52 GEOd RA I'lUr FKA TUIIES OF SO UTHERM PA TA d OXIA 



III many places important streams enter this great longitudinal 

 valley from the eastern plains and discharge their waters into the 

 lakes, which in turn are emptied into the Pacific through rivers inter- 

 secting the main range of the Andes. This is true of all tlie lakes of 

 this region, with the one noted exception of Lake Argentina and its 

 affluents. The upper courses of the great transverse valley's of Pata- 

 gonia are always directly opposite some of the larger of these tributary 

 valleys, so that at such places the continental divide is exceedingly low 

 and inconspicuous. This condition, together with certain glacial ])he- 

 noinena, has led Dr Moreno to advance the theory that formerly all 

 the lakes now found in the eastern longitudinal valley discharged 

 their waters into the Atlantic, and that their diversion to the Pacific 

 has been due to the damming of their eastern outlets with glacial 

 < I rift. 



A careful examination of all the facts does not, I think, justify such 

 an assumption. I have examined with consideral)le care several of 

 the low coutiiuMital divides about the eastern extremities of some of 

 these lakes, and have never found the original rocks there covered to 

 any considerable depth with glacial detritus. The great terminal 

 moraines left l>v the former ice-cap could always be seen crossing the 

 transverse valleys some distance to the eastward of the continental 

 divide, where I have observed them to have a thickness of more than 

 300 feet, as displayed in the bluffs of some of the streams which have 

 cut their way through these moraines in their course to the Atlantic. 



A more plausible explanation, it appears to me, is afforded b}'^ a 

 consideration of the features at present existing throughout Patagonia 

 and Tierra del Fuego in connection with a proper understanding of 

 the relative land and sea areas that existed there during late Tertiary 

 times, with an appreciation 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 Pata- 

 gonian beds we know that during Middle Tertiary times the entire 

 southern extremit}' of the continent excepting the higher peaks of the 

 Andes was submerged beneath a shallow sea. That this sea was no- 

 where very deep is shown l>y the character of the fossils, which are 

 ever3'where 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 char- 

 acteristic shallow-water forms, can only be explained l)y assuming 

 that this region was undergoing a subsidence sufficiently gradual to 



