60 L. M. Parsons — 



and separated only by short distances. It is, of course, quite 

 probable that selective features may be consistent in one area but 

 contradictory in another. Such features appear to depend as much 

 on some inherent factor in the original limestone as upon the time 

 and mode of dolomitization. 



Many of the beds in the Winster district contain chert, and 

 sections across the junction of chert and dolomite show that the 

 latter is not included in the former. From this it appears that 

 silicifi cation occurred before dolomitization. 



One other feature must be mentioned. Where the dolomitic 

 material is seen immediately underneath the upper lava, on Masson 

 Hill, for instance, metamorphic effects are not present in the 

 dolomite. Although the lava might fail to produce alteration in 

 a contemporaneous dolomite underneath, it is far more likely that 

 the absence of metamorphism is due to the dolomitization having 

 taken place at some subsequent period. 



In passing it must be noted that the absence of metamorphism 

 in a dolomite resting on a lava yields no evidence of either con- 

 temporaneous or subsequent alteration. Metamorphic effects or 

 their absence in dolomite in contact with a sill would be more helpful, 

 but the limestones in contact with the sills in the Matlock area consist 

 of undolomitized beds mainly of D^ age. 



The majority of the structural features described above indicate 

 subsequent dolomitization, and thus support the evidences afiorded 

 by field relations. 



Condition of Fossils. 



Very little need be said under this heading. The presence of a 

 rich fauna containing numerous corals negatives the probability 

 of primary precipitation, but the condition of coral and other fossils 

 is so varied in different parts of the dolomitic material that no 

 satisfactory conclusion with regard to either contemporaneous or 

 subsequent dolomitization can be drawn from the condition of 

 organic remains. Contradictory selective phenomena on a large 

 scale occur very similar to those on a microscopic scale described 

 above. 



Chemical Analyses and Porosity. 



Numerous analyses of the material between Gratton and Cromford 

 show that in composition the dolomitic limestones of Winster difier 

 in many respects from the dolomite of Matlock. At Winster the 

 bulk of the material consists of dolomitic limestones in which the 

 proportion of magnesium carbonate is variable and usually small, 

 though material occurs in the escarpment and the Wyn Tor having 

 a much higher proportion of MgCOg. Near Matlock the bulk of the 

 material of Masson and Upperwood consists of dolomite having 

 40 per cent or more of MgCOg, and although some of the portions 

 analysed have yielded a smaller proportion than this, the composition 

 is generally much more uniform than that of the Gratton and 



