of islands. The torrents in the valleys 

 certainly have great power in wearing the rocks ; as 

 could be told by the dull rattling sound of the many 

 fragments night and day hurrying downwards ; and as 

 was attested by the vast size of certain fragments, which 

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