GEOGRA PHIC FEA TURES OF SO UTHERN PA TA G ONI A 53 



just keep pace with the sedimentation going on over tlie bottom of 

 the sea. After a time the rate of subsidence became less rapid or 

 ceased entirely, and the shallow sea was graduall}'^ converted into a 

 series of estuaries, lakes, and dry lands, in and over which were de- 

 posited the Santa Cruz beds of lacustrine and £eolian origin. For a 

 long period, extending over late Miocene and earl}- Pliocene times, 

 this region was elevated above the sea. During this long period of 

 late Tertiary elevation the surface of the land was subjected to ero- 

 sion, and the courses of all the more important valleys and drainage 

 systems now existing were then determined. Toward the close of the 

 Pliocene this entire region was again submerged beneath the sea for 

 a short period, but sufficient for the deposition of the marine Cape 

 Fairweather beds. During this second period of submergence the 

 Andes would appear as a long archipelago of high mountainous 

 islands. 



At the close of the Pliocene there began over this region a process 

 of elevation, which, as has been shown, was much more considerable 

 toward the north than in the south. This difference in the amount 

 of elevation accomplished in the northern and southern regions has 

 determined the presence of the series of fresh-water lakes now found 

 in the north in the same relative positions that are occupied farther 

 south by the fiords and inlets from the Pacific. I have obtained ab- 

 solute proof that this elevation in the north along the Andes has not 

 been less than 5,000 feet, and that it has been much greater in the 

 north than in the south and far greater along the Andes than over 

 the plains. 



As this elevation proceeded, each of the transverse valleys, which, 

 as we have already remarked, had their origin previous to tlie Jast 

 submergence, would appear successively first, as straits connecting the 

 two oceans, and next as valleys, with deep bays along the coast. The 

 Strait of Magellan is the last or most southerly of these great trans- 

 verse valleys, and still exists as a strait connecting the two oceans. 



Turning now to the eastern longitudinal valley, it will be seen that 

 as the elevation ])rogressed it would at first be broken up into a series 

 of fiords and inlets toward the north still communicating with the 

 Pacific through the deeper channels intersecting the main range of 

 the Andes. In time such communications would be severed and the 

 heads and arms of the fiords would })e left as lakes to discharge their 

 waters into the Pacific b}' the last and deepest of the connecting chan- 

 nels. We have thus represented between the Strait of Magellan and 



