602 Summary and Conchision. pabt h. 



sionally feldspabhic streams and abundant mineral 

 •exhalations during the gypseous or cretaceo-oolitic 

 period ; then the eruptions of the Uspallata range, and 

 at an ancient but unknown period, when the sea came 

 up to the eastern foot of the Cordillera, streams of ba- 

 saltic lava at the foot of the Portillo range ; then the 

 old Tertiary eruptions ; and lastly, there are here and 

 there amongst the mountains much worn and appa- 

 rently very ancient volcanic formations without any 

 craters ; there are, also, craters quite extinct, and others 

 in the condition of solfataras, and others occasionally 

 or habitually in fierce action. Hence it would appear 

 that the Cordillera has been, probably with some 

 quiescent periods, a source of volcanic matter from an 

 epoch anterior to our cretaceo-oolitic formation to the 

 present day ; and now the earthquakes, daily recurrent 

 on some part of the western coast, give little hope 

 that the subterranean energy is expended. 



Recurring to the evidence by which it was shown 

 that some at least of the parallel ridges, which together 

 compose the Cordillera, were successively and slowly 

 upthrown at widely different periods; and that the 

 whole range certainly once, and almost certainly twice, 

 subsided some thousand feet, and being then brought 

 up by a slow movement in mass, again, during the old 

 Tertiary formations, subsided several hundred feet, and 

 again was brought up to its present level by a slow and 

 often interrupted movement; we see how opposed is 

 this complicated history of changes slowly effected, to 

 the views of those geologists who believe that this 

 great mountain-chain was formed in late times by a 

 single blow. I have endeavoured elsewhere to show, 1 

 that the excessively disturbed condition of the strata 

 in the Cordillera, so far from indicating single periods 



1 ' Geolog. Transact.,' vol. v. p. 626. 



