1835-J MENDOZA. 241 



March 2gth. We set out on our return to Chile, by the Uspallata 

 pass situated north of Mendoza. We had to cross a long and most 

 sterile traversia of fifteen leagues. The soil in parts was absolutely 

 bare, in others covered by numberless dwarf cacti, armed with formid- 

 able spines, and called by the inhabitants " little lions." There were, 

 also, a few low bushes. Although the plain is nearly three thousand 

 feet above the sea, the sun was very powerful ; and the heat, as well 

 as the clouds of impalpable dust, rendered the travelling extremely 

 irksome. Our course during the day lay nearly parallel to the Cordillera, 

 but gradually approaching them. Before sunset we entered one of 

 the wide valleys, or rather bays, which open on the plain : this soon 

 narrowed into a ravine, where a little higher up the house of Villa 

 Vicencio is situated. As we had ridden all day without a drop of 

 water, both our mules and selves were very thirsty, and we looked out 

 anxiously for the stream which flows down this valley. It was curious 

 to observe how gradually the water made its appearance : on the 

 plain the course was quite dry ; by degrees it became a little damper ; 

 then puddles of water appeared ; these soon became connected ; and 

 at Villa Vicencio there was a nice little rivulet. 



March 30^. The solitary hovel which bears the imposing name oi 

 Villa Vicencio, has been mentioned by every traveller who has crossed 

 the Andes. I stayed here and at some neighbouring mines during the 

 two succeeding days. The geology of the surrounding country is very 

 curious. The Uspallata range is separated from the main Cordillera 

 by a long narrow plain or basin, like those so often mentioned in Chile, 

 but higher, being six thousand feet above the sea. This range has 

 nearly the same geographical position with respect to the Cordillera, 

 which the gigantic Portillo line has, but it is of a totally different origin : 

 it consists of various kinds of submarine lava, alternating with volcanic 

 sandstones and other remarkable sedimentary deposits ; the whole 

 having a very close resemblance to some of the tertiary beds on the 

 shores of the Pacific. From this resemblance I expected to find 

 silicified wood, which is generally characteristic of those formations. 

 I was gratified in a very extraordinary manner. In the central part 

 of the range, at an elevation of about seven thousand feet, I observed 

 on a bare slope some snow-white projecting columns. These were 

 petrified trees, eleven being silicified, and from thirty to forty converted 

 into coarsely-crystallized white calcareous spar. They were abruptly 

 broken off, the upright stumps projecting a few feet above the ground. 

 The trunks measured from three to five feet each in circumference. 

 They stood a little way apart from each other, but the whole formed 

 one group. Mr. Robert Brown has been kind enough to examine the 

 wood: he says it belongs to the fir tribe, partaking of the character 

 of the Araucarian family, but with some curious points of affinity with 

 the yew. The volcanic sandstone in which the trees were embedded, 

 and from the lower part of which they must have sprung, had accumu- 

 lated in successive thin layers around their trunks ; and the stone yet 

 retained the impression of the bark. 



It required little geological practice to interpret the marvellous 



