ORIGIN OF LIMESTONES. 105 



supposed to have been derived originally from the destruction of 

 volcanic rocks and then extracted from the water by various organisms 

 in precisely the same way as carbonate of lime and other substances 

 are assimilated. 



What these organisms are, has not, however, been satisfactorily 

 determined. Many animals, no doubt, would yield on decay an appre- 

 ciable amount of phosphates, if the material could accumulate tran- 

 quilly on the sea-bed, and infiltrate into limestone ; but all these beds 

 give more or less complete evidence of being formed in shallow or 

 comparatively shallow water; and it has been urged that these are 

 the positions where marine plants, which contain much phosphoric 

 acid in the ash, most abound. Being rooted to the ground or growing 

 over a definite area, they would tend to accumulate during a long 

 interval of geological time a considerable quantity of phosphates, such 

 as could be derived from no other source. These submarine forests 

 are, moreover, the chosen feeding-grounds alike of herbivorous and 

 carnivorous animals, and the phosphates set free would all be capable 

 of combining with lime, and would thus explain the accumulation 

 over limited areas of such beds of phosphatic nodules. 



Salt and Gypsum. 



The origin of rock salt from evaporation ol the ocean and salt lakes 

 is sufficiently obvious to need no detailed exposition ; but Mr. Darwin 

 and Mr. David Forbes have described in South America many beds 

 in which, owing to the action of vegetation, other substances have been 

 formed. There are great deposits of nitrate of soda, as well as horizontal 

 layers of gypsum. The nitrate beds generally contain salt, and the 

 nitrate forms from 20 to 75 per cent, of the bed. In drought, most 

 of the streams of the Pampas become saline, and at Bahia Blanca the 

 surface is covered for a quarter of an inch with a deposit that consists 

 of 93 per cent, of sulphate of soda, and 7 per cent, of common salt. 

 In many of the natural salt lakes near the Rio Negro, the salt- is asso- 

 ciated with crystals of selenite and sulphate of soda. There is seen 

 to be every phase of change from chloride of sodium into sulphate of 

 soda, and from sulphate of soda into sulphate of lime, the latter sub- 

 stance being developed wherever shells were sufficiently abundant to 

 furnish the requisite lime, and the waters sufficiently sulphurous. 

 There can, however, be no doubt but that the isolated crystals of 

 selenite, common in all our clays, result from the decay by oxidation 

 of iron pyrites, so that the liberated sulphuric acid combines with 

 the lime of shells. But the great deposits in the Trias of Nottingham, 

 Stafford, and Cheshire, which are often associated with rock salt, 

 require such an explanation as is suggested by Mr. Darwin's observa- 

 tions. 



Limestones, 



Though many ancient limestones have obviously been partly re- 

 constructed out of older deposits, yet every limestone deposit must be 



