Dolomitization in the Carbon if ei'ous Limestone. Ill 



district, and shows apparently thick-bedded dolomite against thin- 

 bedded white limestones. 



It is evident that the transition material formerly on the western 

 hill-tops has been denuded. South of the fault dolomitic stacks 

 surrounded by limestone occur on the eastern side of the valley. 

 A little north of the fault lateral transition of some of the dolomitized 

 beds gives the appearance of interbedding of dolomite and lime- 

 stone. Still higher up the valley more lateral transitions occur on 

 the eastern side, and extensive dolomitization finally dies out just 

 beyond the Roystone Rocks near the roadway from Parwich to 

 Pike Hall. 



The northern boundary of the Rainster mass is certainly faulted ; 

 limestones with chert are brought against dolomite without chert, 

 and the direction of the dip is different. 



In the whole district from Hopton to Roystone the persistent 

 modification of the dolomitic pseudo-strata by the faults is a 

 feature of considerable significance. If the dolomitization were of 

 the type in which faulting had been a factor, it is inconceivable that 

 such a large majority of the faults in the district would show lime- 

 stones one side of each fault plane and dolomite on the other. It is 

 quite definite that the dolomitization must have occurred prior to 

 the dislocations and is independent of them. The metasomatism 

 is of a subsequent type, but of great antiquity, and probably occurred 

 soon after the Carboniferous Limestone was formed. 



With regard to evidences connected with the relation between 

 faulting and dolomitization, certain features may be summarized 

 briefly : — 



In the case of primary, or contemporaneous, or subsequent 

 dolomitization which occurred prior to faulting : — • 



(1) If the throw of a fault is sufficient a " bed " of dolomite is 

 usually against a bed of limestone, and the faulted junction is a 

 sharp line of demarkation. 



(2) With thick dolomites a fault of small throw may j)roduce 

 dolomite against dolomite. 



(3) The outcrops or pseudo-strata are affected by faulting in 

 the usual ways. 



(4) Megascopic crystals of dolomite are not usually found in fault 

 planes and cavities. Should such crystals occur, complex or 

 compound dolomitization may be indicated. 



In cases of subsequent dolomitization occurring after faulting, 

 particularly alteration of the " vein " type : — 



(1) The beds on both sides of the fault are usually affected though 

 those on one side may be more affected than those on the other owing 

 to differences of texture, etc. The line of fault is typically not a 

 sharp line of demarkation. 



(2) Dolomitization is more marked in the immediate vicinity 

 of the fault. 



