288 BULLETIN OF THE 



mentioned valleys, had cut their way as canons through the strata 

 underlying it. It seems therefore possible that even if the former 

 extension of the Pacific Ocean over the tract occupied by the nitrate 

 beds cannot account entirely for the deposition of the salt and ni- 

 trates, it must at any rate have played an important part in their 

 formation. The rivers, taking their rise higher up in the Andes to 

 the eastward, flowing through the basins, are all fresh. The water 

 near the general surface of the basin is saline, but as we go down 

 we soon reach a stratum of absolutely fresh water, showing that if 

 the saline matter were brought down from the mountains it must 

 have all been washed out at the present day, and that the main 

 cause to which the formation of the nitrates has been assigned is no 

 longer active. Certainly neither the number nor size of the extinct 

 and actual river-beds crossing the Nitrate Basin favors the presump- 

 tion that they could have been a sufficient cause for the accumulation 

 of the immense deposits of salt extending over the large area covered 

 by the nitrate and other saline beds. 



From the careful observations made by Darwin on the elevation of 

 the west coast of South America, the positive proof of the recent 

 elevation of the continent (at certain points) to a height of 800 feet is 

 placed beyond doubt, it can, judging from terraces and other some- 

 what less positive proofs, be considered as reasonably certain that this 

 elevation extended to a height of 1,300 feet, while the presence of 

 corals at Tilibiche would seem to leave but little doubt that the 

 continent has gradually been raised within a comparatively recent 

 period to a height of at least 2,900 feet. The presence of extensive 

 saline basins on the west slope of the Andes, at a height of over 

 7,000 feet, flanked on their western edge by low ridges, may be due 

 to a similar cause. But, however this may be, we might almost be 

 tempted to claim that the elevation of the continent can be traced 

 to a still greater height, judging from the presence of eight species 

 of Allorchestes, a genus belonging to a truly marine family of Crus- 

 tacea (Orchestiadae) in Lake Titicaca, at a depth of 66 fathoms, and 

 thus attempt to establish the former connection with the sea of the 

 lake now at a height of 12,500 feet above the level of the Racine. 

 Only eight, and two of these are probably identical species, out of 

 eighty-one known species of this family, as I am informed by Mr. 



