256 L. M. Parsons — DoloTnitization 



The evidences concerning the origin of the Breedon and Breedon 

 Cloud red dolomites indicate quite definitely that the dolomitization 

 was subsequent and associated with, waters percolating through the 

 Trias, which formation rests upon the upturned edges of the red 

 dolomites at these localities. It seems fairly evident that this 

 subsequent dolomitization has attacked previously unaltered lime- 

 stones (2) ^ lying between apparently contemporaneous dolomites 

 stratigraphically lower (1) at the Breedons and higher (4) at 

 Ticknall. 



9. The Bakeen Gkey and Yellow Dolomites or Ticknall and 



Calke. 



About ten feet of bedded dolomites, yellow below but grey above, 

 occur at the very top of the Carboniferous Limestone at Ticknall and 

 Calke. They are succeeded by dark shales which pass up conformably 

 into Millstone Grit. The chemical composition of these dolomites is 

 similar to that of certain rocks described by Professor Skeats as 

 "Dolomites of theoretical composition".* A comparison of the 

 results of analysis makes this evident. 



In the Ticknall dolomite a small amount of free calcite is present, 

 as shown by slides stained with Lemberg's solution. 



These rocks are seen best in the old lime works of Ticknall, but 

 the exposures are very limited in extent, so that the bedded nature 

 of these dolomites and their apparent non-passage into unaltered 

 limestone cannot be used as reliable evidence of their origin (d). 

 There is, on the other hand, not the slightest visible development of 

 patchy dolomitization (/). 



A small fault occurs, but this shows no definite connexion with 

 the origin of the dolomite. The yellow and grey varieties of the 

 rock are very similar in their microscopic characters. Sections show 

 a crystalline mass in which a large number of crystals have been 

 rounded off, presumably by simultaneous development («), but some 

 rhombohedral outlines are retained, particularly in the case of some 

 of the larger crystals (micro-photo, PI. XI, Fig. 3). There are very 

 numerous inclusions, consisting mainly of minute particles of insoluble 

 matter incorporated during crystallization {k), but there are no 

 central nor zonal inclusions of haematite {I), iron oxide in the form 

 of limonite being interstitial. This feature has special significance 

 in view of the fact that reddish rocks of Permian and Triassic ages 

 rest upon the limestone in the north-west corner of the Ticknall 

 exposures. Chert is absent, and organic remains, if present 

 originally, have been completely obliterated (r). According to the 



^ Numbers refer to stratigrapbical positions given in the classification of 

 petrological types. 



2 Q.J.G.S., 1905, p. 105. 



