576 Iquiqiie, Peru. part ii. 



trace of stratification in their central and basal parts : 

 in most cases, I believe this is simply due either to the 

 obliterating effects of metamorphic action, or to such 

 parts having been mainly formed of intrusive por- 

 phyries, or to both causes conjoined; in some instances, 

 however, it appeared to me very probable that the 

 great central unstratified masses of porphyry were the 

 now partially denuded nuclei of the old submarine 

 volcanos, and that the stratified parts marked the 

 points whence the streams flowed. In one case alone, 

 and it was in this Yalley of the Despoblado, I was able 

 actually to trace a thick stratum of purplish porphyry, 

 which for a space of some miles conformably overlay 

 the usual alternating beds of breccia-conglomerates and 

 clay-stone lavas, until it became united with, and blended 

 into, a mountainous mass of various unstratified por- 

 phyries. 



The difficulty of tracing the streams of porphyries 

 to their ancient and doubtless numerous eruptive 

 sources, may be partly explained by the very general 

 disturbance which the Cordillera in most parts has 

 suffered ; but I strongly suspect that there is a more 

 specific cause, namely, that the original points of 

 eruption tend to become the points of injection. This 

 in itself does not seem improbable ; for where the earth's 

 crust has once yielded, it would be liable to yield 

 again, though the liquefied intrusive matter might not 

 be any longer enabled to reach the submarine surface 

 and flow as lava. I have been led to this conclusion, 

 from having so frequently observed that, where part of 

 an unstratified mountain-mass resembled in mineralo- 

 gical character the adjoining streams or strata, there 

 were several other kinds of intrusive porphyries and 

 andesitic rocks injected into the same point. As these 

 intrusive mountain -masses form most of the axes-lines 



