Dolomitization in the Carboniferous Limestone. 53 



rocks result from later alteration of either contemporaneous or 

 subsequent dolomites. 



Compound dolomites in which a primary clastic or precipitated 

 dolomite has been produced by secondary addition of magnesia. 

 It must be noted that types petrologically described under various 

 terms such as dolomite-mudstone, pseudo-breccia, etc., will naturally 

 fall into their respective groups in the classification arranged 

 according to the time and mode of alteration. 



The methods of investigation adopted in connexion with the 

 Derbyshire material are essentially the same as those pursued in 

 the case of the Leicestershire rocks. The evidences obtained from 

 different series of phenomena converge toward definite conclusions. 

 The different series of phenomena are those connected with (a) 

 field relations, (b) inherent structural and mineralogical features, 

 (c) the absence or presence and condition of fossils, (d) chemical 

 analyses. 



A general summary of evidences has been given previously, but 

 additional criticism of certain evidences and amplification of others 

 accompanies the description of the Derbyshire rocks. 



The uniformity or variation in chemical composition of dolomitic 

 rocks is a phase of the subject which has scarcely received the 

 attention it deserves, and in the present work I have made an attempt 

 to develop this side of the question. 



III. The Dolomitic Limestones and Dolomites between 

 Gratton Dale and Cromford. 



The tectonics of the limestone area between Cromford and Gratton 

 Dale are relatively simple in the Gratton Dale district, but become 

 more complicated eastwards in the neighbourhood of Matlock Bath, 

 where there is a series of anticlinal and synclinal folds worked out 

 in detail by Mr. C. B. Wedd.^ With the exception of the lower 

 sequence in Gratton Dale and Longdale, described separately, none 

 of the D| beds between Gratton and Cromford exhibit dolomitization, 

 but in the D., strata north and east of the lower lava dolomitic 

 material occurs in two extensive areas situated in the neighbourhood 

 of Winster and of Matlock Bath respectively. 



Field Relations. 



The dolomitization has attained its greatest vertical and lateral 

 development in the vicinity of Winster. Near Bonsall Moor Farm 

 occurs a well-marked escarpment of dolomitic limestone, just above 

 the lower lava. Northwards the metasomatism extends below and 

 above the upper lava to a position beyond the Wensley road and not 

 far below the Pendleside Shales. Apparently nearly all the D^ 

 beds, and possibly part of the Dg material here consists of dolomitic 

 limestone. A little further to the east, beyond Bonsall Moor Farm, 



1 3Iem. Geol. Surv. : " The Geology of the Northern Part of the Derbyshire 

 Coalfield and Bordering Tracts." 19l3. 



