chap. xv. Porphyritic Formation. 553 



of gypsum. These strata are conformably overlaid by a 

 great thickness of thinly stratified, compact limestone 

 with included crystals of carbonate of lime. At a place 

 called Tierra Amarilla, at the foot of a mountain thus 

 composed, there is a broad vein, or perhaps stratum, of 

 a beautiful and curious crystallised mixture, composed, 

 according to Prof. G. Rose, 1 of sulphate of iron under 

 two forms, and of the sulphates of copper and alumina : 

 the section is so obscure that I could not make out 

 whether this vein or stratum occurred in the gypseous 

 formation, or more probably in some underlying masses 

 [A], which I believe are altered clay-slate. 



Second Axis of Elevation. — After the gypseous 

 masses [B], we come to a line of hills of unstratified 

 porphyry [C], which on their eastern side blend into 

 strata of great thickness of porphyritic conglomerate, 

 dipping eastward. This latter formation, however, here 

 has not been nearly so much metamorphosed as in most 

 parts of centra] Chile ; it is composed of beds of true 

 purple claystone porphyry, repeatedly alternating with 

 thick beds of purplish-red conglomerate with the well- 

 rounded, large pebbles of various porphyries, not blended 

 together. 



Third Axis of Elevation. — Near the ravine of Los 

 Hornitos, there is a well-marked line of elevation, 

 extending for many miles in a NNE. and SSW. direc- 

 tion, with the strata dipping in most parts (as in the 

 second axis) only in one direction, namely, eastward at 

 an average angle of between 30° and 40°. Close to the 

 mouth of the valley, however, there is, as represented 

 in the section, a steep and high mountain [D], composed 

 of various green and brown intrusive porphyries en- 

 veloped with strata, apparently belonging to the upper 

 parts of the porphyritic conglomerate, and dipping 

 1 Meyen's « Reise,' &c. Th. 1. s. 394. 



