and Leicestershire Dolomites. 257 



evidence it seems reasonable to infer that the Ticknall dolomites are 

 of contemporaneous origin. 



10, FOSSILIFEROUS DoLOMITIC LiMESTONES OF TiCENALL AND CaLKE. 



Certain limestones occurring at a slightly lower horizon than that 

 of the barren dolomites of Ticknall, are interesting mainly on 

 account of their representing an incomplete stage in the process of 

 dolomitization. These limestones are best studied at Ticknall, but 

 some of the Calke specimens yield very fine slides showing 

 "selective" phenomena. The amount of magnesium carbonate 

 does not exceed 16 per cent. As in the case of the barren dolomites 

 above, these rocks occur apparently in definite beds and are not 

 associated with faulting of any importance. Chert is absent. 

 Microscopic sections show idiomorphic crystals of dolomite having 

 a fair degree of purity (k), and devoid of zonal haematite in- 

 clusions [I), i The rhombohedra show a decided preference for organic 

 structures (p), both coralline and brachiopod, though dolomitization 

 occurs to some extent in the matrix. Micro-photo, PI. XI, Fig. 4, 

 shows rhombohedra developed in organic structures, the matrix 

 consisting of recrystallized calcite in places. Though the features 

 shown b}^ this photograph are typical, some individual crystals are 

 developed partly in an organism and partly in the matrix. The 

 rock being of a mixed organic nature, there is no knowledge of the 

 relative proportions of aragonite and calcite muds in the original 

 matrix, and even if this point could be decided there would still be 

 the question of the comparative stability of coarser aragonite and of 

 calcite mud to be determined. In one or two cases dolomite crystals 

 are entirely surrounded by recrystallized calcite, from which it 

 appears that either the recrystallization took place after the 

 formation of dolomite or recrystallized calcite has been converted 

 into dolomite. The inference that dolomitization was prior to re- 

 crystallization would tend to support the theory of contemporaneous 

 oi'igin. The selective phenomena exhibited by the Ticknall 

 and Calke dolomitic limestones do not, in my opinion, yield any 

 conclusive evidence concei*ning the period of dolomitization, and 

 though the balance of evidence derived from other sources may be 

 slightly in favour of contemporaneous alteration, it may be wiser to 

 consider the matter " not proven " since some of the evidence is of 

 a conflicting nature. 



Having arrived at the general conclusion that most of the 

 dolomites of the Leicestershire area are of contemporaneous origin, 

 may I suggest that they appear to have been formed in shallow 

 portions of the Carboniferous sea situated in the Charnwood region. 

 The dolomitic nature of these rocks compared with that of the more 

 normal limestones occurring at the same horizons (D^ and Do) in 

 Derbyshire may thus be explained. The case may be somewhat 

 similar to that of the Imyiinosa dolomites of the Bristol area, which 

 are situated about twenty-five miles distant from the more normal 

 limestones of the Mendips. The presence of a rich Lamellibranch 

 fauna in the Do subzone in Leicestershire, and the presence of only 



DECADE VI. — VOL. v. — NO. VI. 17 



