SOME META SOMATIC CHANGES IN LIMESTONES. 51 



named, nor indeed between calcium and magnesium com- 

 pounds in general. Retgers (2), after a full consideration 

 of this question, came to the conclusion that the carbonates 

 ol lime and magnesia examined by him fell under three 

 heads ; viz., calcite (CaC0 3 ) with very little or no magnesia, 

 magnesite (MgC0 3 ) with very little or no lime, and the 

 double salt dolomite of perfectly definite constitution 

 (CaMg2CCX). As a matter of fact, it is not difficult in 

 the less fine-grained magnesian limestones to verify the 

 mixed nature of the rocks by the action of dilute acid, 

 the more soluble calcite being thus separated from the 

 less soluble dolomite. 



Between the magnesian and ferrous carbonates, on the 

 other hand, it seems that there is a true chemical isomorph- 

 ism, just as in the corresponding silicates (both pyroxenes 

 and olivines). Mineral analyses of the rhombohedral car- 

 bonates prove the existence of various connecting links 

 between dolomite and pure ankerite (CaFe2CO,) and 

 probably between magnesite and chalybite (FeC0 3 ). In- 

 deed Bentivoglio (3) goes so far as to maintain that more 

 or less iron is essential to the constitution of dolomite, since 

 in thirty-six analyses of dolomite rhombohedra from various 

 localities, he found ferrous carbonate never absent. 



In speculating on the mode of derivation of dolomitic 

 rocks from ordinary limestones various difficulties arise 

 which cannot be said to have been completely surmounted 

 up to the present time, and it is to these especially that we 

 shall turn our inquiries. These difficulties are connected 

 firstly with the source of the large quantities of magnesia 

 which are supposed in most theories of " dolomitisation " to 

 have been introduced from without, and secondly with the 

 chemical processes by which calcium carbonate may conceiv- 

 ably be converted into the double carbonate, dolomite, under 

 natural conditions. 



As regards the source of the magnesia, various early 

 writers ascribed to it a volcanic origin. Although it is by 

 no means certain that dolomitisation may not in some cases 

 have been effected in this way, it is manifest that such a 

 hypothesis cannot be of general application. Other geolog- 



