I/O The American Geologist. March, i90i. 
waters sweet, but a few of them do in very dry seasons become 
somewhat brackish. 
Scattered all over the Patagonian plains from the strait of 
Magellan to Bahia Blanca are great numbers of salt lakes. Such 
lakes are usually of quite limited area and of exceedingly shal- 
low depth, though they occasionally attain to considerable di- 
mensions. In reference to their origin I have called these salt 
lakes residual lakes. I have elsewhere advanced the theory 
that these lakes have resulted from confined bodies of water, 
cut off from the sea, during the process of elevation, which be- 
gan at the close of the Tertiary and which resulted in the final 
recovery of this region from the ocean. I have held that the 
salt of these lakes has been derived directly from sea water and 
has not resulted by evaporation from the surface of an orig- 
inally fresh water lake with no outlet. No doubt some of the 
salt and other saline matter found in these lakes has been de- 
rived 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 the paleontologic and geologic evidences it 
is apparent that for a considerable 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 im- 
portant 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 characteristically marine fos- 
sils. At the close of the Tertiary a period of very gradual ele- 
vation set in, resulting in the final 'rescue of what is now south- 
ern Patagonia from the sea. As this land-mass graduall}" 
emerged, the higher table lands separating the previously 
eroded water courses would be the first to appear as islands 
and peninsulas separated by narrow channels and bays formed 
by the valleys of the 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 o£ 
the tides. The formation of such bars, together with the grad- 
