246 PATAGONIAN EXPEDITIONS: GEOGRAPHY. 



the surface of an originally fresh-water lake with no outlet. No doubt, 

 some of the salt and other saline matter found in these lakes has been 

 derived in this manner, but I believe that, for the most part, it has resulted 

 directly from the evaporation of confined bodies of sea water. From 

 palaeontologic and geologic evidences it is apparent that, for a consider- 

 able period in late Tertiary times, this region was elevated above the sea 

 and subjected to erosion. During this period of late Tertiary elevation, 

 all the more important of the present drainage systems were outlined. 

 Near the close of the Tertiary there was a subsidence just sufficient to 

 permit the ingress of the sea. This submerged condition prevailed only 

 for a relatively very short period, but sufficient for the deposition, over the 

 previously eroded surface, of a thin layer of sedimentary rocks with char- 

 acteristically marine fossils, the Cape Fairweather beds. At the close of 

 the Tertiary, a period of very gradual elevation set in, resulting in the 

 final rescue of what is now southern Patagonia from the sea. As this 

 land mass gradually emerged, the higher table-lands, between the previ- 

 ously eroded water-courses, would first appear as islands and peninsulas, 

 separated by narrow channels and bays which occupied the valleys of the 

 previously eroded drainage systems mentioned above. As the elevation 

 continued, the bottoms of such valleys would be successively brought 

 above the water level and numerous small bays would be formed in all 

 the smaller tributaries. Across the mouths of such bays bars would be 

 thrown by the action of the tides. The formation of such bars, together 

 with the gradual elevation constantly taking place, would tend to decrease 

 the circulation between the waters of these bays and the ocean. By the 

 combined action of these two agencies the circulation would be more and 

 more impeded, until a stage would be reached in which this circulation 

 would become intermittent. The two bodies of water would then be 

 entirely separated, except during periods of unusually high tides, when 

 the waters of the sea would rise sufficiently to overflow into the bay, or 

 lake, as it might now be more properly termed. At first the obstruction 

 would not be so great but that the biweekly high tides occurring at each 

 full and new moon would produce a flow of water from the sea into the 

 lake, thereby replenishing every two weeks the water lost by surface 

 evaporation with a new supply of sea water. After a time the obstruction 

 would become so great that only the exceptionally high, semi-annual tides 

 would suffice for its submergence, and the replenishing of the waters of 



