88 SECTIONAL ADDRESSES. 



and Clitheroe areas. Other examples occur at Poolvash, near Castleton, 

 Isle of Man, while the Waulsortian of Belgium, part of the Syringothyris 

 beds of Co. Clare, and the well-knowu brachiopod beds of the Midlands^^ 

 (Treak Cliff, Castleton, Park Hill, near Longnor, Thorpe Cloud, in Derby- 

 shire, Narrowdale, near Wetton, and Cauldron,®* in Staffordshire), and 

 those of St. Doulagh, near Dublin, are analogous. 



From South Wales, Dixon ®^ describes in C\ reef dolomites largely of 

 bryozoal origin and a small limestone reef of C2 age, the relations of which 

 to the surrounding rocks of the Zaphrentid-phase are particularly well 

 seen. 



The Waulsortian of Belgium is described by Vaughan®® as ' composed 

 of thick irregular accumulations of powdery limestone often brecciated 

 and seamed with calcite veins ; its most striking aspect is a massive lime- 

 stone, mottled with bluish blotches rich in Fenestellids. Every now and 

 then the beds swell out into mushroom-shaped lenticles of unstratified 

 reefs (the " knolls ") roofed over by the next stratified deposit, which 

 spread over the uneven floor.' 



Both in Britain®'^ (S. Wales and Clitheroe) and in Belgium^® a peculiar 

 laminar structure occurs associated with the Fenestellids of the reef 

 dolomites. The significance of this is not yet understood. Fenestellids, 

 though generally characteristics of reef-knoll limestones, are not common 

 in the Cracoe knolls. 



It was observed by Tiddeman,®^ who first described them, that the 

 knolls do not all lie on the same horizon. He divided them into two 

 groups — an upper series which he grouped with the Pendleside lime- 

 stones, and a lower series included in the Clitheroe limestone. Vaughan'''' 

 maintained that Tiddeman's upper group is of D2 age, and showed that 

 his lower group included two distinct series, an upper series of S age and 

 a lower series of C age. Dr. Parkinson's recent work is in accord with 

 that of Vaughan as to the age of the Clitheroe knolls. 



With few exceptions recent workers are agreed that Tiddeman's 

 original views as to the origin of reef-knolls are essentially correct. He 

 held that they are due to the original deposition of the remains of calcareous 

 organisms in an area undergoing depression, not to any subsequent packing 

 by earth movements, as was maintained by Marr.''^ Tiddeman's theory 

 was originally supported by Dakyns, and received weighty confirmation 

 from Vaughan, who showed that there is a special reef-facies easily recog- 

 nisable as such, whatever the level may be from which the specimens are 

 derived. Gastropods and lamellibranchs, sometimes bryozoa and trilo- 

 bites, and in particular certain persistent brachiopods {Piignax, Schizo- 

 phoria, Martinia), which often attain an exceptional size, abound in these 

 reef deposits, but corals, with the exception of Amplexus, are rare. 



" Q.J.G.S., vol. Ixiv. (1908), p. 49. 



«« Oeol. Mag., vol. Iviii. (1921), p. 367. 



^ Pembroke and Tenby Memoir, p. 127. 



«8 Q.J.G.S., vol. Ixxi. (1915), p. 12. 



*' Dixon, Pembroke and Tenby Memoir, p. 127. 



«» Delepine MS. 



«5 Rep. Brit. Ass., Newcastle, 1889, p. 602, and Bradford, 1900, p. 740. 



'" Proc. Yorks Oeol. Soc, vol. xix. (1916), pp. 41-60. 



'1 Q.J.O.S., vol. Iv. (1899), pp. 327-58. 



