﻿I860.] 



FORBES BOLIVIA AND PERU. 



59 



present in some is the last mineral element to crystallize and become 

 solid ; it seems probable that, during this cooling, the consequent 

 expansion due to the crystallization of the constituents has forced 

 those components (quartz, along with iron-pyrites, gold, &c, which 

 latter, from their very low fusibility, would remain longest of all in 

 a fluid state), still fluid, into the fissures of the neighbouring rocks, 

 and so formed such auriferous quartz-veins, which observation shows 

 are only developed in the slate-rocks at no very great distance from 

 granitic eruptions, either visible or such as, though hidden, may 

 reasonably be inferred to exist. This granite is the same which 

 is everywhere met with in the diluvium of the eastern plateau, as 

 large blocks, frequently used as a building-material where solidity is 

 required*. 



Although we do not meet with any actual granite on the direct 

 line of section without diverging (as I have done in order to show the 

 granitic outburst of the Silla Tuncari), we find, a little to the west of 

 Unduavi, these beds broken through by a fault, probably due to this 

 eruption. The granite has also caused an anticlinal in the strata, 

 which hitherto, from the summit of the pass, had constantly dipped 

 to the east, but now become inclined to the westward, and continue 

 so to San Felipe, near which place, however, after presenting some 

 contortions in their bedding, they again resume their western dip, 

 having at the Angostura, below San Felipe, a strike of about N. 30° 

 E., with 50° westwardly dip. 



At San Felipe the hard sandy shales, of a blue colour and slightly 

 micaceous, contained frequent Annerid-tracks and -burrows, and a 

 great number of small nail-shaped bodies like the spines of an Echinus, 

 also others horn-shaped, with concentric rings, both of which Mr. 

 Salter attributes to Annelids. Along with these were frequent spe- 

 cimens of the Cruziana Unduavi (PLY .fig. 7) ; and I also noticed several 

 specimens of the Boliviano, bipennis (fig. 11). Traces of other fossils 

 were everywhere frequent, but too indistinct to permit determination. 



About half a mile further down the valley, Annelid-tracks were 

 found in abundance in the hard blue siliceous slate, as well as im- 

 prints of Cruziana. About one mile further down, the thin-bedded 

 sandy and highly-indurated rocks were literally covered in all 

 directions with Annelid-tracks and -burrows, the same nail-headed 

 bodies previously described, and a variety of other and peculiar 

 markings. I also found some indistinct specimens of Boliviano Melo- 

 cactus and Cruziana Cucurbita, and several better ones of Cruziana 

 Unduavi, which last were found a little lower down the valley. 

 Eipple-marks are everywhere visible in these beds. 



In another valley, called the " Quebrada de Aceromarka," situated 

 a little to the south of Unduavi, in similar beds, specimens of the 

 shapeless Cruziana Cucurbita were found in abundance, in company 

 with the Boliviano Melocactus, and a single specimen of the Boliviano 



* The corinthian columns of the new cathedral at La Paz are hewn out of 

 this granite, and are most creditable to the architect, especially when it is con- 

 sidered that the cutting of this hard material has been entirely executed by the 

 Aymara Indians of the district. 



