Studies in the Calcite Group. 247 



lines. It will be noted that these two sets of lines cross each other 

 in this case, at quite sharp angles. Figs. 8 and 9 show respectively 

 the MgCOa and the FeCOg ends of the MgCOg-FeCOg series. 

 Here the two sets of lines become practically parallel to each 

 other. It should be stated that the lines of equal refractive index 

 and specific gravity give the percentages of CaCOg, MgCOg and 

 FeCOg and do not consider the possible presence of MnCOg. 

 However, the position of the lines would not be very materially 

 changed, even if a considerable amount of manganese carbonate 

 should be present, since its characters are fairly close to those 

 of ferrous carbonate. 



While no claim can be made that the exact composition of a 

 mineral of this group can be determined by means of its physical 

 constants and the use of these figures, it should still be possible 

 from them, to place approximately the composition of the speci- 

 men, especially if a further guide was provided by making quali- 

 tative chemical tests upon it. In any event these figures show 

 in simple graphical form the relations that exist between the 

 chemical composition and the specific gravities and refractive 

 indices of the members of the Calcite Group. 



