OPPONENTS AND OBJECTIONS. 203 



fact, that it was preserved by the introduction of serpentine at 

 the time of its growth. Above the unbroken Eozoon reefs are 

 limestones made up apparently of the c'ebris of Eozoon thus 

 preserved by serpentine, and there is no doubt that this cal- 

 careous rhizopod, growing in water where serpentine was not 

 in process of formation, might, and probably did, build up pure 

 limestone beds like those formed in later times from the ruins 

 of corals and crinoids. Nor is there anything inconsistent in 

 this with the assertion which Messrs. King and Eowney quote 

 from me, viz., that the popular notion that all limestone forma- 

 tions owe their origin to organic life is based upon a fallacy. 

 The idea that marine organisms originate the carbonate of 

 lime of their skeletons, in a manner somewhat similar to that 

 in which plants generate the organic matter of theirs, appears 

 to be commonly held among certain geologists. It cannot, 

 however, be too often repeated that animals only appropriate 

 the carbonate of lime which is furnished them by chemical 

 reaction. Were there no animals present to make use of it, 

 the carbonate of lime would accumulate in natural waters till 

 these became saturated, and would then be deposited in an in- 

 soluble form ; and although thousands offset of limestone have 

 been formed from the calcareous skeletons of marine animals, 

 it is not less true that great beds of ancient marble, like many 

 modern travertines and tufas, have been deposited without 

 the intervention of life, and even in waters from which living 

 organisms were probably absent. To illustrate this with the 

 parallel case of silicious deposits, there are great beds made 

 up of silicious shields of diatoms. These during their lifetime 

 extracted from the waters the dissolved silica, which, but for 

 their intervention, might have accumulated till it was at length 

 deposited in the form of schist or of crystalline quartz. In either 

 case the function of the coral, the rhizopod, or the diatom is 

 limited to assimilating the carbonate of lime or the silica from 

 its solution, and the organised form thus given to these sub- 

 stances is purely accidental. It is characteristic of our 

 authors, that, rather than admit the limestone beds of the 

 Eozoon rocks to have been formed like beds of coralline lime- 

 stone, or deposited as chemical precipitates like travertine. 



