Studies ill the Calcife Group. 219 



13.7 respectively. The siderite molecule, FeCO^, was present in 

 twenty-one analyses, its three hig-hest percentages being 5.93, 

 6.0 and y.;^. The magnesite molecule, MgCO,, was present in 

 twenty analyses ; in only three cases, however, did its percentage 

 rise above three per cent, namely 3.0, 3.8 and 7.4. 



Since dolomite is in many ways distinct from the other mem- 

 bers of the group and needs a rather more extended considera- 

 tion, the description of its characters will be postponed for the 

 present and will be given in detail further on. The other 

 members of the group can be discussed now. 



The siderite and magnesite molecules may mix together in all 

 proportions and the same is apparently true of the siderite and 

 rhodochrosite molecules. These facts are shown g-raphically 

 in Fig. 2. Points along the vertical lines represent percentages 

 of MgCOg, FeCOg and MnCO, respectively. The cross lines 

 represent different analyses of these minerals which give the 

 percentages shown at the ends of the lines. The horizontal lines 

 represent those analyses that contained only the two different 

 molecules, while the slanting lines belong to those that showed 

 more than two isomorphous constituents. For the sake of 

 clearness a few lines have been omitted in the most congested 

 portions of the figure. 



It is clearly seen that the FeCOg-MgCOg series is practically 

 complete, such gaps as do exist being small and probably 

 accidental. In the case of the FeCOg-MnCO, series there is one 

 considerable break between 30-50 per cent FeCOg and 50-70 per 

 cent AlnCO,. It is difficult to say at present whether this break 

 has any real significance or is again accidental. 



If we consider as rhodochrosite all those analyses that show 

 more than 50 per cent of MnCOg we can study the amounts of 

 the CaCOg and MgCOg molecules that may occur in it. We 

 have some twenty-three such analyses for examination. The 

 calcium carbonate molecule occurs in twenty-one of these 

 analyses with an average percentage of 4.9. More than half of 

 the analyses show percentages considerably below this average 

 while in only three cases does the percentage rise above 10 per 

 cent, namely, 10.4, 10.5 and 13. i. Eleven of these analyses 

 showed the presence of MgCOg with an average of 2.7 per cent. 

 In only one case, 7.2 per cent, did the amount present exceed 

 five per cent. Arguing from the analytical data at hand it. would 



