chap. xv. Illapel to Los Homos. 537 



is (judging from several specimens) all coniferous. 

 Some of the layers of the black siliceous slate contained 

 irregular angular fragments of imperfect pitchstone, 

 which I believe, as in the Uspallata range, has origin- 

 ated in a metamorphic process. There was one bed of 

 a marly tufaceous nature, and of little specific gravity. 

 Veins of agate and calcareous spar are numerous. The 

 whole of this gypseous formation, especially the upper 

 half, has been injected, metamorphosed, and locally 

 contorted by numerous hillocks of intrusive porphyries 

 crowded together in an extraordinary manner. These 

 hillocks consist of purple clay-stone and of various other 

 porphyries, and of much white feldspathic greenstone 

 passing into andesite ; this latter variety included in 

 one case crystals of orthitic and albitic feldspar touch- 

 ing each other, and. others of hornblende, chlorite, and 

 epidote. The strata surrounding these intrusive hil- 

 locks at the mines of Los Hornos, are intersected by 

 many veins of copper-pyrites, associated, with much 

 micaceous iron-ore, and by some of gold : in the neigh- 

 bourhood of these veins the rocks are blackened and 

 much altered. The gypsum near the intrusive masses 

 is always opaque. One of these hillocks of porphyry 

 was capped by some stratified porphyritic conglomerate, 

 which must have been brought up from below, through 

 the whole immense thickness of the overlying gypseous 

 formation. The lower beds of the gypseous formation 

 resemble the corresponding and probably contempora- 

 neous strata of the main Cordillera ; whilst the upper 

 beds in several respects resemble those of the Uspallata 

 chain, and possibly may be contemporaneous with 

 them ; for I have endeavoured to show that the Uspal- 

 lata beds were accumulated subsequently to the gyp- 

 seous or Neocomian formations of the Cordillera. 



This pile of strata dips at an angle of about 20° to 



