C— GEOLOGY. 93 



area,^^ that in Sj at Bury Hill being one of the finest in the Bristol district. 

 In the Burrington section algal layers were tound only in S.^, and in the 

 Sodbury^'^ section in CVSi and So. 



The algal development of the Whitehead limestone at Mitcheldean is 

 perhaps the most remarkable in the Soixth of England. In the Pembroke 

 and Tenby district^^ algal limestones occur in the K and C^-Sj beds, while 

 the Sg pisolites of the Kidwelly area are doubtless algal. 



Dolomitisatiou.^^ 



This subject would by itself afford sufficient material for several 

 presidential addresses. I make no attempt to deal with the general 

 question, and in particular cannot discuss the important results obtained 

 by American investigators of coral-reef dolomitisation. My aim is merely 

 to summarise some recent work on British Carboniferoiis dolomites and 

 provide a brief statement as to their distribution. 



The dolomites of South Wales were very carefully studied by Dixon, ^' 

 while those of the Midlands are fully described by Parsons. ^^ These 

 authors agree as to the general classification of dolomites, Parsons dividing 

 them into : — 



1. Primary, including 



(a) Those deposited as clastic rocks derived from some pre-existing 



dolomite. 

 (6) Those chemically precipitated as dolomite. 



2. Secondary, including 



(a) Contemporaneous, i.e. rocks deposited as ordinary limestones but 

 dolomitised soon after deposition. 



(6) Subsequent, i.e. rocks dolomitised at some later period. 

 Primary dolomites are of comparatively slight importance among 

 Avonian rocks, but, according to Dixon, ^^ include dolomite-mudstones 

 and reef dolomites. Dolomite-mudstones occur in the Athyris glabristria 

 zone of the North-west Province. The reef dolomites of the Tenby dis- 

 trict are grey unbedded dolomite-mudstone showing a peculiar association 

 with Fenestellids, which is to be matched exactly in the knoll limestones of 

 Clitheroe and the Waulsortian of Belgium. It is now generally admitted 

 that most dolomites are secondary and contemporaneous, having been 

 produced in part by a process of leeching out of the lime and subsequent 

 concentration of the magnesia, in part by a process of replacement of 

 lime by magnesia derived from sea-water. The occurrence over a wide 

 area of dolomite of constant character points to its contemporaneity, 

 e.g. the Zaminosa-dolomite of the South-west Province, while patchy 

 dolomite such as occurs on a large scale in the D beds of Derbyshire is 

 subsequent. The lateral and often abrupt passage into unaltered lime- 

 stone such as occurs in Derbyshire is also characteristic of subsequent 



•1 Proc. Bristol Nat.Soc, 4th series, vol. vi., pt. 3 (1926, issued for 1925), p. 246i 



•2 Geol. Mag., vol. Ix. (1923), p. 117. 



•' Dixon, Pembroke and Tenby 3Iemoir, p. 69. 



•* See F. W. Clarke, The Data of Geochemistry (1908), p. 480. Full references. 



•5 Swansea Memoir (1907), p. 11, and Pembroke and Tenby Memoir (1921), p. 70i 



»« Qeol. Mag., vol. Ux. (1922), pp. 51 and 104. 



•' Pembroke and Tenby Memoir (1921), p. 70. 



