C— GEOLOGY. 87 



In the Gower paper ^' P is used primarily in a chronological sense for 

 strata succeeding D,,,.,, while Sibly^^ suggests its use in a purely phasal 

 sense. 



The zone P was re-defined by Bisat^^ in a strictly chronological sense as 

 the zone of Goniatites crenistria, and probably that of any member of the 

 genus Goniatites {sensu stricto). This use is somewhat more restricted than 

 that of the Loughshinny paper, but wider than that of the Gower paper. 

 It is thus clear that there has been no uniformity in the use of P, and 

 probably it would really be best if, as Dixon®" suggested, the symbol as 

 used in a chronological sense were dropped, and, as Sibly proposed, it were 

 used purely in a phasal sense for Avonian rocks displaying the Culm or 

 goniatite development. The successive zones might then (as Dixon pro- 

 posed) be alluded to by the zonal fossil, as is done with the graptolites of 

 the Lower Palaeozoic and the ammonites of the Mesozoic. 



It cannot, however, be expected that such a course should commend 

 itself to the workers on the Culm-goniatite development, particularly in 

 view of the already extensive literature based on Mr. Bisat's work. It 

 seems to me that both zone P and Yoredalian are north-country 

 denominations, and that in the south it would be more convenient to 

 designate their equivalents D^p and D.jy. 



Reef Knolls 



61 



The general facts regarding reef knolls are well known. They are 

 highly fossiliferous limestone, in certain cases dolomite, hills or mounds, 

 the fossils being exceptionally well preserved. ' The term reef as used in 

 these cases implies that, though not composed of corals, the masses have 

 undoubtedly caused local elevations at the bottom of the Carboniferous 

 sea ' (Dixon). ®^ In some cases the stratification shows a quaquaversal 

 dip, and the massive and frequently crystalline limestone of which the 

 reefs are composed is succeeded in all directions by rocks of the 

 Zaphrentid phase — thinly-bedded limestones and shales. 



Mr. Hudson, in describing the Cracoe knolls, says that the characteristics 

 of reef limestone are seen in its irregular non-bedded nature, in the con- 

 temporaneous breccias, in the frequent shell-beds of rolled and broken 

 Productids, and in the abundant pockets packed with shells in perfect 

 preservation, often showing colour bands and evidently in the position 

 of growth. He remarks that the frequent occurrence of sheets of 

 calcareous tufa points to deposition in shallow water or even to emergence. 



Such reefs are met with in the Lower Carboniferous rocks at a number 

 of localities in the British Isles, but particularly in West Yorkshire, in 

 the South Craven area between Skipton and Malham, and in the Cracoe 

 and Burnsall areas between Skipton and Grassiugton, and in the Bowland 



•' Cf. table facing p. 105. 



" Proc. Oeol. Ass., xxxi. (1920), p. 81. 



■''■> Op. cit., pp. 43-5. 



'^ Southampton Report, sub-report i. 



^1 Note. — Dr. D. Parkinson's paper on the ' Faunal Succession in the Carboniferous 

 Limestone of Clitheroe,' Q.J.O.S., vol. Ixxxii. (1926), pp. 188-249, contains an import- 

 ant account of the Clitheroe knolls. 



" Pembroke and Tenby Memoir, p. 68. 



