C— GEOLOGY. 97 



nodular type, but, as Mr. Wedd informs me, are a cherty silicification of 

 silts and shales, sometimes accompanied by patches of chert, in calcareous 

 sandstones. Beekitisation of fossils may also be a marked feature of the 

 Flint development. 



Chert occurs in the Pendleside limestone of Pendle Hill. In the 

 Settle^i^ district chert occurs in the upper part of the Bryozoa series (Sj) 

 in the districts between the faults, and is typically developed in the 

 OrionaslrvBa bed (base of Yoredalian). In Wensleydale and other York- 

 shire dales cherts are well developed at several levels in the upper 

 Yoredalian, particularly just above the Undersett limestone. 



In the North-west Province chert isnot characteristic ; Garwood^^* states 

 that it is met with locally, especially in the higher part of D, but never 

 in any great quantity. Replacement of fossils by beekite is not unusual 

 at various horizons. Cherty limestones occur in the Botany Beds, the 

 highest calcareous beds in the Avonian of the North-west Province. 

 Chert occurs low in the series near the bottom of the Seventh limestone 

 (upper S.J in the Whitehaven^^" district, and again in D.^ in the upper 

 part of the Fourth limestone. 



Classification of Cherts. 



The cherts alluded to in the above list fall into three groups, which 

 practically correspond with the three phases of deposition of Vaughan's 

 Winnipeg report. 



(a) The irregular nodular chert often forming impersistent bands at 

 various levels throughout the South-west Province, and in the Dj beds 

 of the Midlands and Yorkshire, is specially characteristic of the standard 

 limestones. 



(fe) The great development of bedded cherts in D,, of the Midlands 

 and North Wales and in the Bishopston beds (' Black Lias ') of Gower is 

 intermediate in character between those of groups (a) and (c), and is 

 specially characteristic of the Zaphrentid and Cyathaxonid phase. The 

 laminated cherts of Z.^ at Vallis, Somerset, should probably be placed here. 



Note.—h\ alluding to these cherts as bedded, it is not intended to imply 

 that they themselves show traces of bedding. They are lenticular or 

 nodular cherts developed along the bedding planes. The laminated 

 cherts mentioned below are those frequently alluded to as banded 

 cherts, but the term laminated is substituted in view of the fact that 

 the term banded has often been applied to concentrically zoned flints. 



(c) The laminated cherts of the lagoon-phase deposits of Dixon are 

 represented by those of the North Devon Culm, by those of the P beds 

 (Dap) of Gower, and by those at the base of the Millstone Grit of the 

 Pembroke area. The deposits in which they occur belong to the third 

 phase of Vaughan's table {Modiola and Posidonomya phases and other 

 shallow-water deposits). 



"' Q.J.O.S., vol. Ixxx. (1924), p. 206. 



"8 Ibid., vol. Ixviii. (1912), p. 551. 



"• See Edmonds, Geol. Mag., vol. lis. (1922), pp. 78 and 81. 



1926 2 



