and Leicester s /lire Dolomites. 255 



the yellow dolomite of Breedon, etc., is undoubtedly in favour of 

 contemporaneous dolomitization. 



8. The Eed Ferruginous Dolomites of Breedon and 

 Brkedon Cloud. 



The uppermost portion of the Carboniferous Limestone sequence 

 seen at Breedon and Breedon Cloud is composed of several feet of 

 thinly bedded red dolomites quite distinct from the more massive 

 yellow dolomites below. These red dolomites cannot be seen to pass 

 laterally into unaltered limestone [d), but it must be remembered 

 that the outcrop is not very extensive. The rock is patchy in the 

 sense that the proportion of magnesium carbonate varies in any 

 particular stratum, but there are no external appearances analogous 

 to those of pseudo-brecciation (n). Faulting occurs at Breedon 

 Cloud, though I infer that dislocation has not been a factor of the 

 dolomitization for the following reasons: (1) no faulting occurs at 

 Breedon-on-the-Hill where these red dolomites are otherwise 

 precisely similar to the corresponding rocks at Breedon CloUd, (2) 

 the faulting at Breedon Cloud is also associated with the yellow 

 dolomites yielding very strong evidence of contemporaneous 

 dolomitization (A). 



Microscopic sections exhibit a structure characterized by idio- 

 morphic, fairly large, and verj^ impure rhombohedra («', j^, and h). 

 Well-marked central and zonal inclusions of haematite occur in the 

 crystals of dolomite forming these beds (/) (micro-photo, PL XI, 

 Fig. 1). The outer zones of the crystals are free from haematite, 

 but contain other inclusions similar to those in the rhombohedra of 

 the yellow dolomites. A few streaks and patches of recrystallized 

 calcite are present, and it is probable that this recrystallization 

 took place prior to dolomitization. 



It has yet to be proved whether coarsely crystalline calcite, either 

 original or recrystallized, is under any particular conditions immune 

 from alteration to dolomite. In connexion vpith this question, the 

 condition of crinoid stems and ossicles in these red dolomites is 

 interesting. Micro-photo, PI. XI, Fig. 2, taken from a specimen 

 of the Breedon rock, shows a crinoid ossicle invaded by haematite- 

 bearing dolomite near the central passage and around the external 

 margin, which is badly corroded by the alteration. 



Other organic structures are obscure. That other fossils were 

 present in these rocks originally, is shown by the occasional 

 occurrence of coral and Brachiopod casts. It appears that we have 

 here an instance of undoubted subsequent dolomitization in which 

 both the matrix and organic structures, with the exception of 

 encrinites, have been completely altered. Even crinoid remains, a 

 most stable form of calcite, have been altered to some extent. With 

 regard to the matrix, there is no wav of ascertaining its original 

 condition. It may have been calcite or ai'agonite, or a mixture of 

 the two ; or it maA"- have been calcite recrystallized prior to 

 dolomitization. Evidently this rock affords an illustration of the 

 f;i.ct that conclusions are not easily made from phenomena connected 

 with selective dolomitization. 



