C— GEO LOG Y. 75 



The rocks grouped here include china stones and otlior calcile mud- 

 stones, dolomite mudstone, oolite, i)isolite, algal limestones, ferruginous 

 bryozoal and crinoidal limestone of the type of the well-known Bryozoa 

 bed (hor. a) of the South-west Province. 



The characteristic fossils of the Modiola phase are small lainellibranchs, 

 ostracods, spirorbid annelids, foraminifera and calcareous alga;. The 

 brachiopod Seminula is also characteristic. 



Bands of standard limestone witli the normal fauna are intercalated 

 at certain localities and horizons, probably indicating invasion of the 

 lagoon area by the open sea. 



(2) Rcdiolarian phase=GheTty lagoon-phase. 



These rocks show far less lithological variability than the calcareous 

 lagoon-phase rocks, and are far less abundant. They consist of chert 

 bands alternating with shales. 



Dixon shows that radiolarian cherts are not, as was claimed by Hinde, 

 necessarily deep-sea deposits, and that they may be interbedded with 

 shales wliich are clearly of shallow-water origin. The finely laminated 

 and current-bedded character of many cherts is inconsistent with a deep- 

 water origin. The evidence for the existence of these radiolarian lagoon 

 phases was obtained by Dixon in Gower, and he points out that, though 

 absent in the Bristol area, such phases occur throughout the whole area 

 south of the Pembroke coalfield. He claims that the radiolarian cherts 

 of the Culm of North Devon were accumulated under lagoon-phase conditions. 

 Dixon in the Gower paper recognises four Modiola phases, three of which, 

 those of Km, G,, and top of S,,, are very widely traceable throughout the 

 South-west Province. They are essentially similar in the Avon, Sodbury 

 andTytherington sections, tlie S, development being particularly prominent 

 at Sodbury. In the Burrington section Modiola phases are not con- 

 spicuous, the Km beds being ill-exposed, C, being mainly standard 

 limestones, and the S,, development relatively thin. The fourth Gower 

 lagoon phase is at the base of P. 



In the eastern part of the South Wales coalfield Morftok-phase conditions 

 start earlier than in the Gloucestershire sections, and a lower Modiola 

 phase (Ci-G,), 250 ft. thick, is separated by 100 ft. of dolonaite from an 

 upper Modiola phase (S,) at Cefn On, to the north of Cardiff. 



The rocks of the isolated mass of Cannington Park^'^ (S,) near Taunton 

 appear to be mainly of Modiola-\)hase type. 



In the Forest of Dean the Km Modiola phase is not well developed, and 

 includes bands of standard limestone, but the Whitehead limestone (C.,) 

 constitutes a well-marked Modiola phase, the algal development bemg 

 one of the most remarkable in Britain. 



In the outcrop south of the Pembroke coalfield (Tenby, &c.) Modiola 

 phases are thinner than in most of the country farther east, but occur 

 at the same general levels Km, C, and top of S,. The Km phase is recog- 

 nisable throughout except at Freshwater West; the C, phase is thin along 

 the northern outcrop (West Williamstown and Tenby), and is absent 

 south of Tenby. The S, phase is present throughout, but poorly developed 

 in the southernmost outcrop. 



"■■i F. S. Wallis, Geol. Mag., vol. l.xi. (1924), p. 218. 



