Studies in the Calcitc Group. 



245 



go in predicting the composition of a given specimen, having 

 determined these physical characters. This is best done by 

 means of triangular projections. Fig. 5 shows the complete 

 triangular projection for CaC03-MgC03-(Fe,Mn)C03, with the 

 positions indicated upon it of all the analyses of these minerals 

 that have been collected. The heavy broken lines show the 

 probable limiting boundaries of the various isomorphous mix- 

 tures. We need, therefore, to consider only these portions of the 



CaCOi 



CO- i.bsa 



fkCOs 



Fig. 6. 



entire projection. Fig. 6 shows the calcite end of the series 

 with the lighter solid lines within the figure indicating the loci 

 of various combinations of molecules that would possess the 

 same refractive index. In a similar way the heavier solid lines 

 indicate the specific gravities. The refractive index lines have 

 the same direction and spacing in all portions of the triangular 

 projection but the lines of the specific gravities change in the 



