49 8 Section of the Portillo Chain, past n. 



upheaved and tilted from the mountains : of this con- 

 clusion I can now entertain not the smallest doubt. 



At the mouth of the valley, within the cliffs of the 

 above lava-field, there are remnants, in the form of 

 separate small hillocks and of lines of low cliffs, of a 

 considerable deposit of compact white tuff (quarried for 

 filtering-stones), composed of broken pumice, volcanic 

 crystals, scales of mica, and fragments of lava. This 

 mass has suffered much denudation, and the hard mica- 

 schist has been deeply worn, since the period of its 

 deposition ; and this period must have been subsequent 

 to the denudation of the basaltic lava-streams, as at- 

 tested by their encircling cliffs standing at a higher 

 level. At the present day, under the existing arid 

 climate, ages might roll past without a square yard of 

 rock of any kind being denuded, except perhaps in the 

 rarely moistened drainage-channel of the valley. Must 

 we then look back to that ancient period, when the 

 waves of the sea beat against the eastern foot of the 

 Cordillera, for a power sufficient to denude extensively, 

 though superficially, this tufaceous deposit, soft al- 

 though it be ? 



There remains only to mention some little water- 

 worn hillocks [B B], a few hundred feet in height, and 

 mere mole-hills compared with the gigantic mountains 

 behind them, which rise out of the sloping, shingle- 

 covered margin of the Pampas. The first little range 

 is composed of a brecciated purple porphyritic clay- 

 stone, with obscurely marked strata dipping at 70° to 

 the SW. ; the other ranges consist of — a pale-coloured 

 feldspathic porphyry, — a purple clay-stone porphyry 

 with grains of quartz, — and a rock almost exclusively 

 composed of brick-red crystals of feldspar. These 

 outermost small lines of elevation extend in a NW. by 

 W. and SE. by S. direction. 



