5 1 8 Stratification of paet n. 



on the altered feldspathic clay-slate, whereas the por- 

 phyritic conglomerate formation alternates with and 

 rests conformably on it. These porphyries, moreover, 

 with the exception of the one blackish stratum, and of 

 the one indurated, white tufaceous bed, differ from the 

 beds composing the Uspallata range in the line of the 

 Villa Vicencio Pass. 



I will now give, first, a sketch of the structure of the 

 range, as represented in the coloured section, and will 

 then describe its composition and interesting history. 

 At its western foot, a hillock [N] is seen to rise out of 

 the plain, with its strata dipping at 70° to the west, 

 fronted by strata [0] inclined at 45° to the east, thus 

 forming a little north and south anticlinal axis. Some 

 other little hillocks of similar composition, with their 

 strata highly inclined, range NE. and SW., obliquely 

 to the main Uspallata line. The cause of these dislo- 

 cations, which, though on a small scale, have been vio- 

 lent and complicated, is seen to lie in hummocks of 

 lilac, purple and red porphyries, which have been in- 

 jected in a liquefied state through and into the under- 

 lying clay-slate formation. Several dikes were exposed 

 here, but in no other part, that I saw of this range. 

 As the strata consist of black, white, greenish, and 

 brown coloured rocks, and as the intrusive porphyries 

 are so brightly tinted, a most extraordinary view was 

 presented, like a coloured geological drawing. On the 

 gently inclined main western slope [P P], above the 

 little anticlinal ridges just mentioned, the strata dip at 

 an average angle of 25° to the west ; the inclination in 

 some places being only 10°, in some few others as much 

 as 45°. The masses having these different inclinations, 

 are separated from each other by parallel vertical faults 

 [as represented at P a], often giving rise to separate, 

 parallel, uniclinal ridges. The summit of the main 



