376 GEOLOGY. 



than that of the carbonate. The more important fact, however, is 

 that marine plants and animals use lime carbonate freely for skeletal 

 and housing purposes. It is held by some that they get their lime 

 from the sulphate, but if so they convert it into carbonate before it 

 takes the form of shells, coral, etc., the sulphuric acid set free in the 

 process reproducing, directly or indirectly, more sulphate. The secre- 

 tion of lime carbonate by organisms is not dependent on the saturation 

 of the water, but may be carried on when the amount in solution is 

 very small. 



There can be little doubt that the chief deposits of lime carbonate 

 have been and are being made through the agency of plants and animals 

 in the form of shells, coral, bones, teeth, and other devices for support- 

 ing, stiffening, 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 sub- 

 ject to concentration from evaporation, simple precipitation takes 

 place freely. There is some difference of opinion as to the importance 

 of these two classes of deposits, past and present. The debated point 

 is whether simple precipitation takes place in any appreciable degree 

 under the usual oceanic conditions. There is much more evidence of 

 solution by sea-water than of precipitation from it. The ocean appears 

 to be under-saturated with Hme carbonate on the whole, though it is still 

 possible that deposition may take place in favorable situations, as, for 

 example, where the very calcareous waters of rivers are spread out in 

 thin sheets on the surface of the heavier salt water, and thus exposed 

 to exceptional evaporation, or where there is very exceptional agitation 

 and aeration.^ 



Gypsum appears to be deposited in quantity only in the closed 

 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, closed seas, or other situations 

 favorable to great concentration. This is usually achieved only in 

 notably arid regions, and in basins that receive httle or no drainage 

 from the land. 



Deposits of salt usually, therefore, signify highly arid conditions, 

 and where they occur over wide ranges in latitude and longitude, as 



^ Willis. Jour, of Geol, Vol. I, p. 500, where the evidences for deposition are fiilly 

 set forth. 



