SOME METASOMATIC CHANGES IN 

 LIMESTONES. 



THE origin of dolomitic rocks has engaged the attention 

 of geologists and chemists from an early time. Some 

 of the older chemical theories may be dismissed without 

 discussion as having at most a very limited application, 

 and we may confine ourselves chiefly to those views in 

 which the rocks in question are regarded as ordinary 

 calcareous deposits, owing their magnesian character to 

 accidents in the main posterior to their accumulation as 

 strata. 



In their present condition these rocks seem to consist 

 in general of dolomite or of a mixture of dolomite and 

 calcite, with which may be associated siliceous, argillaceous, 

 or other impurities in varying amount. Those examples 

 richest in dolomite are styled dolomite-rocks, or simply 

 "dolomites"; those in which the mineral named is sub- 

 ordinate to calcite are correctly termed dolomitic or mag- 

 nesian limestones, though the latter name has been applied 

 (as in the Permian of the North of England) to strata 

 consisting essentially of dolomite. Cotta fixed the limit 

 at 23 per cent of magnesium carbonate, which corresponds 

 to equal molecular proportions of dolomite and calcite. 

 This percentage should be reckoned, as pointed out by 

 Hopkins (1), on the total carbonates, excluding non- 

 calcareous impurities. We must not exclude the pos- 

 sibility of the simple magnesium carbonate, magnesite, 

 also occurring in some of these rocks, but there seems 

 to be little positive information on this point. However 

 this may be, it seems probable that rocks which on analysis 

 are found to consist of calcium and magnesium carbonates 

 in other than equal molecular proportions, are composed of 

 grains each of which has a definite constitution, viz., that 

 of calcite or that of dolomite (or more doubtfully magnesite). 

 All actual investigations tend to the belief that no true 

 chemical isomorphism exists between the three minerals 



