chap. xiv. Sectio7i by the Portillo Pass. 485 



Beyond the junction of the Yeso and Volcan, the 

 porphyritic strata appear to dip towards the hillocks of 

 andesite at an angle of 40° ; but at some distant points 

 on the same ridge they are bent up and vertical. 

 Following the valley of the Yeso, trending NE. (and 

 therefore still unfavourable for our transverse section), 

 the same porphyritic conglomerate formation is pro- 

 longed to near the Cuestadel Indio, situated at the 

 western end of the basin (like a drained lake) of Yeso. 

 Some way before arriving at this point, distant lofty 

 pinnacles capped by coloured strata belonging to the 

 great gypseous formation could first be seen. From 

 the summit of the Cuesta, looking southward, there is 

 a magnificent sectional view of a mountain-mass, at 

 least 2,000 feet in thickness [E], of fine andesitic 

 granite (containing much black mica, a little chlorite 

 and quartz), which sends great white dikes far into the 

 superincumbent, dark-coloured, porphyritic conglome- 

 rates. At the line of junction the two formations are 

 wonderfully interlaced together : in the lower part of 

 the porphyritic conglomerate, the stratification has 

 been quite obliterated, whilst in the upper part it is 

 very distinct, the beds composing the crests of the 

 surrounding mountains being inclined at angles of 

 between 70° and 80°, and some being even vertical. 

 On the northern side of the valley, there is a great 

 corresponding mass of andesitic granite, which is 

 encased by porphyritic conglomerate, dipping both 

 on the western and eastern sides, at about 80° to west, 

 but on the eastern side with the tips of the strata bent 

 in such a manner, as to render it probable that the 

 whole mass had been on that side thrown over and 

 inverted. 



In the valley-basin of the Yeso, which I estimated 

 at 7,000 feet above the level of the sea, we first reach 



