Ixxviii PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



by two rivers passing through breaches in the mountains to the east. 

 On the banks of one of them, the Mendoza, the plain is seen to be 

 composed of a great accumulation of stratified shingle estimated at 

 400 feet in thickness. The origin of these accumulations of gravel 

 Mr. Darwin considers to be inexplicable by debacles or ordinary al- 

 luvial action. He supposes that the sea formerly occupied the val- 

 leys of the Chilian Cordillera, in precisely the same manner as it now 

 does in the more southern parts of the continent, where deep wind- 

 ing creeks penetrate into the very heart of, and quite through this 

 great range ; that the mountains were upraised in the same slow 

 manner as the eastern and western coasts have been upraised within the 

 pleistocene period ; that on this view every part of the bottom of 

 each valley will have long stood at the head of a sea-creek, into which 

 the then existing torrents would deliver fragmicnts of rocks, which, by 

 the action of the tides, would be rolled, rudely stratified, and the surface 

 of the mass levelled into successive sea-beaches ; as the slow rising of 

 the Cordillera would probably be interrupted by long periods of rest. 

 He considers this to have been one of the most important conclusions 

 to which his observations on the geology of South America have led 

 him, " for we thus learn that one of the grandest and most symmiC- 

 trical mountain-chains in the world, with its several parallel lines, have 

 been together uplifted in mass between 7000 and 9000 feet, in the same 

 gradual manner as have been the eastern and western coasts*." 



On the western flank of the Uspallata range, at a height of 7000 feet 

 above the level of the sea, Mr. Darwin discovered, in an argillaceous 

 sandstone, a group of fifty-two stems of trees, part of them silicified, 

 but the greater number changed into carbonate of lime, with cavities 

 lined by quartz crystals. They project between two and five feet 

 above the ground, and stand at exactly right angles to the strata in 

 which they are contained, and which are inclined at an angle of 25°. 

 Specimens which he brought home he submitted to the examination 

 of Mr. Robert Brown, who pronounced the wood to be coniferous, 

 partaking of the character of the Araucarian tribe, with some curious 

 points of afiinity with the Yew. The stems have in general nearly 

 the same diameter, about fifteen inches, some twelve, others eighteen 

 inches ; they are grouped in a clump within a space of about sixty 

 yards, and all stand at the same level. The strata in which they 

 are contained rest on a thick bed of submarine lava ; they are covered 

 by indurated tuifs, passing upwards into a fine-grained purplish sedi- 

 mentary rock, the united thickness of the argillaceous sandstone and 

 tuffs being from 400 to 500 feet, and upon them is another mass of 

 fine-grained basalt 1000 feet thick ; and above this basalt Mr. Darwin 

 could clearly distinguish five conformable alternations, each several 

 hundred feet in thickness, consisting of sedimentary rocks and lavas. 



What a wonderful chapter is this spot in the history of the earth ! 

 what a tale it tells of repeated elevations and depressions of the land 

 on a vast scale, and all within a comparatively modern period in geo- 

 logical chronology ! It is a document written in characters so clear, 

 so intelligible, as to admit of no doubt of their true meaning. They 



* Page 67. 



