ORIGIN OF THE SALT LAKES. 247 



the lake would then occur only once every six months. After this a stage 

 \\ould be reached when ordinary spring tides would no longer suffice, and 

 only an exceptionally high tide, brought on by a continued strong easterly 

 wind, acting in conjunction with the sun and moon at the period of 

 spring tide, would pile up the waters of the sea sufficiently high to over- 

 flow the isthmus separating it from the lake. Such conditions would, of 

 course, occur only at irregular intervals and would constantly become less 

 and less frequent, until finally all communication would cease and the 

 smaller body of water would become entirely separated as an inland salt 

 lake, gradually diminishing in area after the last overflow, by evaporation 

 from its surface, until a point would be reached when the loss by evapo- 

 ration would just balance the gain from tributary streams and springs, 

 which latter, in the lakes of the region now being considered, is exceed- 

 ingly slight. 



It was during these stages of intermittent communication that the salt 

 deposits were accumulated. These deposits are now often found covering 

 the bottoms and adjacent shores of the lakes to a depth of several feet. 

 During periods when communication between the lakes and the sea was 

 suspended, the volume of water in the former would be greatly reduced 

 by evaporation, thus increasing its salinity, until an over-saturated solution 

 would be attained, resulting in the precipitation of considerable quantities 

 of salt. With the next ingress of the sea a fresh supply of salt would be 

 introduced in solution, to be deposited in the same manner during the 

 next period of suspended intercommunication. Such conditions, con- 

 tinued over a long period, have resulted in the deposition of the consider- 

 able bodies of salt now found in and about these lakes. 



In the manner just described, series of salt lakes were formed and may 

 still be seen occupying slight depressions over the bottoms of all the 

 abandoned water courses of Patagonia, while every stage in the process 

 of the formation of such lakes may be observed in and about the heads 

 of the different inlets all along the coast. Exceptional advantages for 

 studying the origin of these salt lakes are offered at the head of the Bay 

 of San Julian and in the valley extending from the bay into the interior 

 for a distance of one hundred miles. In the bottom of this valley are 

 numerous salt lakes, while, in the region about the head of the bay there 

 is a succession of lakes and inlets, showing every stage in the process of 

 lake formation as detailed in the foregoing lines. 



