514 Section by the Cumbre Pass, paet ii. 



truly extraordinary. I saw one mountain of whitish 

 porphyry, from which two huge dikes, thinning out, 

 branched downwards into an adjoining blackish por- 

 phyry. Another hill of white porphyry, which had 

 burst through dark-coloured strata, was itself injected 

 by a purple, brecciated, and recemented porphyry, both 

 being crossed by a green dike, and both having been 

 upheaved and injected by a granitic dome. One brick- 

 red porphyry, which above the Jaula forms an isolated 

 mass in the midst of the porphyritic conglomerate 

 formation, and lower down the valley a magnificent 

 group of peaked mountains, differs remarkably from 

 all the other porphyries. It consists of a red feldspathic 

 base, including some rather large crystals of red feldspar, 

 numerous large angular grains of quartz, and little bits 

 of a soft green mineral answering in most of its charac- 

 ters to soap-stone. The crystals of red feldspar resemble 

 in external appearance those of orthite, though, from 

 being partially decomposed, I was unable to measure 

 them ; and they certainly are quite unlike the variety, 

 so abundantly met with in almost all the other rocks of 

 this line of section, and which, wherever I tried it, 

 cleaved like albite. This brick-red porphyry appears 

 to have burst through all the other porphyries, and 

 numerous red dikes traversing the neighbouring moun- 

 tains have proceeded from it : in some few places, 

 however, it was intersected by white dikes. From this 

 posteriority of intrusive origin, — from the close general 

 resemblance between this red porphyry and the red 

 granite of the Portillo line, the only difference being 

 that the feldspar here is less perfectly granular, and 

 that soap-stone replaces the mica, which is there im- 

 perfect and passes into chlorite, — and from the Portillo 

 line a little southward of this point appearing to blend 

 (according to Dr. Gillies) into the western ranges, — I 



