Doloonitization in the Carboniferous Limestone. 59 



With regard to impurities, haematite is relatively uncommon, and 

 occurs merely as surface stains associated with recent weathering. 

 Limonite is more common, and is typically interstitial, but certain 

 specimens, particularly some from Gratton, exhibit zonal inclusions 

 of limonite in dolomite crystals. PI. IV, Fig. 3 shows two distinct 

 phases of dolomitization. The wall of the coral {Lithostrotion 

 portlocki), a portion of the interior, and parts of the matrix are 

 replaced by fine-grained dolomite free from zonal growths, but the 

 matrix mainly consists of dolomite larger in texture and containing 

 many central or zonal limonite inclusions. The inference that the 

 dolomitization of the coral occurred at a different period from that 

 of the limonite-bearing portion of the matrix iippears to be legitimate. 



At Gratton and other localities two (or perhaps more) periods 

 of metasomatism have produced dolomitic rocks which may be 

 described as complex-subsequent. 



PI. IV, Fig. 3 also illustrates certain features of " selective " 

 dolomitization. A considerable portion of the interior of the coral 

 is occupied by coarsely recrystallized calcite. General examination 

 of the slide and hand-specimen shows that coral structures are partly 

 replaced by fine-grained dolomite and partly obliterated by the 

 recrystallized calcite. Most of the matrix consists of the coarser 

 limonite-bearing dolomite. These features indicate three distinct 

 phases of alteration : — 



(1) The recrystallization of portions of the coral and intervening 

 spaces. 



(2) Dolomitization of the coral tissues and perhaps a portion of the 

 matrix. (There is nothing to indicate whether or not the coral 

 tissues had been previously recrystallized by calcite of a finer 

 grain.) 



(3) Simultaneous dolomitization and limonitization of the larger 

 part of the matrix. 



It is very probable that (1) preceded (2), but it is difficult to say 

 whether the bulk of the matrix was altered before or after the 

 dolomitization of coral structures. 



The transition limestones of Winster (PI. IV, Fig. 2) and of 

 Jughole (PI. IV, Fig. 4) show that a matrix mostly non- 

 recrystallized is a striking feature of the unaltered limestone in these 

 Do beds. It might be assumed that, in the case described above, the 

 matrix was dolomitized before the organic structures. But con- 

 clusions from " selective " phenomena must be made with care, 

 and as a discussion of this matter was given in my earlier paper it 

 will be sufficient if it is stated here that features observed in Derby- 

 shire fully confirm my opinions expressed at that time. At Winster 

 and Gratton both corals and matrix are often found equally 

 dolomitized. In some of the Gratton specimens I have found that 

 the dolomitization has affected the matrix more than coral structures. 

 Thus, in the district under consideration, selective phenomena vary 

 considerably in material taken from exposures along similar horizons 



