T. Crook — On Dedolomitization. 



343 



are better developed than in any of the other specimens which the 

 writer has examined. The turbidity of the calcite as compared with 

 the dolomite is a pronounced feature in the Port Shepstone specimen. 



Several other specimens of metamorphosed dolomite from various 

 localities, including Glenelg and Iona in Scotland, exhibit the same 

 tendency to the development of coronas of dolomite around the grains 

 of the secondary magnesium minerals (olivine and serpentine). The 

 Iona specimen examined by the writer is of considerable interest as 

 showing a streakiness or somewhat foliated character, arising from 

 pressure-flow subsequent to thermal metamorphism. This pressure-flow 

 has been accompanied by partial destruction of the enclosing shells of 

 dolomite ; but there can be no doubt that the texture originally 

 produced by thermal metamorphism in this case was essentially 

 similar to that of the Ceylon and Natal rocks described above. 



Fig. 2. Metamorphosed dolomitic limestone, Verona, Ontario ; showing 

 chondrodite with an almost complete corona of dolomite. Stippled portion 

 calcite. x 12. 



Typical ' Eozoon ' seems to afford a good example of a metamorphosed 

 dolomite in which the dedolomitization is almost complete. Yery 

 little dolomite remains behind in the writer's specimen ; this is some- 

 times in the form of scattered grains in the calcite matrix, but more 

 commonly it forms partial coronas around the coalesced serpentine 

 grains. 



The petrography of ' Eozoon ' strongly suggests that it was formerly 

 a forsterite marble. Adams & Barlow, however, writing apparently 

 with reference to the ' Laurentian ' limestones of North America as 

 a whole, state that " Olivine has never been found in these limestones ; 

 nor is there any reason to believe that it ever exists or ever did exist 

 in them ".* They hold the view that the serpentine in all cases has 



1 Geology of the Halibtirton and Bancroft Areas, Province of Ontario. 

 F. D. Adams & A. E. Barlow, Memoir No. 6, Department of Mines, Canada, 

 p. 214, 1910. 



