230 William E. Ford, 



groups therefore, that may make up the composition of the 

 mineral can be written as CaMg(C03)o, CaFe(C03)2, CaMn- 

 (CO3),, CaCaCCOa), and possibly Mg-MgCCOa)^. The mole- 

 cules CaMg(C03)2 and CaFe(C03)2 must be considered to form 

 a true isomorphous series since they can apparently replace each 

 other in any proportion. It is true that the end of the series 

 toward CaFe(C03)2 has not been observed among minerals, but 

 that is probably due to lack of favorable conditions of formation 

 rather than to the impossibility of its occurrence. Foote and 

 Bradley^ have recently investigated the question as to the amount 

 of excess CaCOg that dolomite may contain. They analyzed 

 dolomites that had been crystallized upon calcite and that, there- 

 fore, must have been formed from solutions that were saturated 

 with calcium carbonate. Such dolomites should carry the maxi- 

 mum amount of isomorphous calcium carbonate. They found 

 that this excess amount of CaCOa could reach as high as 20 

 per cent. Their results have been clearly confirmed by the study 

 of all available dolomite analyses. Over one himdred such 

 analyses were collected and recalculated into their various 

 isomorphous molecules. It seems unnecessary to quote these 

 analyses in detail here, but it will be of interest to give in tabular 

 form (Table III) the pei-centages that they show of'MgCOs and 

 CaCO, in excess of tliat required in the usual molecules. 



'Am. Jour. Sci., 37, 339, 1914. 



