484 Section by the Portillo Pass. paet n 



belong to the andesitic class. These are succeeded by 

 some rugged Mils [B] of dark-green, crystalline, feld- 

 spatliic and in some parts slaty rocks, winch I be- 

 lieve belong to the altered clay-slate formation. From 

 this point, great mountains of purplish and greenish, 

 generally thinly stratified, highly porphyritic conglo- 

 merates, including many strata of amygdaloidal and 

 greenstone porphyries, extend up the valley to the 

 junction of the rivers Yeso and Yolcan. As the valley 

 here runs in a very southerly course, the width of the 

 porphyritic conglomerate formation is quite conjectural ; 

 and from the same cause, I was unable to make out 

 much about the stratification. In most of the exterior 

 mountains the dip was gentle and directed inwards; 

 and at only one spot I observed an inclination as high 

 as 50°. Near the junction of the R. Colorado with the 

 main stream, there is a hill of whitish, brecciated, 

 partially decomposed feldspathic porphyry, having a 

 volcanic aspect but not being really of that nature : at 

 Tolla, however, in this valley, Dr. Eleven l met with a 

 hill of pumice containing mica. At the junction of 

 the Teso and Yolcan [D] there is an extensive mass, in 

 white conical hillocks, of andesite, containing some 

 mica, and passing either into andesitic granite, or into 

 a spotted, semi-granular mixture of albitic (?) feldspar 

 and hornblende : in the midst of this formation Dr. 

 Meyen found true trachyte. The andesite is covered 

 by strata of dark-coloured, crystalline, obscurely por- 

 phyritic rocks, and above them by the ordinary por- 

 phyritic conglomerates, — the strata all dipping away at 

 a small angle from the underlying mass. The surround- 

 ing lofty mountains appear to be entirely composed 

 of the porphyritic conglomerate, and I estimated its 

 thickness here at between 6,000 and 7,000 feet. 



1 « Eeise urn Erde,' Th. 1. SS. 338, 341. 



