LAVA PLAIN. I I I 



at this point did I see any shingle upon the surface of the lava. It was, 

 therefore, clear that this particular stream of lava had been ejected subse- 

 quent to the deposition of the great- shingle formation of Patagonia. The 

 fact that no shingle or other detritus is found upon the surface of the lava 

 beds throughout most of Patagonia appears to me as conclusive evidence 

 that these lavas were not poured out over the bottom of the sea as sup- 

 posed by Darwin. All these matters will be discussed later when I come 

 to treat of the geology of Patagonia. 



On the fifteenth of January we left our camp at th? bottom of the canon 

 and, pulling slowly up the eastern slope, finally gained the summit. To 

 the north of us lay a series of low, terraced benches, or steps, each rising 

 successively higher, until at a distance of some twenty or twenty-five 

 miles the summit of the high, level pampa appeared. From our point 

 of vantage on the east side it was evident that we had acted wisely in 

 crossing the canon at this point, for not only would it have been quite 

 impossible to have efi'ected a crossing for many miles above, but the 

 nature of the surface on the opposite side was such as would have offered 

 insurmountable obstacles to our further progress. Far away to the north 

 and west, on the western side .of the gorge, there extended a continuous 

 and uninterrupted lava sheet. As far as the eye could reach there appeared 

 only the broken and uneven surface of black and barren rocks. The sur- 

 face of the plain at the higher levels on the east side of the canon was free 

 from lava and afforded a fairly practicable highway to the northern coun- 

 try, for which we were headed. The topography of the country and the 

 distribution of the lava bore positive evidence that the greater part of the 

 canon had been cut prior to the outflow which had submerged the plain 

 to the west. A number of old craters seen in the distance had undoubtedly 

 been the source of the lava. From these it had flowed down in great 

 streams over the gently inclined surface, until coming in contact with the 

 eastern wall of the canon which had confined and restricted the flow in 

 that direction. 



By keeping along near the eastern crest of the canon wall we" found 

 fairly good travelling, and ascended the successive terraces as they were 

 encountered. Although constantly increasing our altitude, the bottom 

 of the canon below was rising even more rapidly, so that at a distance 

 of twenty miles, or just before gaining the broad level surface of the 

 higher pampa, the previously rugged nature of the canon disappeared 



