342 T. Crook — On Dedolomitization. 



An examination of numerous sections of this Ceylon specimen 

 showed a plentiful development of forsterite grains surrounded by 

 coronas of dolomite, these being set in a matrix of calcite. The 

 coronas are sometimes complete, sometimes broken, and often show 

 attachment to adjacent patches of dolomite (Fig. 1). Here and there 

 is seen a tendency to a linear arrangement, the adjacent coronas of 

 dolomite coalescing in a manner which suggests eozoonal structure. 



The secondary calcite contains numerous inclusions of dolomite. 

 These inclusions are sometimes arranged in parallel layers (Fig. 1). 

 Instead of parallel layers we may get a micropegmatite-like inter- 

 growth l of the two minerals, or the dolomite may be scattered in the 

 calcite in disconnected granules. 



A feature of special note observed in the sections is that the streaks 

 of dolomite intergrown with the calcite are sometimes connected with 

 the dolomite coronas in such a way as to suggest that they served 

 as channels along which magnesia was supplied to the coronas during 

 the growth of the enclosed grain of forsterite. 



The specimen of chondrodite marble from Ontario differs markedly 

 from the Ceylon specimens just described in that the original rock was 

 clearly a dolomitic limestone rather than a true dolomite rock, 

 the amount of magnesia being small compared with that of lime. 

 Here again, however, the secondary minerals which have been 

 produced by metamorphism are free from lime (excepting calcite), the 

 minerals observed being chondrodite, spinel, phlogopite, graphite, and 

 pyrrhotite. As in the forsterite rock from Ceylon, these secondary 

 minerals show a decided preference to be embedded in dolomite, 

 though the amount of dolomite present is comparatively small. This 

 is particularly the case with the grains of chondrodite. In the 

 sections examined, each of the numerous grains of chondrodite had its 

 corona of dolomite (see Fig. 2). Here again, in addition to the 

 coronas, the sections show scattered grains and patches of dolomite in 

 the calcitic matrix ; and the simultaneous extinction of neighbouring 

 patches of this dolomite, in some cases, suggests that they are portions 

 of an anastomosing growth of dolomite. A pronounced feature in this 

 specimen is the turbidity of the calcite as compared with the dolomite ; 

 this is a point which has been emphasized by previous workers who 

 have described similar rocks. 



Another specimen examined was kindly supplied by Dr. Hatch, from 

 the outer ophicalcite zone of the Port Shepstone occurrence dealt with 

 in the paper by Hatch & Rastall already referred to. This specimen 

 is one of critical importance, owing to the definiteness of certain of the 

 conditions of metamorphism in that particular case. As previously 

 mentioned, it was shown by these authors that the inner reaction zone 

 in the Port Shepstone instance was marked by practically complete 

 dedolomitization. In the outer zone, however, one would expect the 

 change to be far from complete ; and the examination of a specimen of 

 the ophicalcite proves this to be the case. Moreover, the coronal 

 layers of dolomite around the grains of serpentine in this ophicalcite 



1 These peculiar intergrowths of dolomite in calcite appear to be identical 

 with those described and figured by Coomaraswamy (' ' Crystalline Limestones of 

 Ceylon " : Q.J.G.S., vol. lviii, p. 412, 1902). 



