356 



PRINCIPLES OF STRATIGRAPHY 



River into the former upper end of the Gulf of California, cut oflf 

 the head of this water body from the remainder of the Gulf, convert- 

 ing it into a separate lake. This was at first converted into a fresh or 

 brackish water lake by the continued influx of the river water, with 

 a corresponding rise of the level as indicated by the traces of old 

 shore-lines 40 feet or so above the present sea-level, the deposit of 

 calcium carbonate and the presence of numerous shells of fresh or 

 brackish water molluscs. The Colorado repeatedly changed its 

 mouth, discharging at intervals into the Gulf of California. When- 

 ever this occurred the lake began to shrink, the waters evaporating 

 under the drying influence of the Westerlies descending from the 

 high coastal ranges. The Salton-sink represents the present concen- 



FiG. 69. Sketch map of the Salton Sink Region. (After Davis.) 



tration of these waters, the bottom of this lake being 273.5 ^^^t 

 below mean sea-level (Freeman and Bolster -17 :^(5-57). Before the 

 recent overflows of the Colorado (1891, 1905, 1906-07) the waters 

 of the Salton Lake were intensely saline and its shores were fringed 

 with broad belts covered with white crusts of salt. The author 

 (19:5, 4) has estimated that the salt resulting from the complete 

 evaporation of the sea waters originally cut off from the head of the 

 Gulf of California would form a bed 12.5 m. thick over the area 

 of the present Salton Lake. (Fig. 69.) 



NON-CALCAREOUS TERRESTRIAL PRECIPITATES. 

 Salt Lakes and Salinas. 



Salt lakes and salinas are characteristic features of the arid re- 

 gions of the earth, and they occur in nearly every country. Those 

 of the drainless basin of the Caspian have long been well known. 



