82 AQUEOUS AGENCIES. 



in Utah, is also a saturated brine depositing salt, as is proved by the 

 incrustations of salt about its margin in dry seasons ; but the deposit 

 has not progressed so far in this case as in the preceding. The great 

 extent to which the waters of this lake have dried away and become 

 concentrated is further shown by old lake-margins far beyond the 

 limits, and several hundred feet above the level, of the present shore- 

 line. Similar phenomena are observed about other salt lakes, especially 

 about the Caspian Sea (Murchison). 



In the case of salt lakes, either formed entirely, or modified, by 

 river-water, the deposits are probably much more complex and various 

 — sometimes salt, sometimes carbonate of lime, and sometimes sulphate 

 of lime. Immense deposits, mostly of carbonate of lime, are found 

 about the salt lakes of Nevada. They form a conspicuous feature of 

 the scenery about Pyramid Lake. 



Deposits are also sometimes formed in lakes which are not salt. 

 For example : the Solfatara Lake, Italy, is formed by the accumulation 

 of the water from warm carbonated springs, similar to those of San 

 Filippo and San Vignone. In this lake, therefore, deposits of traver- 

 tine are forming. Although these deposits take place in a lake, they 

 properly belong to deposits from springs, since they do not take place 

 entirely by concentration, but partly also by escape of carbonic acid. 



Chemical Deposits in Seas. 

 Concerning these little is known. It is certain, however, that all 

 rivers carry to the sea carbonate of lime in solution, and some of them 

 in considerable quantities. There is scarcely any river-water which 

 contains less carbonate of lime than sea-water; many rivers contain 

 four times as much.* This carbonate of lime thus constantly carried 

 into the sea must eventually deposit in some form. Usually, however, 

 sea- water is kept below the saturating point for this substance, by its 

 constant withdrawal by shells and corals, as will be explained under 

 Organic Agency. But in shallow bays nearly cut off from the sea, or 

 in salt lagoons on the sea-margin near the mouths of rivers in dry 

 climates, and subject to occasional overflows by the sea and floodings 

 by rivers, carbonate of lime and sulphate of lime may deposit by evapo- 

 ration. At the mouths of many rivers, whose waters contain much 

 carbonate of lime, as, for instance, the Khine, the delta deposit is 

 cemented into hard rock by means of th^s substance. On shores of 

 coral seas, as upon the Keys of Florida, the coast of the West India 

 Islands, and the islands of the Pacific, shore-material is consolidated 

 into hard rock by the same means. On many shores in tropical regions, 

 the waves, being driven up on flat beaches far inland, leave sea-water 

 inclosed in shallow pools, which by evaporation give rise to calcareous 



* Biscbof, Chemical and Physical Geology, vol. i, p. 179. 



