THE WORK OF THE OCEAN. 377 



in certain periods of the past, unusual aridity is inferred. Where con- 

 fined to limited areas, their chmatic 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 to be regarded as 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 con- 

 siderable 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-hquor," or ' ^bittern," is drawn off to prevent 

 these '^ bitter" salts from mixing with the common salt. The mag- 

 nesium salts are among the last to be precipitated, not only because 

 they are readily soluble, but because their quantity is small; yet in the 

 original rock from which all the sea-salts came, there is at least as much 

 magnesium as sodium, while in the sea there is about five times as nmch 

 sodium as magnesium. Just what becomes of the remaining magnesium 

 is not yet well understood. It has a notable disposition to form double 

 salts with some other constituent, as noted above. In the earher marine 

 strata, dolomite, that is, limestone composed partly or wholly of the 

 double carbonate of hme and magnesia, (CaMg)C03, abounds. This 

 appears to have been formed by a gradual substitution of molecules of 

 magnesium for those of calcium, but just how and when and why it was 

 done has not been fully worked out. It appears to be a case where the 

 saline matter of the sea made its contribution to the sedimentary deposits 

 by chemical reaction upon them, rather than by precipitation because 

 of saturation. 



The relatively small amount of potash in the sea-water is probably 

 due to its disposition to remain united with the clays and earths of the 

 mantle rock and of the shaley deposits. 



To some extent the salts in solution act directly on the earthy mat- 

 ter brought down into the sea by rivers, but where sedimentation is 



