49 2 Section of the Portillo Chain, paet n. 



bouring pinnacles must, I conceive, rise to nearly 16,000 

 feet above the sea. The river draining the intermediate 

 valley of Tenuyan, passes through the Portillo line. 

 To return to our section : — shortly after leaving the 

 lower beds [P 2 ] of the gypseous formation, we come to 

 grand masses of a coarse, red conglomerate [V], totally 

 unlike any strata hitherto seen in the Cordillera. This 

 conglomerate is distinctly stratified, some of the beds 

 being well defined by the greater size of the pebbles : the 

 cement is calcareous and sometimes crystalline, though 

 the mass shows no signs of having been metamorphosed. 

 The included pebbles are either perfectly or only par- 

 tially rounded : they consist of purplish sandstones, of 

 various porphyries, of brownish limestone, of black 

 calcareous, compact shale precisely like that in situ in 

 the Peuquenes range, and containing some of the same 

 fossil shells ; also very many pebbles of quartz, some of 

 micaceous schist, and numerous, broken, rounded crys- 

 tals of a reddish orthitic or potash feldspar (as deter- 

 mined by Professor Aliller), and these from their size 

 must have been derived from a coarse-grained rock, 

 probably granite. From this feldspar being orthitic, 

 and even from its external appearance, I venture posi- 

 tively to affirm that it has not been derived from the 

 rocks of the western ranges ; but, on the other hand, 

 it may well have come, together with the quartz and 

 metamorphic schists, from the eastern or Portillo line, 

 for this line mainly consists of coarse orthitic granite. 

 The pebbles of the fossiliferous slate and of the purple 

 sandstone, certainly have been derived from the Peu- 

 quenes or western ranges. 



The road crosses the valley of Tenuyan in a nearly 

 east and west line, and for several miles we have on 

 both hands the conglomerate, everywhere dipping west 

 and forming separate great mountains. The strata, 



