94 SECTIONAL ADDRESSES. 



dolomites. In contemporaneous dolomites, which are normally dark grey, 

 the crystals interlock with one another and produce a granular mosaic 

 of allotriomorphic crystals, while in subsequent dolomites, which are 

 normally light grey or reddish, the crystals as a rule tend to be relatively 

 large and idiomorphic. In Derbyshire, however, the thick dolomites 

 claimed by Parsons as subsequent are mainly allotriomorphic. The inter- 

 bedding of a dolomite with non-dolomitised strata is not necessarily a 

 proof of contemporaneous alteration, for, as Parsons points out, subsequent 

 dolomitisation is capable of afiecting a bed while leaving those above and 

 below unaltered. 



Organic remains, particularly those which, like crinoidal 'ossicles,' are 

 composed of calcite in fairly large crystals, tend to be more resistant to 

 dolomitisation than the matrix, which is acted on with special readiness 

 if consisting originally of aragonite mud. Dixon^* shows that in con- 

 temporaneous dolomites corals and ooliths are resistant as compared with 

 the matrix, while in subsequent dolomites the corals and ooliths are 

 affected first. In several districts where Triassic rocks rest or have rested 

 on the Avonian, the dolomite crystals are seen to contain haematite, 

 presumably derived from the Trias, and affording proof that the dolomite 

 is subsequent ; Parsons®^ figures an excellent case of this from Breedon, 

 in Leicestershire. 



Selective dolomitisation is a term employed by him when the formation 

 of dolomite occurs in certain portions of the rock, tending to produce a 

 mottled or brecciated appearance, e.g. the pseudobreccias of the South- 

 west Province. Cases are known where there is evidence of the secondary 

 addition of Mg. at more than one period ; such are termed by Parsons 

 complex dolomites. 



Distribution of Dolomites. 



South-ivestern Province. — There is progressive increase in dolomiti- 

 sation as one passes northwards from the Mendips and eastward from 

 Pembrokeshire, the phenomenon reaching its maximum in the region 

 south and east of the South Wales coalfield. It is not easy to trace any 

 relation between the amount of dolomitisation and the shore-lines of the 

 period.^"" 



In the Burrington section lower Cj is dolomitised, and there is some 

 dolomite at various levels in S^ 



In the Avon section^"^ dolomitisation is widely prevalent throughout 

 Z and y, without, however, leading to the obliteration of the fossils. Lower 

 Cj and C.2 are almost completely dolomitised, while there is a good deal 

 at various levels in S. The Sodbury section^"^ is essentially the same as 

 the Avon section as regards dolomitisation. 



In the Chepstow area Z and C are greatly dolomitised, while there is 

 a good deal in S, especially Sj. 



"8 Q.J.G.S., Ixxiii. (1917), p. 110. 

 " Ibid., pi. X., fig. 4. 



^'"' See Vaughan, ' Shift of the Western Shore-lines in England and Wales during 

 the Avonian period,' Eep. Brit. Ass., Manchester, 1915, p. 429. 

 "1 Oeol. Mag., vol. Iviii. (1921), p. 544. 

 "- Ibid., vol. Ix. (1923), pp. 112-3. 



