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THE ROYAL SOCIETY OF CANADA 



from which he concluded that dolomite was not a member of an 

 isomorphous series of which calcite and magnesite would be the end 

 members, but an isolated compound. He also considered calcite and 

 magnesite to be practically immiscible. His conclusions seemed amply 

 substantiated b)^ the discovery that dolomite crystallises in a less sym- 

 metrical class of the hexagonal system than calcite and magnesite, and 

 by the large number of analyses of the three minerals that show only 

 very small amounts of impurities. The result has been that analyses 

 showing both CaCOg and MgCO« have been thrown aside as representing 



**••• J *" 



Figure 5 



mechanical mixtures of two minerals. The advances made in the study 

 of isomorphous relationships during the last decade have been such 

 that the evidence of specific gravity values alone would not now be 

 taken as conclusive. Besides, the rigid application of Retger's law of 

 the direct proportionality of specific gravity to molecular weight in 

 the case of an isomorphous series is hardly justifiable.* It assumes that 

 the two components of the mixed crystal occupy each the same volume 

 in the isomorphous mixture that they would occupy as independent 

 crystals. In the case of Type III of Roozeboom (Fig. 5.) it may happen 



*Wallerant. Fortschritte d. Minéralogie, Krystallographie u. Pétrographie 

 II, 1912, 86. cf. also Day and Allen: the isomorphism and thermal properties of the 

 felspars, p. 73. 



