390 



PASSAGE OF CORDILLERA. March, 1835. 



fore likewise the repubhcs of Chile and Mendoza. To the 

 eastward, a mountainous and elevated region separates it 

 from the second range (called the Portillo) overlooking the 

 Pampas. The streams from the intermediate tract find a 

 passage a little way to the southward through this second 

 line. 



I will here give a very brief sketch of the geological 

 structure of these mountains: first, of the Peuquenes, or 

 western line; for the constitution of the two ranges is 

 totally different. The lowest stratified rock is a dull red or 

 purple claystone porphyry, of many varieties, alternating 

 with conglomerates, and breccia composed of a similar 

 substance : this formation attains a thickness of more than 

 a mile. Above it there is a grand mass of gypsum, which 

 alternates, passes into, and is replaced by, red sandstone, 

 conglomerates, and black .calcareous clay-slate. I hardly 

 dare venture to guess the thickness of this second division ; 

 but I have already said some of the beds of gypsum alone 

 attain a thickness of at least two thousand feet. Even at 

 the very crest of the Peuquenes, at the height of 13,210 

 feet, and above it, the black clay-slate contained numerous 

 marine remains, amongst which a gryphsea is the most abun- 

 dant, likewise shells, resembling turritellee, terebratulee, and 

 an ammonite. It is an old story, but not the less wonder- 

 ful, to hear of shells, which formerly were crawling about 

 at the bottom of the sea, being now elevated nearly four- 

 teen thousand feet above its level. The formation probably 

 is of the age of the central parts of the secondary series of 

 Europe. 



These great piles of strata have been penetrated, up- 

 heaved, and overturned, in the most extraordinary manner, 

 by masses of injected rock, equalling mountains in size. On 

 the bare sides of the hills, complicated dikes, and wedges 

 of variously-coloured porphyries and other stones, are seen 

 traversing the strata in every possible form and direction ; 

 proving also by their intersections, successive periods of 

 violence. The rock which composes the axis of these great 



