506 Section by the Cumbre Pass, paet n, 



ranges N, by W. and S. by E., forming a chain of 

 mountains, apparently little inferior in height to the 

 Cumbre : the strata, as we have seen, dip at an average 

 angle of 30° to the west. The flanks of the mountains 

 are here quite bare and steep, affording an excellent 

 section ; so that I was able to inspect the strata to a 

 thickness of about 4,000 feet, and could clearly dis- 

 tinguish their general nature for 1,000 feet higher, 

 making a total thickness of 5,000 feet, to which must 

 be added about 1,000 feet of the inferior strata seen a 

 little lower down the valley. I will describe this one 

 section in detail, beginning at the bottom. 



1st. The lowest mass is the altered clay- slate de- 

 scribed in the preliminary discussion, and which in this 

 line of section was here first met with. Lower down 

 the valley, at the R. de las Vacas, I had a better oppor- 

 tunity of examining it ; it is there in some parts well 

 characterised, having a distinct, nearly vertical, tortuous 

 cleavage, ranging NW. and SE., and intersected by 

 quartz veins : in most parts, however, it is crystalline 

 and feldspathic, and passes into a true greenstone often 

 including grains of quartz. The clay-slate, in its upper 

 half, is frequently brecciated, the embedded angular 

 fragments being of nearly the same nature with the 

 paste. 



2nd. Several strata of purplish porphyritic con- 

 glomerate, of no very great thickness, rest conformably 

 upon the feldspathic slate. A thick bed of fine, purple, 

 clay-stone porphyry, obscurely brecciated (but not 

 of metamorphosed sedimentary origin), and capped 

 by porphyritic conglomerate, was the lowest bed 

 actually examined in this section at the Puente del 

 Inca. 



3rd. A stratum, eighty feet thick, of hard and very 

 compact impure whitish limestone, weathering bright 



