chap. xiy. Uspallata Range. 531 



streams are very compact; none being vesicular, and 

 only a few amygdaloidal : the effects which some of 

 them, especially those low in the series, have produced 

 on the tufaceous beds over which they have flowed is 

 highly curious. Independently of this local meta- 

 morphic action, all the strata undoubtedly display an 

 indurated and altered character ; and all the rocks of 

 this range — the lavas, the alternating sediments, the 

 intrusive granite and porphyries, and the underlying 

 clay-slate — are intersected by metalliferous veins. The 

 lava-strata can often be seen extending for great dis- 

 tances, conformably with the under- and over-lying 

 beds ; and it was obvious that they thickened towards 

 the west. Hence the points of eruption must have been 

 situated westward of the present range, in the direction 

 of the main Cordillera : as, however, the flanks of the 

 Cordillera are entirely composed of various porphyries, 

 chiefly claystone and greenstone, some intrusive, and 

 others belonging to the porphyritic conglomerate for- 

 mation, but all quite unlike these submarine lava- 

 streams, we must in all probability look to the plain of 

 Uspallata for the now deeply buried points of eruption. 

 Comparing our section of the Uspallata range with 

 that of the Cumbre, we see, with the exception of the 

 underlying clay- slate, and perhaps of the intrusive rocks 

 of the axes, a striking dissimilarity in the strata com- 

 posing them. The great porphyritic conglomerate 

 formation has not extended as far as this range ; nor 

 have we here any of the gypseous strata, the magnesian 

 and other limestones, the red sandstones, the siliceous 

 beds with pebbles of quartz, and comparatively little of 

 the conglomerates, all of which form such vast masses 

 over the basal series in the main Cordillera. On the 

 other hand, in the Cordillera, we do not find those end- 

 less varieties of indurated tufts, with their numerous 



