and Leicestershire Dolomites. 253 



present originally. In other words, the final stage of secondary 

 dolomitization might produce the complete alteration of a rock 

 which may have exhibited selective phenomena in its early stages of 

 alteration. 



(r) The presence of fossils in a dolomite is a little more significant, 

 particularly if they are at all numerous. The argument is then 

 against the theory of primary precipitation. In a rock showing no 

 selective features the condition of fossil structures as casts or as 

 replacements in dolomite, appears to yield very little evidence of 

 contemporaneous or subsequent alteration since both casts and 

 replacements of the same class of organisms may occur in the same 

 bed of dolomite. 



The perfect replacement by dolomite of coral structures is certainly 

 suggestive of contemporaneous alteration, since such replacements have 

 been found to occur in modern reefs before calcitic recrystallization 

 of coral tissues took place. ^ 



Paet II : The Leicestershire Dolomites. 



A series of faulted inliers of Carboniferous dolomites extends in 

 a north-westerly direction from the edge of Charnwood, commencing 

 with small patches of dolomite at the village of Osgathorpe, 

 and ending to the north in the mass forming Breedon Hill, 

 a landmark for many miles. Between these limits of tbe series 

 are situated the dolomite hill known as Breedon Cloud and the 

 much smaller though petrologically similar inlier called Barrow 

 Hill. In each, of these cases the Carboniferous Limestone is 

 surrounded by unconformable Keuper. A few miles to the west of 

 Breedon there are small valley inliers of Lower Carboniferous rocks 

 containing bedded dolomites anddolomitic limestones at Ticknall and 

 Calke Park. At these localities the Lower Carboniferous is succeeded 

 conformably by Millstone Grit, which is overstepped at one or two 

 places by Trias. 



6. Classification of Petrological Types. 



Disregarding details of stratigraphy and palceontology which I have 

 described in another paper,^ and considering the formations purely 

 from a petrological point of view without assuming the mode of 

 origin of the dolomite, we may distinguish in the area different 

 types of dolomitic rocks as follows: — 



/Dense yellow dolomites of Breedon and Breedon 

 Dolomites proper, containing Cloud (l).''^ 



a proportion of magnesium < Red ferruginous dolomites of Breedon and 



carbonate approaching 40% Breedon Cloud (2). 



I Barren grey and yellow dolomites of Ticknall (4) . 

 Dolomitic limestones con- C 



taining a relatively small | Fossiliferous dolomitic limestones of Ticknall 



percentage of magnesium"! and Calke (3). 



carbonate 



1 See Cullis, " The Atoll of Funafuti " : Report of Coral Reef Committee, 

 Royal Society, 1904, section xiv, p. 407. 



- See Abstracts of the Proc. Geol. Soc. of London, No. 1004, March 14, 

 1917. 



' The numbers in parentheses indicate relative stratigraphical positions in 

 ascending order. 



