VIII. ] CHEMISTRY OF LIMESTONES AND DOLOMITES. 81 



ary 17, 1862. In this remarkable memoir, which has for its 

 title, On the Origin of the Calcareous Eocks which do not be- 

 long to the Primordial Crust (De Vorigine des roches calcaires, 

 etc.), the author gives his views upon the formation of limestone 

 and dolomite. He rejects Von Buch's theory of dolomitiza- 

 tion, which still finds some supporters, and which supposes 

 that the magnesia was introduced subsequent to the deposition 

 of the sediments, by a " certain mysterious action of intrusive 

 pyroxenic rocks " which have been ejected in the vicinity of 

 deposits of pure limestone. Mr. Cordier also combats the idea 

 that these last have been formed entirely of the debris of testa- 

 cea and zoophytes, which, according to him, form but a small 

 proportion of limestone-formations. Going back still further, 

 he finds the source alike of the carbonate of lime of these 

 organic remains, and of the great mass of calcareous rocks, in 

 certain chemical reactions. The pure limestones, according to 

 him, pass into magnesian limestones by an admixture of dolo- 

 mite, and form thus a transition to the pure dolomites, so that 

 we must admit a common origin for all these rocks. The source 

 of the two carbonates which compose them, according to Mr. 

 Cordier, is to be found in the reaction of carbonate of soda upon 

 the chlorides of calcium and magnesium in sea- water ; the car- 

 bonate of soda being derived from the decomposition of feld- 

 spars, from alkaline springs, and from plutonic emanations. 

 This alkaline salt, reacting upon the salts of sea-water, would 

 give rise to chloride of sodium and carbonate of lime, and, under 

 certain conditions, to calcareo-magnesian precipitates. . From 

 this reaction must result a secular variation in the composition 

 of the ocean, which corresponds to the progressive changes in 

 the marine fauna of successive geological epochs. 



Such is the theory of Mr. Cordier, which is now published, 

 for the first time, in 1862 ; and which I have thus noticed in 

 order to call the attention of the Academy to my own published 

 papers, in which I have maintained similar views for the last 

 four years. [See, for the origin of carbonate of lime, the first 

 paper in the present volume, an abstract of which was given 

 in the letter of which this is a part.] 



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