C— GEOLOGY. 95 



In the ' Main Limestone ' (Z,— y) of the Forest of Dean dolomitisation 

 is very complete,^"^ and while chiefly contemporaneous, in the hsematite- 

 bearing beds where it is at its maximum, subsequent (vein) dolomitisation 

 has been added to the contemporaneous. Along the south-eastern margin 

 of the South Wales coalfield in the TaflE Valley, and to the east, there is an 

 almost unbroken series of dolomites extending throughout the whole 

 section from the base of Z to the Millstone Grit. The change is mainly 

 contemporaneous, but in C.^ and S.^ there has been vein dolomitisation. 

 West of the TafE Valley, as at Ruthin, dolomitisation is in the main 

 restricted to Zj and Cj.^''* 



Throughout Gower"^ C, retains the character of the laminosa-dolomite 

 of the Avon section, and there is much dolomite in Z^. In B. Gower Zj is 

 also dolomitised, as are to some extent the pseudobreccias of D. There is 

 less dolomite in the S beds of W. Gower than in those of B. Gower. 



In the Tenby district,!"*' except for the fact that there is little dolomite 

 in Z, dolomites prevail at much the same levels as in Gower, viz., Cj, various 

 levels in C^ and in the pseudobreccias of D^. The peculiar reef-dolomite 

 of Ci has been alluded to above. On the whole, in Pembrokeshire, as in 

 the eastern part of the South-west Province, dolomitisation of the 

 Tournaisian increases northwards.^"^ Both in Pembrokeshire and along 

 the north crop of the main South Wales coalfield it is almost wholly 

 absent from the Visean. 



Midland Area and North Wales. 



In the D limestones of the Midland area, as Parsons ^^^ points out, 

 dolomitisation shows two contrasting types. In the main limestone mass 

 of central Derbyshire it is wholly subsequent, while in the marginal deposits 

 of the Leicester coalfield it is almost or wholly contemporaneous. 



In North Wales there are no horizons of widespread dolomitisation, but 

 Greenlyi"" states that local dolomitisation is not uncommon in Anglesey, 

 and appears to have been, partly at any rate, contemporaneous. On the 

 other hand the masses of dark-brown dolomite of Seiriol and Peumon 

 (top of D.j) are subsequent. 



North of England. 



In Yorkshire and the North-west Province dolomitisation is com- 

 paratively slight. No contemporaneous dolomite is mentioned as occur- 

 ring in the Settle district, though extensive subsequent dolomitisation has 

 taken place locally in relation to the Craven faults.^^" 



Dolomitisation is not a characteristic feature of the North-west 

 Province, but the dolomite mudstones of the Solenopora sub-zone (Oi) of 

 the Shapi" area are characteristic examples of primary dolomites. 



103 Geol. Mag., Dec. v., vol. ix. (1912), p. 419. 



"1 Dixey and Sibly, Q.J.G.S., vol. Ixxiii. (1917), p. 122. 



105 Gower paper, Q.J.G.S., vol. Ixvii. (1911), table facing p. 505. 



io« Pembroke and Tenby Memoir, p. 70. 



107 Dixon, Sum. Prog. (1906), p. 54. 



108 Geol. Mag., lix. (1922), p. 115. 

 io» Anglesey Memoir, ii., p. 606. 



110 Q.J.O.S., Ixxx. (1924), pp. 210-212. 



"1 Ibi'l., vol. Ixviii. (1912), pp. 456 and 487. 



