324 GEOLOGY 



The chief deposits of lime carbonate have been made through 

 the agency of plants and animals, in the form of shells, coral, bones, 

 teeth, and other devices for supporting, housing, protecting, and 

 arming themselves; but while it is agreed that the larger part of the 

 lime carbonate deposited in the open sea is of organic origin, it is 

 equally clear that in closed seas subject to concentration from 

 evaporation, simple precipitation may take place. There is differ- 

 ence of opinion as to the quantitative importance of this last class 

 of deposits. 



Gypsum appears to be deposited in quantity only in the basins 

 of arid regions where concentration reaches an advanced state. 

 Since normal sea-water is far from saturation with common salt, 

 the latter is precipitated only in lagoons, in closed seas, or other 

 situations favorable to great concentration. This is, as a rule, 

 only in regions which are notably arid. It follows that deposits of 

 salt usually signify highly arid conditions, and where they occur 

 over wide ranges in latitude and longitude, as in certain periods 

 of the past, general aridity of climate is inferred. Where confined 

 to limited areas, their climatic significance is less, for topographic 

 conditions may determine local aridity. The total area where salt 

 is now being precipitated is small, though on the whole the present 

 is probably a rather arid period of the earth's history. On the 

 other hand, ancient deposits of salt preserved in the sedimentary 

 strata show that the area of salt deposition has been much more 

 considerable than now at one time and another in the earth's 

 history. The salt and gypsum deposits of the past seem, therefore, 

 to tell an interesting tale of the climates of the past. 



The magnesium salts are among the last to be thrown down as 

 the sea-water is evaporated, and they most commonly take the form 

 of sulphates and chlorides. They often form double salts with 

 potassium, a relatively small and soluble constituent of sea-water. 

 In the artificial evaporation of salt water to obtain common salt, 

 the process is usually stopped before the saturation point for the 

 magnesium salts is reached, and the residue, the "mother-liquor," 

 or "bittern," is drawn off to prevent these "bitter" salts from 

 mixing with the common salt. The magnesium salts are among 

 the last to be precipitated, not only because they are readily soluble, 



