C— GEOLOGY. 



89 



Viiugliau'- gives a list of species characteristic of reef knolls in general, 

 and further lists of those characterising Tournaisian and Visean knolls 

 respectively. It remains to be seen whether any comparison can be made 

 between the physiographic relation of Avonian reefs and those of the 

 coral reefs of our own time. 



Distribution of Reef Knolls and Limestone. 



/■South Craven Area. 



Cracoe neighbourhood : Swinden, Skeltertou, Stebden, 



Butterhaw, Garden. 

 Burnsall neighbourhood : Elbolton, Thorpe Kail, Hart- 



lington Kail. 

 Grassington neighbourhood: Thorpe to Hebden.^^ 

 Gargrave neighbourhood : Fogger,^^ Crag Laithe.^^ 

 D \ Malham neighbourhood : Cawden, Wedber. 

 Settle neighbourhood : Scaleber. 



Midland Area. 



Wetton, Thorpe Cloud. 



Isle of Man. 

 Poolvash. 



BowLAND Area. 



Ashnott. 



Bowland Area. 



Dunmow, near Slaidburn, Knowlmere. 



Clitheroe Area. 



Salt Hill, Crow Hill, Worsaw, Withgill, Gerna, Sykes, 

 Twiston. 



Clitheroe Area. 



White Croft Wood, Waddow, Coplow. 



Pembrokeshire. 

 Freshwater West. 



D or 



high S i 



S 



Pseudobreccia. 



This well-known rock type, originally described by Tiddeman^* and 

 subsequently and much more fully by Dixon, ^^ is very characteristic of 

 the D beds, particularly of Dj, throughout the whole South-western 

 Province. While, perhaps, best developed in Gower, where these rocks 



" Q.J.Q.S., vol. Ixxi. (1915), p. 13. 



'* Mr. Hudson informs me that these are not true knolls, but are local develop- 

 ments of limestone of reef type. Similar developments occur at other places in the 

 Craven lowlands. 



'* Swansea Memoir, p. 10. 



''^ Q.J.G.S., vol. Ixvii. (1911), p. 507. 



