340 T. Crook — On Dedolomitization. 



represented in the widespread pre-Cambrian rocks of Canada, and they 

 also occur in the United States. In India and Ceylon they occur 

 freely among the rocks of the fundamental complex. In Africa they 

 are met with in the ancient and highly metamorphosed basement rocks 

 of Nigeria, Rhodesia, Nyasaland, Natal, and other areas. In Britain, 

 highly metamorphosed dolomites of Cambrian age occur in North-West 

 Scotland, and others of uncertain age are also met with among the 

 rocks of the Dalradian complex. 



There appear to be very few, if any, exceptions to the rule that 

 these ancient dolomitic rocks, especially those of the pre-Cambrian, 

 have been thermally metamorphosed. The characteristic feature of 

 this metamorphism is the production of secondary minerals in which 

 magnesia plays a supreme part. Very often, indeed, lime does not 

 enter at all into these secondary minerals, e.g. olivine, serpentine, 

 mica (phlogopite or biotite, generally the former), periclase, brucite, 

 spinel, and chondrodite. 1 In tremolite, which is fairly common as 

 a secondary silicate in these rocks, magnesia predominates over lime, 

 whilst in diopside magnesia and lime play equally important parts. 

 It is of interest to note that picroilmenite occurs as a constituent 

 of metamorphosed dolomite in Ceylon, associated with forsterite, 

 phlogopite, and spinel. In all probability geikielite occurs in the 

 same way. 



Wollastonite, idocrase, and garnet (andradite and grossularite), 

 which are common as secondary minerals in non-dolomitic crystalline 

 limestone, are notable for their absence, at least as a rule, in 

 metamorphosed dolomites ; and it seems, indeed, highly probable that 

 they cannot develop as the result of the direct thermal metamorphism 

 of dolomite. 



The production of secondary magnesian minerals, or minerals 

 in which magnesium predominates over calcium, in thermally 

 metamorphosed dolomite, necessitates the formation of calcite, as has 

 been shown by Teall. 2 In those portions of the rock where sufficient 

 silica was present, dedolomitization may be complete, or almost 

 complete, and in such a case a rock may be produced consisting 

 essentially of forsterite embedded in calcite, with very little residual 

 dolomite. The forsterite marbles of Sutherland and Skye described 

 by Teall 3 and Harker i afford examples of this type. 



Hatch & Rastall have described a unique and extremely interesting 

 case of Jbhe partial metamorphism of a lump of granite in dolomite, in 

 which the inner reaction zone showed complete dedolomitization, 

 accompanied by the formation of olivine, spinel, and brown phlogopite. 5 

 It appears, however, that dedolomitization so complete as this, in 



# 



1 It is convenient to use the name chondrodite in a general sense for the 

 members of the humite series when dealing with scattered granules in these 

 rocks, since it is not practicable to identify any given irregular small grain with 

 safety as belonging to any particular member of the series. 



2 See particularly The Geological Structure of the North-West Highlands 

 (Memoirs of the Geological Survey of Great Britain), pp. 453-62, 1907. 



3 Loc. cit. 



4 The Tertiary Igneous Rocks of Skye (Mem. Geol. Surv.), pp. 144-51, 1904. 



5 Q.J.G.S., vol. lxvi, p. 507, 1910. 



