chap. xiy. Gypseous Formation, 489 



must be added, as will be immediately seen, a great 

 overlying mass of red sandstone. 



In descending the eastern slope of this great central 

 range, the strata, which in the upper part dip eastward 

 at about an angle of 40°, become more and more curved, 

 till they are nearly vertical; and a little farther onwards 

 there is seen on the farther side of a ravine, a thick 

 mass of strata of bright red sandstone [T], with their 

 upper extremities slightly curved, showing that they 

 were once conformably prolonged over the beds [S] : 

 on the southern and opposite side of the road, this red 

 sandstone and the underlying black shaly rocks stand 

 vertical, and in actual juxtaposition. Continuing to 

 descend, we come to a synclinal valley filled with 

 rubbish, beyond which we have the red sandstone [T 2 ] 

 corresponding with [T], and now dipping, as is seen 

 both north and south of the road, at 45° to the west ; 

 and under it, the beds [S 2 , R 2 , Q 2 , and I believe P 2 ] 

 in corresponding order and of similar composition, with 

 those on the western flank of the Peuquenes range, but 

 dipping westward. Close to the synclinal valley the 

 dip of these strata is 45°, but at the eastern or farther 

 end of the series it increases to 60°. Here the great 

 gypseous formation abruptly terminates, and is suc- 

 ceeded eastward by a pile of more modern strata. 

 Considering how violently these central ranges have 

 been dislocated, and how veiy numerous dikes are in 

 the exterior and lower parts of the Cordillera, it is 

 remarkable that I did not here notice a sino-le dike. 

 The prevailing rock in this neighbourhood is the black, 

 calcareous, compact shale, whilst in the valley-basin of 

 the Yeso the purplish-red sandstone or mudstone pre- 

 dominates, — both being associated with gypseous strata 

 of exactly the same nature. It would be very difficult 

 to ascertain the relative superposition of these several 



