52 L. M. Parsons — 



Since mapping is indispensable if the nature of metasomatism 

 is to be fully appreciated, I have traced the dolomitization on the 

 6 in. sheets of the Ordnance Survey. Certain features of the maps, 

 particularly lateral transitions and kernels of dolomite or of .lime- 

 stone, are obliged to be more or less diagrammatic, since it is 

 impossible to show some of them true to scale. In mapping 

 metasomatism, all parts of the surface within the boundaries should 

 be examined in detail, and it is possible that some remnants of 

 limestone or dolomite may have been overlooked. 



The chemical composition may vary in an area of dolomitization, 

 consequently the shading on the maps represents the type, dolomite, 

 or dolomitic limestone, as the case may be, which predominates 

 in any area. 



In tracing changes of horizon I have found the zonal map of 

 Mr. C. B. Wedd ^ to be of the greatest help in connexion with the 

 study of the country east of Grange Mill. In the absence of a zonal 

 map of the limestone west of Grange Mill, I have studied the fauna 

 in the dolomitic material itself, and in, the limestone adjacent to the 

 dolomitized rocks. The results supply the information necessary 

 concerning the horizons of the metasomatism. 



Detailed zonal subdivisions are not shown by any method of 

 shading on the maps, since the variable position in the sequence of 

 •chert in the country west of Grange Mill makes it inadvisable to give 

 a map similar to that of the Matlock district by Mr. Wedd. The only 

 difierentiation possible in the neighbourhood of the dolomite is 

 between D^ and D.j, and the maps are less burdened by detail if 

 such simple differentiation is expressed, where necessary, by the 

 corresponding symbols. 



Numbers representing altitudes are given, so that the relation 

 between dolomitization and topography may be appreciated. 



The Leicestershire Dolomites having been described previously, 

 the present paper deals almost entirely with the Derbyshire material, 

 but since the two districts exhibit such contrasting types of 

 metasomatism, and as a comparison could not be made before the 

 material in Derbyshire had been examined, I have given in the last 

 section an outline of my conclusions with regard to the Midland 

 Carboniferous area as a whole. 



II. Note on Classification and Evidences. 



A classification of magnesian rocks based upon chemical com- 

 position alone is unsatisfactory, and a simple classification based 

 upon the time and mode of introduction of magnesia was given in 

 my earlier paper. The recognition of the following more com- 

 plicated types appears to be necessary : — 



Complex dolomites or dolomitic limestones yielding evidences of 

 more than one period of secondary addition of magnesia. Such 



1 Mem. Geol. Surv. : " The Geology of the Northerr Part of the Derbyshire 

 Coalfield and Bordering Tracts," 1913, p. 15. 



