chap. xit. Section of the Portillo Chain. 493 



where first met with, after leaving the gypseous forma- 

 tion, are inclined westward at an angle of only 20°, 

 which farther on increases to about 45°. The gypseous 

 strata, as we have seen, are also inclined westward : 

 hence, when looking from the eastern side of the valley 

 towards the Peuquenes range, a most deceptive appear- 

 ance is presented, as if the newer beds of conglomerate 

 dipped directly under the much older beds of the 

 gypseous formation. In the middle of the valley, a 

 bold mountain of unstratified lilac-coloured porphyry 

 (with crystals of hornblende) projects ; and farther on, 

 a little south of the road, there is another mountain, 

 with its strata inclined at a small angle eastwards, which 

 in its general aspect and colour, resembles the porphy- 

 ritic conglomerate formation, so rare on this side of the 

 Peuquenes line and so grandly developed throughout 

 the western ranges. 



The conglomerate is of great thickness : I do not 

 suppose that the strata forming the separate mountain- 

 masses [V, V, V] have ever been prolonged over each 

 other, but that one mass has been broken up by several, 

 distinct, parallel, uniclinal lines of elevation. Judging 

 therefore of the thickness of the conglomerate, as seen 

 in the separate mountain-masses, I estimated it at least 

 from 1,500 to 2,000 feet. The lower beds rest con-' 

 formably on some singularly coloured, soft strata [W], 

 which I could not reach to examine ; and these again 

 rest comformably on a thick mass of micaceous, thinly 

 laminated, siliceous sandstone [X], associated with a 

 little black clay-slate. These lower beds are traversed 

 by several dikes of decomposing porphyry. The lami- 

 nated sandstone is directly superimposed on the vast 

 masses of granite [Y, Y] which mainly compose the 

 Portillo range. The line of junction between this 

 latter rock, which is of a bright red colour, and the 



