502 THE JOURNAL OF GEOLOGY. 



and relatively so ineffective in structures whose delicate anatomy 

 can still be traced even to microscopic details? Thin sections 

 of limestone which show a mass of interferant crystals suggest 

 that this was the primary structure of the rock, and organic 

 remains appear to be foreign bodies which are accidentally 

 of the same substance as the matrix. If this view be correct, 

 then only the alteration of the organic carbonate is the 

 measure of the alteration of the rock-mass. If it can be 

 shown that limestones now forming by chemical precipitation 

 possess a crystalline structure, which resembles that of ancient 

 limestones, the resemblance will constitute a presumption in favor 

 of similarity of origin for the modern and ancient formations. 

 And the fact that limestone is now being precipitated would, if it be 

 established, leave the geologist free to weigh the evidence in the 

 case of any ancient limestone for and against its organic or chem- 

 ical origin. It is not proposed here to argue that limestones are 

 prevailingly of one origin or the other, but only to show that the 

 assumption of organic origin for all the calcareous deposits of the 

 stratified series is too sweeping. To this end it is desirable to 

 consider the chemical and mechanical conditions which affect the 

 precipitation of carbonate of lime, to estimate the solubility 

 of the carbonate in salt water, to review the conditions under 

 which lime is contributed to, and distributed in, the sea, and 

 to describe several cases of modern limestone formation by 

 precipitation. 



Schloesing made a number of experiments on the solubility 

 of carbonate of lime in carbonic acid and water ; he thus describes 

 his method and results. z 



" Experiments : — The method adopted was to cause to pass 

 through pure water, which was maintained at a constant temper- 

 ature and contained an excess of carbonate of lime, a mixture of 

 air and carbonic acid, of a composition varied at will, but con- 

 stant, for each experiment; this mixture was constantly supplied 

 until a perfect equilibrium was established between the substances 



'Comptes Rendus, Vol. 74, 1872, pp. 1552-56, and Vol. 75, p. 70. 



