March, 1835. geology. 391 



lines of dislocation, at a distance very closely resembles 

 granite, but on examination, it is found rarely to contain 

 any quartz ; and instead of ordinary felspar, albite. 



The nietamorphic action has been very great, as might 

 have been expected from the close proximity of such grand 

 masses of rock, which were injected when in a liquefied 

 state from heat. When it is known, first, that the stratified 

 porphyries have flowed as streams of submarine lava under 

 an enormous pressure, and that the mechanical beds sepa- 

 rating them owe their origin to explosions from the same 

 submarine craters ; secondly, that the whole mass in the 

 lower part has generally been so completely fused into one 

 soHd rock by metamorphic action, that the lines of division 

 can only be traced with much difficulty ; and thirdly, that 

 masses of porphyry, undistinguishable by their mineralogical 

 characters from the two first kinds, have been subsequently 

 injected; — the extreme compUcation of the whole will readily 

 be beheved. 



We now come to the second range, which is of even 

 greater altitude than the first. Its nucleus in the section 

 seen in crossing the Portillo pass, consists of magnificent 

 pinnacles of coarsely-crystalhzed red granite. On the eastern 

 flank, a few patches of mica slate still adhere to the un- 

 stratified mass ; and at the foot a stream of basaltic lava has 

 burst forth at some remote period, — perhaps when the sea 

 covered the wide surface of the Pampas. On the western 

 side of the axis, between the two ranges, laminated fine sand- 

 stone has been penetrated by immense granitic dikes proceed- 

 ing from the central mass, and has thus been converted into 

 granular quartz rock. The sandstone is covered by other 

 sedimentary deposits, and these again by a coarse conglome- 

 rate, the vast thickness of which I wiU not attempt even to 

 estimate. All these coarse mechanical beds dip from the 

 red granite directly towards the Peuquenes range, as if 

 they passed beneath it ; though such is not the case. 

 On examining the pebbles composing this conglomerate 

 (which, to my surprise, betrayed no signs of metamorphic 



