" CRYSTALLINE LIMESTONES " OF CANADA. 81 



We may now summarize the evidences and considerations 

 which have been brought forward by way of substantiating our 

 view as to the origin of the Archaean " crystalline limestones " 

 of Canada that they were silacid metamorphics which have 

 become chemically and otherwise changed or methylosed into 

 calcitic masses (hemithrenes) , it having been shown : 



That the calcite of the calcareous structures of " Eozoon " 

 in the ophite of the Canadian Laurentians is a replacement 

 of serpentine and other mineral silicates ; 



That the mineral silicates, augite, hornblende, &c. (character- 

 istic of hemithrenes), are often pseudomorphosed into 

 calcite ; 



That only under extremely limited conditions are chemically 

 precipitated limestones formed ; 



That no reliable evidences have yet been adduced proving the 

 existence, during the Archaean periods, of lime-elaborating 

 organisms (the only ether kind of agency admissible on our 

 part), through whose intervention the "crystalline lime^ 

 stones " could have been formed ; 



That the peculiar convoluted or tortuous lamination which 

 distinguishes many of the " crystalline limestones " is only 

 explicable, in the absence of cases to the contrary, on the 

 idea of its being a superinduced phenomenon ; 



That the " crystalline limestones," also the associated ophites, 

 are most abundant where they are interbedded with silacid 

 rocks (" pyroxenytes ") , whose essential minerals contain a 

 large percentage of calcium (and magnesium) silicate ; 



That the gneissose rocks of St. -Philippe (Vosges) and of Glas- 

 sillaun (Cleggan Bay) have been converted into hemithrenes 

 by chemical action ; finally, 



That the great Archaean limestone formation of Canada 

 consists of silacid gneisses which " pass insensibly " into 

 hemithrenes through decrease of mineral silicates and 

 increase of carbonate of lime, that this formation is inter- 

 sected by "granitic" and (e calcareous veins," between 

 which " it is easy to trace a gradual change," that the 

 " pyroxenytes " and beds of limestone, also the " granitic " 

 and " calcareous veins," include in a general sense identical 

 crystalline siliceous minerals, with their surfaces more or 

 less corroded by the dissolving action of heated water con- 



