Dolomitization in the Carboniferous Limestone. 115 



vi. — summaey of conclusions and comparison with the 

 Leicestershire Dolomites. 

 The Carboniferous Limestone of the Midlands exhibits two con- 

 trasting types of dolomitization : — • 



(1) Wholly subsequent in the main limestone mass in Central 

 Derbyshire, and 



(2) Almost wholly contemporaneous in the marginal deposits 

 north of the Leicestershire Coalfield. 



In neither case is there any extensive development of typical 

 " vein " dolomitization. 

 In Central Derbyshire : — • 



(1) Magnesian ground waters appear to have been the agent of 

 metasomatism, but the source of the magnesium is not definitely 

 indicated. 



(2) The dolomitization is of more than one period, as shown by 

 certain complex-subsequent types, and as suggested by the relation 

 between the metasomatism and the topography. There is little to 

 indicate the period at which dolomitization took place in the 

 Winster, Gratton Dale, and Matlock districts, but in the case of the 

 widespread alteration in the Harborough area further south, the 

 dolomitization must have occurred prior to the extensive faulting 

 there seen. 



(3) It is shown that subsequent dolomitization is capable of 

 affecting certain beds, while others above and below may be left 

 unaffected, hence apparent interbedding is not necessarily a proof 

 of contemporaneous alteration. Different horizons are affected in 

 different parts of the area, the affected horizons in the district as a 

 whole being Middle D,, top of Dj, lower D„, and locally near Winster, 

 Upper D2 ; Upper Dj beds are affected only around Harborough. 

 Within these portions of the sequence the metasomatism varies 

 horizon, and the surface areas of dolomitization are pseudo-strata. 



(4) Extensive dolomitization in Middle Dj beds is generally 

 restricted to the valleys of Giatton Dale and Long Dale, but that on 

 higher horizons is mostly confined to hill-tops and moors. 



(5) On all horizons, lateral transitions into limestone are abrupt, 

 and the transition limestones exhibit feeble or only moderate calcific 

 r eery stallization . 



(6) On all horizons the dolomitic material exhibits a general 

 tendency to allotriomorphism, and thus contrasts strongly with 

 some subsequent dolomites of other districts. 



(7) Dolomitization has generally produced an increase in size of 

 grain, but in spite of this increase the size of grains and crystals 

 are frequently smaller than those of some contemporaneous 

 dolomites of other areas. Neither allotriomorphism nor size of 

 grain can be accepted as unimpeachable evidence of the mode of 

 magnesiation. Such features appear to be closely related to the 

 original texture and mineralogical constitution of the rock, and to 

 the intensity of the magnesian factor in the agent of metasomatism. 



