LAKES OF SOUTHERN PATAGONIA. 245 



or even longer. None of them has as yet been fully explored or accu- 

 rately charted. All of them are, except on their eastern shores, sur- 

 rounded by lofty, precipitous mountains. The summits of many of the 

 latter are covered with immense fields of snow and ice, from which descend 

 glaciers that occasionally extend quite down the mountain slopes into the 

 waters of the lakes. Huge blocks are often detached from the front of the 

 glaciers and float off into the lake as icebergs of considerable size. 



The basins occupied by these lakes are largely of tectonic origin and 

 they are chiefly due to the unequal folding of the strata that took place 

 during the elevation of the southern Andes in late Tertiary times. 



With the exception of Lakes Viedma and Argentino, these lakes all dis- 

 charge their waters into the Pacific, notwithstanding the fact that they lie, 

 entirely to the eastward of the main range of the Andes, and that the 

 eastern extremities of most of them project even into the great plain of 

 eastern Patagonia. 



Just to the eastward of this series of lakes of tectonic origin and situated 

 on the plains, entirely without the foothills of the Andes, there is a second 

 series of lakes evidently of glacial origin. For the most part, these lakes 

 are of small size and of minor importance, though some of them, like 

 Laguna Blanca, Lake Cardiel, and Lakes Colhue and Musters, are of con- 

 siderable dimensions. These lakes have, for the most part, originated 

 from the damming of pre-Glacial drainage systems with glacial detritus, 

 during the recession of the glaciers, that occupied these valleys at the 

 close of the Glacial period. Like the lakes just mention'ed, they contain fresh 

 water. Although, as a rule, they have no surface outlet, the circulation per- 

 mitted by the confining glacial drift is usually sufficient to keep the waters 

 sweet, but a few of them do, in very dry seasons, become somewhat brackish. 



Scattered all over the Patagonian plains, from the Straits of Magellan to 

 Bahia Blanca, are great numbers of salt lakes. Such lakes are usually of 

 quite limited area and of exceedingly shallow depth, though they occa- 

 sionally attain to considerable dimensions. In reference to their origin, I 

 have called these salt lakes residual lakes. I have elsewhere advanced 

 the theory that these lakes have resulted from confined bodies of water, 

 cut off from the sea, during the process of elevation, which began at the 

 close of the Tertiary and which resulted in the final recovery of this 

 region from the ocean. I have held that the salt of these lakes has been 

 derived directly from sea water and has not resulted by evaporation from 



