chap. xiv. Section by the Cumbre Pass. 5 1 3 



and looking to the west or backwards, the Cordillera 

 appear composed of huge, square, nearly horizontal, 

 tabular masses : so wide a space, with such lofty moun- 

 tains so equably elevated, is rarely met with within the 

 Cordillera. In this line of section, the interval between 

 the Puente del Inca and the neighbourhood of the 

 Cumbre, includes all the chief axes of dislocation. 



The altered clay-slate formation, already described, 

 is seen in several parts of the valley as far down as Las 

 Vacas, underlying the porphyritic conglomerate. At 

 the Casa de Pujios [L], there is a hummock of (ande- 

 sitic ?) granite ; and the stratification of the surrounding 

 mountains here changes from W. by S. to SW. Again, 

 near the R. Vacas there is a larger formation of (ande- 

 sitic ?) granite [M], which sends a mesh-work of veins 

 into the superincumbent clay-slate, and which locally 

 throws off the strata, on one side to NW. and on the 

 other to SE. but not at a high angle : at the junction, 

 the clay-slate is altered into fine-grained greenstone. 

 This granitic axis is intersected by a green dike, which 

 I mention, because I do not remember having elsewhere 

 seen dikes in this lowest . and latest intrusive rock. 

 From, the R Vacas to the plain of Uspallata, the valley 

 runs NE., so that I have had to contract my section; 

 it runs exclusively through porphyritic rocks. As far 

 as the Pass of Jaula, the clay-stone conglomerate forma- 

 tion, in most parts highly porphyritic, and crossed by 

 numerous dikes of greenstone-porphyry, attains a great 

 thickness : there is also much intrusive porphyry. From 

 the Jaula to the plain, the stratification has been in 

 most places obliterated, except near the tops of some of 

 the mountains ; and the metamorphic action has been 

 extremely great. In this space, the number and bulk 

 of the intrusive masses of differently coloured porphyries, 

 injected one into another and intersected by dikes, is 



