292 PATAGONIAN EXPEDITIONS! GEOGRAPHY. 



and sea areas that existed there during late Tertiary times, with an appre- 

 ciation of the greater elevation which has taken place over northern than 

 over southern Patagonia in recent times. 



From the present distribution of the rocks forming the marine Patago- 

 nian beds, we know that, during Middle Tertiary times, the entire south- 

 ern extremity of the continent, excepting, perhaps, the higher peaks of the 

 Andes, was submerged beneath a shallow sea. That this sea was nowhere 

 very deep is shown by the character of the fossils, which are everywhere 

 extremely abundant, and all belong to shallow water and littoral forms. 

 The accumulation of the 900 feet of rocks now forming the Patagonian 

 beds, containing throughout the fossil remains of characteristic shallow- 

 water forms, can only be explained by assuming that this region was 

 undergoing a subsidence sufficiently gradual to just keep pace with the 

 sedimentation going on over the bottom of the sea. After a time the 

 rate of subsidence became less rapid or ceased entirely, and the shallow 

 sea was gradually converted into a series of estuaries, lakes, and dry 

 lands, in and over which were deposited the Santa Cruz beds of lacustrine 

 and aeolian origin. For a long period, extending over late Miocene and 

 early 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 erosion, 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 absolute proof that this eleva- 

 tion 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 



