T. Crook — On Dedolomitization. 341 



which the secondary magnesium minerals are left embedded in calcite, 

 is comparatively rare. Usually the change is much less complete, 

 and then the rule seems to be that the secondary magnesium minerals 

 are embedded in dolomite and not in calcite. 



Two specimens which serve as good examples of this have been 

 examined by the writer and are worthy of description — one a forsterite- 

 phlogopite marble from Peradeniya, Ceylon, the other a chondrodite 

 marble from Verona, Ontario, Canada. In both cases the texture 

 developed as a consequence of thermal metamorphism appears to have 

 persisted in an undisturbed condition. 



The Ceylon specimen l is one of massive coarsely crystalline dolomite, 

 traversed by a band of secondary silicates consisting almost wholly of 

 forsterite in the form of small greyish and rounded grains. Phlogopite 

 is also present, but the amount is very small. Spinel, which is 

 frequently present in Ceylon dolomites as a product of meta- 

 morphism, appears to be absent in this particular case. On 

 either side of the forsterite band the rock consists of fairly pure 

 dolomite, with only here and there a microscopic patch of calcite, and 

 a few scattered granules of forsterite, which are embedded, not in the 

 calcite, but in the dolomite. The carbonate substance of the forsterite 



Fig. 1. Metamorphosed dolomite, from Peradeniya, Ceylon ; showing forsterite 

 in dolomite. Stippled portion represents calcite which was stained by the 

 Lemberg solution, x 15. 



band itself is largely but not completely dedolomitized ; and even 

 here, where the grains of forsterite are thickly set, they are never 

 exclusively surrounded by calcite, and are generally embedded in 

 dolomite (see Fig. 1). Judging from the relative amounts of calcite 

 and forsterite present, it seems tolerably certain that the calcite 

 present in the specimen has arisen entirely by dedolomitization, and 

 that before metamorphism the carbonate consisted wholly of dolomite. 



1 This is a specimen of the Ceylon Mineral Survey collection at the Imperial 

 Institute. 



