540 Coqiiimbo, Arqueros. p^et ii. 



euritic base. In proceeding to the mines of Arqueros, 

 the strata of porphyrinic conglomerate are at first nearly 

 horizontal, an unusual circumstance, and afterwards 

 they dip gently to SSE. After having ascended to a 

 considerable height, we come to an undulatory district 

 in which the famous silver mines are situated : my 

 examination was chiefly confined to those of S. Rosa. 

 Most of the rocks in this district are stratified, dipping 

 in various directions, and many of them are of so 

 singular a nature, that at the risk of being tedious I 

 must briefly describe them. The commonest variety is 

 a dull-red. compact, finely-brecciated stone, containing 

 much iron and innumerable white crystallised particles 

 of carbonate of lime and minute extraneous fragments. 

 Another variety is almost equally common near S. Rosa ; 

 it has a bright green, scanty basis, including distinct 

 crystals and patches of white carbonate of lime, and 

 grains of red. semi-micaceous oxide of iron : in parts 

 the basis becomes dark green, and assumes an obscure 

 crystalline arrangement, and occasionally in parts it 

 becomes soft and slightly translucent like soapstone. 

 These red and green rocks are often quite distinct, and 

 often pass into each other : the passage being sometimes 

 affected by a fine brecciated structure, particles of the 

 red and green matter being- mingled together. Some 

 of the varieties appear gradually to become porphyritic 

 with feldspar ; and all of them are easily fusible into 

 pale or dark-coloured beads, strongly attracted by the 

 magnet. I should perhaps nave mistaken several of 

 these stratified rocks for submarine lavas, like some of 

 those described at the Puente del Inca, had I not ex- 

 amined, a few leagues eastward of this point, a fine series 

 of analogous but less metamorphosed, sedimentary beds 

 belonging to the gypseous formation, and probably de- 

 rived from a volcanic source. 



