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RELICS OF PRIMEVAL LIFE 



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more recently taken up this fact in the way of 

 experiment, and finds that, while in the case of 

 ordinary calcite this action is slow and imperfect, 

 with the aragonitc which constitutes the calcareous 

 framework of certain corals,* and at temperatures of 

 60"" or over, it is very rapid and complete, producing 

 a mixture of calcium and magnesium carbonates, 

 from which a pure dolomite more or less mixed 

 with calcite may subsequently result.^ 



I regard these observations as of the utmost im- 

 portance in reference to the relations of dolomite 

 with fossiliferous limestones, and especially with those 

 of the Grenville series. The waters of the Lauren- 

 tian ocean must have been much richer in salts of 

 magnesium than those of the present seas, and the 

 temperature was probably higher, so that chemical 

 changes now proceeding in limited lagoons might 

 have occurred over much larger areas. If at that 



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* Aragonite, like ordinary limestone, is calcium carbonate, but 

 its atoms seem to be differently arranged, so as to make it a 

 less stable compound, and it has a different crystalline form. 

 Some calcareous organisms are composed of aragonite, others 

 of ordinary calcite. 



* "Bulletin Geol. Soc. Belgium," vol. ix. (1895, p. 3). Also 

 notice in GeoL Mag.^ July, 1895, p. 329. 



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