502 Section by the Cumbre Pass, part ii. 



Passage of the Andes by the Cumbre or Uspallata, 



Pass. 



This Pass crosses the Andes about sixty miles north 

 of that just described : the section given in Plate I. 

 fig. 2, is on the same scale as before, namely, at one- 

 third of an inch to a mile in distance, and one inch to 

 a mile (or 6,000 feet) in height. Like the last section, it 

 is a mere sketch, and cannot pretend to accuracy, though 

 made under favourable circumstances. We will com- 

 mence as before, with the western half, of which the 

 main range bears the name of the Cumbre (that is the 

 Eidge), and corresponds to the Peuquenes line in the 

 former section ; as does the Uspallata range, though on 

 a much smaller scale, to that of the Portillo. Near the 

 point where the river Aconcagua debouches on the basin 

 plain of the same name, at a height of about 2,300 feet 

 above the sea, we meet with the usual purple and 

 greenish porphyritic clay-stone conglomerate. Beds of 

 this nature, alternating with numerous compact and 

 amygdaloidal porphyries, which have flowed as sub- 

 marine lavas, and associated with great mountain- 

 masses of various, injected, non-stratified porphyries, 

 are prolonged the whole distance up to the Cumbre or 

 central ridge. One of the commonest stratified por- 

 phyries is of a green colour, highly amygdaloidal with 

 the various minerals described in the preliminary dis- 

 cussion, and including fine tabular crystals of albite. 

 The mountain-range north (often with a little westing) 

 and south. The stratification, wherever I could clearly 

 distinguish it, was inclined westward or towards the 

 Pacific, and, except near the Cumbre, seldom at angles 

 above 25°. Only at one spot on this western side, on a 

 lofty pinnacle not far from the Cumbre, I saw strata 

 apparently belonging to the Gypseous formation, and 



