chap. xiv. the Uspallata Range. 519 



range is broad and undulatory, with the stratification 

 undulatory and irregular : in a few places granitic and 

 porphyritic masses [Q] protrude, which, from the small 

 effect they have locally produced in deranging the 

 strata, probably form the upper points of a regular, 

 great underlying dome. These denuded granitic points, 

 I estimated at about 9,000 feet in height above the sea. 

 On the eastern slope, the strata in the upper part are 

 regularly inclined at about 25° to the east, so that the 

 summit of this chain, neglecting small irregularities, 

 forms a broad anticlinal axis. Lower down, however, 

 near Los Hornillos [R], there is a well-marked synclinal 

 axis, beyond which the strata are inclined at nearly the' 

 same angle, namely from 20° to 30°, inwards or west- 

 ward. Owing to the amount of denudation which this 

 chain has suffered, the outline of the gently inclined 

 eastern flank scarcely offers the slightest indication of 

 this synclinal axis. The stratified beds, which we have 

 hitherto followed across the range, a little farther down 

 are seen to lie, I believe unconformably, on a broad 

 mountainous band of clay-slate and grauwacke. The 

 strata and laminse of this latter formation, on the ex- 

 treme eastern flank, are generally nearly vertical; 

 further inwards they become inclined from 45° to 80° 

 to the west : near Villa Vicencio [S] there is apparently 

 an anticlinal axis, but the structure of this outer part 

 of the clay-slate formation is so obscure, that I have not 

 marked the planes of stratification in the coloured sec- 

 tion. On the margin of the Pampas, some low, much 

 dislocated spurs of this same formation, project in a 

 north-easterly line, in the same oblique manner as do 

 the ridges on the western foot, and as is so frequently 

 the case with those at the base of the main Cordillera. 



I will now describe the nature of the beds, begin- 

 ning at the base on the eastern side. First, for the 



