XIII.] ORIGIN OF CRYSTALLINE ROCKS. 311 



or when required for the needs of living plants or animals, 

 which are dependent for their supply of calcareous matter on 

 the carbonate of lime produced, in part by the process just de- 

 scribed, and in part by the action of carbonic acid on insoluble 

 lime-compounds of the earth's solid crust. So many limestones 

 are made up of calcareous organic remains, that a notion exists 

 among many writers on geology that all limestones are, in some 

 way, of organic origin. At the bottom of this lies the idea of 

 an analogy between the chemical relations of vegetable and 

 animal life. As plants give rise to beds of coal, so animals are 

 supposed to produce limestones. In fact, however, the syn- 

 thetic process by which the growing plant, from the elements 

 of water, carbonic acid, and ammonia, generates hydrocarbona- 

 ceous and azotized matters, has no analogy with the assimilative 

 process by which the growing animal appropriates alike these 

 organic matters and the carbonate and phosphate of lime. 

 Without the plant, the synthesis of the hydrocarbons would 

 not take place ; while, independently of the existence of coral 

 or mollusk, the carbonate of lime would still be generated by 

 chemical reactions, and would accumulate in the waters until, 

 these being saturated, its excess would be deposited as gypsum 

 or rock-salt are deposited. Hence, in such waters, where, from 

 any causes, life is excluded, accumulations of pure carbonate 

 of lime may be formed. In 1861 I called attention to the 

 white marbles of Vermont, which occur intercalated among 

 impure and fossiliferous beds, as apparently examples of such 

 a process.* 



It is by a fallacy similar to that which prevails as to the or- 

 ganic origin of limestones, that Daubeny and Murchison were 

 led to appeal to the absence of phosphates from certain old 

 strata, as evidence of the absence of organic life at the time of 

 their accumulation, f Phosphates, like silica and iron-oxide, 

 were doubtless constituents of the primitive earth's crust, and 

 the production of apatite crystals in granitic veins or in crys- 

 talline schists is a process as independent of life as the forma- 



* American Journal of Science (2), XXXI. 402. 

 t Siluria, 4th ed., pp. 28 and 537. 



