642 NEW YOEK STATE MUSEUM 



the plants. 2) If only a small percentage of lime is present, and 

 in the form of bicarbonate, the latter may be converted into the 

 simple carbonate by the action of the oxygen set free by the 

 plants. 



The greater number of limestone deposits are probably of or- 

 ganic origin, that is they result from the accumulation on the 

 ocean bottom of the calcareous remains of marine organisms, 

 such as the shells of mollusks, cases of foraminifera and skeletons 

 of corals, etc. 



Some writers have put forth the theory that many limestones, 

 specially those showing no trace of organic remains, have been 

 formed by chemical precipitation; this, it is argued, has been 

 caused by reaction of alkaline carbonates on lime salts, or by 

 the breaking up of bicarbonate of lime on exposure to the air, 

 this salt ha^dng been often brought to the sea in river water. 

 Dr T. Sterry Hunt was an earnest advocate of the precipitation 

 theory. ■"■ 



G. Bischof was an active opponent of this theory, arguing^ 

 that lime carbonate would not be precipitated under the condi- 

 tions existing in sea water. To cause its precipitation in a man- 

 ner similar to gypsum, Bischof reasons that 75^ of the ocean 

 water would have to be evaporated, in order to produce sufficient 

 concentration. 



The presence of crystalline grains of carbonate of lime inter- 

 mingled with the shell fragments suggests the possibility of two 

 xiauses, viz, organic and chemical, acting at the same time in the 

 building up of the calcareous deposits. This may, however, be 

 explained by the fact that calcium carbonate crystallizes very 

 readily, even at ordinary temperatures, and that portions of the 

 shell remains in the limestone may have been dissolved and re- 

 precipitated. 



1 Chem. and geol. essays. Ed. 4. 1891. p. 82, 311. See also, Lapparent, 

 Albert de. Traitg de geologie, p. 685; also Zirkel. Lehrbuch der petro- 

 graphie, 3: 482. 



2 Chem. and phys. geol. 1: 581. 



