chap. xiv. Section by the Cumbre Pass. 503 



conformably capping a pile of stratified porphyries. 

 Hence, both, in composition and in stratification, the 

 structure of the mountains on this western side of 

 the divortium aquarum, is far more simple than in the 

 corresponding part of the Peuquenes section. In the 

 porphyritic clay-stone conglomerate, the mechanical 

 structure and the planes of stratification have generally 

 been much obscured and even quite obliterated towards 

 the base of the series, whilst in the upper parts, near 

 the summits of the mountains, both are distinctly dis- 

 played. In these upper portions the porphyries are 

 generally lighter coloured. In three places [X, Y, Z] 

 masses of andesite are exposed: at [Y], this rock con- 

 tained some quartz, but the greater part consisted of 

 andesitic porphyry, with only a few well-developed crys- 

 tals of albite, and forming a great white mass, having 

 the external aspect of granite, capped by much dark 

 unstratified porphyry. In many parts of the moun- 

 tains, there are dikes of a green colour, and other white 

 ones, which latter probably spring from underlying 

 masses of andesite. 



The Cumbre, where the road crosses it, is, according 

 to Mr. Pentland, 12,454 feet above the sea ; and the 

 neighbouring peaks, composed of dark purple and 

 whitish porphyries, some obscurely stratified with a 

 westerly dip, and others without a trace of stratification, 

 must exceed 13,000 feet in height. Descending the 

 eastern slope of the Cumbre, the structure becomes very 

 complicated, and generally differs on the two sides of 

 the east and west line of road and section. First we 

 come to a great mass [A] of nearly vertical, singularly 

 contorted strata, composed of highly compact red sand- 

 stones, and of often calcareous conglomerates, and pene- 

 trated by green, yellow, and reddish dikes ; but I shall 

 presently have an opportunity of describing in some 



