344 T. Crook — On Dedolomitization. 



been derived from pyroxene, a view which many will find it difficult 

 to accept. 



In making the above observations, the calcite and dolomite were 

 distinguished from one another by staining with the well-known 

 Lemberg solution — an aqueous solution of logwood and aluminium 

 chloride. This is quite the safest means of discriminating between 

 calcite and dolomite in rock slices, and never fails to give satisfactory 

 results when properly managed. One should remember, however, 

 that magnesite acts like dolomite in this respect, and remains 

 unstained. The suspicion that some of the unstained portions of 

 the slides might be magnesite led the writer to make density tests in 

 several cases ; but the results indicated that, in the specimens tested, 

 the unstained material was dolomite. 



To summarize the foregoing remarks, it appears from the specimens 

 examined by the writer that one may represent the process of 

 dedolomitization in three stages, viz. : — 



1 . Where the amount of silica is small we may get the magnesium 

 silicates, etc., embedded in dolomite, with no calcite in their immediate 

 vicinity. The calcite in such a case may be seen scattered in small 

 patches in the dolomite, but noticeably remote from the secondary 

 magnesium minerals ; and in some cases we may get sections showing 

 a considerable development of serpentine with practically no calcite. 



2. A more advanced stage in dedolomitization is represented by 

 the condition in which dolomite forms coronas round the grains of the 

 secondary magnesium minerals. The calcific matrix at this stage is 

 usually interspersed with dolomite, which may be in the form of 

 scattered granules or connected intergrowths. If the rock is coarse- 

 grained the dolomite may be in the form of platy intergrowths 

 showing parallel bands in section ; and in such a case there appears to 

 be a connexion between the platy intergrowths of dolomite and the 

 dolomite coronas surrounding^ the grains of the secondary magnesium 

 minerals. 



3. In the most advanced stage, where the amount of silica has 

 been sufficient to combine with the whole or almost the whole of the 

 magnesia of the dolomite, the secondary magnesium minerals are left 

 embedded in calcite. Any small amount of residual dolomite present 

 in such a case may be in the form of scattered granules, or groups of 

 granules, in the calcific matrix ; or may be left as broken remnants 

 of the coronas around the grains of the secondary minerals. 



The facts thus summarized are of considerable interest, partly as 

 indicating the prevalence in thermally metamorphosed dolomites of 

 certain textural features which have not been hitherto described, and 

 partly also as throwing some light on the process of dedolomitization. 

 There can be little or no doubt that such minerals as olivine, 

 serpentine, chondrodite, phlogopite, spinel, and other magnesium 

 minerals characteristic of crystalline dolomites and dolomitic lime- 

 stones, are as a rule secondary to the dolomite. At least this has been 

 clearly proved in many cases. There is, moreover, a close similarity 

 between the facts of paragenesis and texture in all cases; and there 

 seems to be no ground for Fermor's inference r that the dolomite and 

 1 " Petrology of Ckhinchvdra " : Eec. Geol. Surv. Ind., vol. xxxiii, pt. iii, 1906. 



