5 1 o Section by the Cumbre Pass, paet n. 



and well rounded pebbles, sometimes of large size, of 

 quartz, — from any other section hitherto described in 

 Chile. From these peculiarities, and from the lens- 

 form of the strata, it is probable that this great pile of 

 strata was accumulated on a shallow and very uneven 

 bottom, near some pre-existing land formed of various 

 porphyries and quartz-rock. The formation of por- 

 phyritic clay-stone conglomerate does not in this section 

 attain nearly its ordinary thickness ; this may he partly 

 attributed to the metamorphic action having been here 

 much less energetic than usual, though the lower beds 

 have been affected to a certain degree. If it had been 

 as energetic as in most other parts of Chile, many of 

 the beds of sandstone and conglomerate, containing 

 rounded masses of porphyry, would doubtless have been 

 converted into porphyritic conglomerate; and these 

 would have alternated with, and even blended into, 

 crystalline and porphyritic strata without a trace of 

 mechanical structure, — namely, into those which, in the 

 present state of the section, we see are unquestionably 

 submarine lavas. 



The beds of gypsum, together with the red alternat- 

 ing sandstones and conglomerates, present so perfect 

 and curious a resemblance with those seen in our former 

 section in the basin-valley of Yeso, that I cannot doubt 

 the identity of the two formations : I may add, that a 

 little westward of the P. del Inca, a mass of gypsum 

 passed into a fine-grained, harb , drown sandstone, which 

 contained some layers of black, calcareous, compact, 

 shaly rock, precisely like that seen in such vast masses 

 on the Peuquenes range. 



Near the Puente del Inca, numerous fragments of 

 limestone, containing some fossil remains, were scattered 

 on the ground : these fragments so perfectly resemble 

 the limestone of bed No. 3, in which I saw impressions 



