106 L. M. Parsons— 



cavities and vesicular structures are often on a more pronounced 

 scale, and the material being of D^ age is comparatively 

 unfossiliferous. 



Microscopic sections typically exhibit grains and crystals generally 

 rather allotriomorphic and of varying size, mixed with residual 

 calcite. Generally the grain is much coarser than that of the 

 Winster or Matlock material, typical dimensions being a minimum 

 diameter of '09 mm. and a maximum diameter of '9 mm, a common 

 size being -4 mm. A certain amount of limonite is interstitial, but 

 I have not found any zonal growths of iron oxide. Chert is absent 

 from this part of the sequence. There are no selective phenomena. 



Thin sections of the transition limestones show that the original 

 material, if similar, consisted of fine-grained foraminiferal limestone 

 {see PI. VI, Fig. 1), containing a moderate amount of recrystallized 

 calcite, though the recrystallization might have occurred at a later 

 date. I have not found alternating zonal growths of calcite and 

 dolomite suggesting that calcification and dolomitization may have 

 occurred simultaneously. Analyses show that the bulk of the altered 

 material consists of dolomitic limestone having on an average about 

 15 per cent of magnesium carbonate, though sometimes the pro- 

 portion is as low as 8 or 9 per cent. Specific gravity and porosity 

 also vary a good deal, many of the vesicular varieties having a 

 porosity as high as 12 or 13 per cent. Considerable leaching must 

 have occurred, though it must be remembered that shrinkage due 

 to molecular change during metasomatism helps in the production 

 of vesicular structures. 



The evidences outlined above amply justify the conclusion that 

 the metasomatism is of subsequent origin and produced by 

 magnesian ground waters, but it is difficult to say at what period 

 or periods the alteration took place and in precisely what manner 

 it was brought about. The dolomitization being persistent only in 

 the valleys might suggest that magnesian waters formerly occupying 

 the dales may have reacted only with certain beds on account of 

 some inherent quality rendering those particular beds susceptible 

 to alteration. This is merely a suggestion, and more definite evidence 

 is necessary before it can be accepted as a proved explanation of 

 the dolomitization. The D^ beds of Gratton prove quite con- 

 clusively that subsequent dolomitization can afiect certain beds 

 and leave others below and above unaltered. Such differential 

 subsequent dolomitization being possible, caution is necessary with 

 regard to conclusions drawn from apparent interbedding of lime- 

 stone and dolomite seen in small exposures or restricted areas, 



V. The Dolomites between Parwich Moor and Wirksworth. 



South of Grange Mill highly dolomitic material is extensively 

 developed in two areas, which appear to have formed originally 

 one large outcrop. The Brassington strike fault has let down the 

 western part of the dolomitized mass, the two portions being 



