chap. xv. of the Cordillera. 593 



the trachytic lavas of the Cordillera. The andesitic 

 rocks have in most cases been the last injected ones, 

 and they probably form a continuous dome under this 

 great range : they stand in intimate relationship with 

 the modern lavas ; and they seem to have been the 

 immediate agent in metamorphosing the porphyritic 

 conglomerate formation, and often likewise the gypseous 

 strata, to the extraordinary extent to which they have 

 suffered. 



With respect to the age at which the several parallel 

 ridges composing the Cordillera were up thrown, I have 

 little evidence. Many of them may have been con- 

 temporaneously elevated and injected in the same 

 manner 1 as in volcanic archipelagoes lavas are contem- 

 poraneously ejected on the parallel lines of fissure. 

 But the pebbles apparently derived from the wear and 

 tear of the porphyritic conglomerate formation, which 

 are occasionally present in the upper parts of this same 

 formation, and are often present in the gypseous for- 

 mation, together with the pebbles from the basal parts 

 of the latter formation in its upper strata, render it 

 almost certain that portions, we may infer ridges, of 

 these two formations were successively upheaved. In 

 the case of the gigantic Portillo range, we may feel 

 almost certain that a pre-existing granitic line was up- 

 raised (not by a single blow, as shown by the highly 

 inclined basaltic streams in the valley on its eastern 

 flank) at a period long subsequent to the upheavement 

 of the parallel Peuquenes range. 2 Again, subsequently 



1 See the latter part of Chapter VI. 



2 I have endeavoured to show in my 'Journal ' (2nd edit. p. 321), 

 that the singular fact of the river, which drains the valley between 

 these two ranges, passing through the Portillo and higher line, is 

 explained by its slow and subsequent elevation. There are many 

 analogous cases in the drainage of rivers : see ' Edinburgh New Phil. 

 Journal,' vol. xxviii. pp. 33 and 44. 



