47 8 PhUonic Rocks of the Cordillera, paet n. 



itic conglomerate being purple or reddish, even when 

 alternating with dusky-coloured or bright green por- 

 phyries and amygdaloids, is probably an analogous 

 circumstance to the scorige of the blackish basalts be- 

 ing often bright red. The ancient submarine orifices 

 whence the porphyries and their fragments were ejected 

 haying been arranged in a band, like most still active 

 volcanos, accounts for the thickness, the narrowness, and 

 linear extension of this formation. 



This whole great pile of rock has suffered much 

 metamorphic action, as is very obvious in the gradual 

 formation and appearance of the crystals of albitic 

 feldspar and of epidote — in the blending together 

 of the fragments — in the appearance of a lami- 

 nated structure in the feldspathic slate — and, lastly, 

 in the disappearance of the planes of stratification, 

 which could sometimes be seen on the same mountain 

 quite distinct in the upper part, less and less plain on 

 the flanks, and quite obliterated at the base. Partly 

 owing to this metamorphic action, and partly to the 

 close relationship in origin, I have seen fragments of 

 porphyries — taken from a metamorphosed conglomerate 

 — from a neighbouring stream of lava — from the nucleus 

 or centre (as it appeared to me) of the whole submarine 

 volcano — and lastly from an intrusive mass of quite 

 subsequent origin, all of which were absolutely undis- 

 tinguishable in external characters. 



One other rock, of plutonic origin, and highly im- 

 portant in the history of the Cordillera, from having 

 been injected in most of the great axes of elevation, and 

 from having apparently been instrumental in meta- 

 morphosing the superincumbent strata, may be con- 

 veniently described in this preliminary discussion. It 

 has been called by some authors Andesite : it mainly 



