54 GEOGRA PIIK' FK. 1 TURES OF SO UTIfERX PA TA GONIA 



Lake Argentina eveiy stage in the (leveloi)inent of the ])resent lake 

 systems of tlie southern Andes. 



A glance at one of Fitzroy's charts of the Magellan Strait instantly 

 reveals the fact that it is much deeper in its western than in its eastern 

 course. In fact, it is extremely shallow throughout its entire course 

 from Useless Bay eastward to Cape Virgin, and onlj' a comparatively 

 slight elevation would iiere suffice to bring its bottom above sea-level 

 and convert it into a valley connecting Tierra del Fuego with the 

 mainland, and changing Useless Ba}' first into a fiord, and later into a 

 lake as the elevation increased, sending its waters to the Pacific by 

 way of the much deeper western channels of the straits. 



The same conditions that exist today in the Strait of Magellan have 

 existed at some previous time over all the great transverse valleys of 

 Patagonia, and an elevation similar to that which has taken place 

 more to the northward would produce conditions along the course of 

 this strait identical with those now existing farther north. So also 

 an elevation of the region south of Ijake Argentina similar to that 

 which has taken place north of this lake would convert Last Hope 

 Inlet, Obstruction Sound, Skyring Water, and Otway Water from 

 marine fiords connecting directly witli the Pacific into a series of 

 fresh-water lakes discharging their waters into the same ocean. 



At i)resent Otway Water is separated from C'abeza del Mar, a small 

 bay extending inland from the eastern extension of the Magellan 

 Strait, by a narrow neck of land only eight miles in width, and with 

 a maximum altitude of perhaps less than 100 feet. Notwithstanding 

 tliis low altitude, the low blurts extending along the heads of both of 

 these l)ays are largeh' comjiosed of 'sedimentary rocks covered over 

 with only a thin layer of glacial detritus, proving conclusively that 

 the former connection that doubtless existed between these two bodies 

 of water has been broken not by a damming up b}' glacial materials, 

 l)Ut by an elevation sutticient to bring the sedimentary rocks at the 

 bottom above the water level. 



From the observations and conditions already referred to, and 

 many other facts l)earing directly upon these questions, I believe that 

 the longitudinal valley's separating tiie main range from the two lat- 

 eral ranges of the Andes, and also the great transverse valleys cross- 

 ing Patagonia from east to west, had their origin previous to the last 

 submergence, which took place over this region in late Pliocene times 

 and continued only for a relatively short })eriod. This submergence 

 was greater over the Avestern than over the eastern Andes, thus ren- 



