Studies in the Calcitc Group. 221 



small amount of MgCO.^ observed in the mineral is due rather 

 to this than to its inability to replace MnCO^ in the molecular 

 structure. It should also be stated that the four analyses show- 

 ing the highest percentages of CaCO.j are all old analyses and 

 that there is no way of testing their accuracy. Between the one 

 of these showing the lowest percentag'e of CaCOg (9.0) there 

 is a jump of more than three per cent to the next percentage 

 (5.9). If these four analyses could be eliminated it would bring 

 the facts much nearer what would seem to be the logical theory. 

 It should also be stated that there is one exceptional analysis of 

 rhodochrosite recorded that has not been included in the above 

 list. The mineral came from Franklin, N. J., and showed, 

 ZnCOa 2.28, FeCOa 0.35, MgCOg 3.74, and CaCO,, 20.37.- This 

 is clearly a very unusual case. Other analyses that are quoted 

 sometimes under rhodochrosite and show large amounts of cal- 

 cium carbonate are probably to be considered as manganiferous 

 dolomites. 



In the case of siderite we have some fifty analyses that show 

 more than 50 per cent of FeCOg. As already indicated both 

 MgCO;j and MnCOs occur in isomorphous mixtures with FeCOo 

 in practically all proportions. Twenty- four of the analyses show 

 also the presence of CaCOg. In all cases, except five, this occurs 

 in amounts less than 2 per cent. In these exceptional cases the 

 percentages are 3.1, 3.2, 7.0, lo.o and 11.9. It would seem as 

 if there was justification in rejecting the last three values since 

 none of the specimens appears to have been crystalline, two of 

 them being of the variety known as sphserosiderite. Without 

 including these the average content of CaCOg becomes i.o per 

 cent. 



There were fifty analyses of magnesite that showed over 50 

 per cent of MgCOg. One half of these showed the presence of 

 CaCOs with an average amount of 2.5 per cent. Eleven of the 

 analyses showed less than 2 per cent CaCOg and in only two cases 

 did this rise above 4 per cent, namely 6.9 and 9.7. There is one 

 analysis of magnesite quoted with some 23 per cent of calcium 

 carbonate. This, in view of the above facts, was rejected as an 

 extremely doubtful case. Thirteen of these analyses showed the 

 presence of MnCOg, which with one exception occurred in per- 

 centages less than 3.5. The exception showed 13.3 per cent 



' Browning, Am. J. Sc, 40, 375, 1890. 



