116 Dolojnitization in tlie Carboniferous Limestone. 



(8) In this area " selective " features are varied and cannot be' 

 accepted as a source of evidence of the mode of niagnesiation. In 

 the production of such features, the original constitution of the rock 

 is probably the chief factor. 



(9) Chemical composition is varied even within small portions of 

 the affected areas, the dolomitic gradients being in some cases steep, 

 in others more gentle. The intensity of magnesiation is greater in 

 the regions around Harborough and Matlock Bath, the magnesian 

 contents generally decreasing southwards and westwards towards 

 Parwich, and north-westwards towards Gratton. 



(10) Porosity is intimately related to dolomitization, the general 

 connexion being that the greater the degree of magnesiation the 

 higher the porosity. Porosity is due to dolomitic re constitution of 

 the rock and to post-metasomatic leaching. Pronounced vesicular 

 weathering is a feature associated with subsequent dolomitization. 



Bordering the Leicestershire Coalfield : — 



(1) The bulk of the sequence consists of some 800 feet of well- 

 bedded contemporaneous dolomites without alternation with lime- 

 stones. The surface areas of dolomitization are not pseudo-strata. 

 These rocks constitute a shallow-water facies formed during long- 

 continued subsidence. 



(2) The horizons affected by this contemporaneous alteration 

 include all of the visible D^ beds and nearly all of the Do-g part of 

 the sequence. These dolomites, therefore, are in Central Derbyshire 

 represented by deposits originally normal limestones, some of which 

 have been subsequently dolomitized. How far northwards from 

 Leicestershire the contemporaneous metasomatism extends is not 

 known, the Carboniferous limestone being obscured by Trias. 



(3) Lateral transitions into limestone and many other features 

 found in Central Derbyshire are absent. 



(4) The dolomitic material on all horizons affected by con- 

 temporaneous alteration is generally allotriomorphic and fine 

 grained. 



(5) " Selective " features are absent, matrix and organisms 

 including corals being completely dolomitized, 



(6) Chemical composition is comparatively uniform, there being 

 slight variation in magnesian contents, but no marked dolomitic 

 gradients similar to those of the Winster or Longcliffe districts. 



(7) Porosity is small, evidences of extensive leaching are absent, 

 ^nd there is no vesicular or differential weathering. 



(8) A small portion of the sequence towards the top of Dg-g 

 exhibits a variety of types. Some of these beds exhibit con- 

 temporaneous alteration, but have been subsequently dolomitized 

 by waters associated with the Trias. Still higher beds showing very 

 incomplete metasomatism and inconclusive " selective " features 

 cannot definitely be assigned to any particular mode of magnesiation. 

 The highest beds, at Ticknall, are composed of completely 

 dolomitized material apparently of contemporaneous origin. 



