494 Section of the Portillo Chain. part n. 



whitish sandstone was beautifully distinct ; the sand- 

 stone being penetrated by numerous, great, tortuous 

 dikes branching from the granite, and having been 

 converted into a granular quartz rock (singularly like 

 that of the Falkland Islands), containing specks of an 

 ochery powder, and black crystalline atoms, apparently 

 of imperfect mica. The quartzose strata in one spot 

 were folded into a regular dome. 



The granite which composes the magnificent bare 

 pinnacles and the steep western flank of the Portillo 

 chain, is of a brick-red colour, coarsely crystallized, and 

 composed of orthitic or potash feldspar, quartz, and im- 

 perfect mica in small quantity, sometimes passing into 

 chlorite. These minerals occasionally assume a laminar 

 or foliated arrangement. The fact of the feldspar being 

 orithic in this range, is very* remarkable, considering 

 how rare, or rather, as I believe, entirely absent, this 

 mineral is throughout the western ranges, in which 

 soda-feldspar, or at least a variety cleaving like albite, 

 is so extremely abundant. In one spot on the western 

 flank, and on the eastern flank near Los Manantiales 

 and near the crest, I noticed some great masses of a 

 whitish granite, parts of it fine-grained, and parts con- 

 taining large crystals of feldspar ; I neglected to collect 

 specimens, so I do not know whether this feldspar is 

 also orthitic, though I am inclined to think so from its 

 general appearance. I saw also some syenite and one 

 mass which resembled andesite, but of which I likewise 

 neglected to collect specimens. From the manner in 

 which the whitish granites formed separate mountain- 

 masses in the midst of the brick-red variety, and from 

 one such mass near the crest being traversed by nume- 

 rous veins of flesh-coloured and greenish eurite (into 

 which I occasionally observed the brick-red granite 

 insensibly passing), I conclude that the white granites 



