Vol. 67.] rHB CARBONIFEROUS SUCCESSION IN GOWER. 521 



anomalous characters to deposition under lagoon- 

 conditions. 1 



In the application of this general principle to particular cases, 

 before a rock-group that satisfies the conditions of exceedingly fine 

 texture and shallow-water origin is regarded as a lagoon-phase, its 

 geographical extent should, if possible, be determined, and for the 

 following reason. For the maintenance of lagoon-conditions it 

 appears to be necessary that the shallow area be of wide extent (see 

 ' Conditions of Deposition of the Modiola Phases,' pp. 517-18) ; and 

 certainly their size has been a feature of the areas of deposition of 

 those lagoon-phases that are known. 



The Grower radiolarian cherts a lagoon -phase. — Iu 

 the case of the radiolarian-chert group of Gower and Carmarthen- 

 shire, a shallow-water origin is inferred from the stratigraphical 

 evidence cited above, and accords with the presence, in interbedded 

 shales, of lamellibranchs (p. 551) known elsewhere only from 

 shallow-water formations. Also, the group has a wide geographic 

 extent: it crops out in Carmarthenshire at a considerable distance 

 from Gower, in a direction at right angles to the trend of the Avonian 

 coast ; and is known through much of South Pembrokeshire, in a 

 direction from Gower parallel to that coast. Consequently, we 

 conclude that it constitutes a lagoon-phase, and, in essentials, differs 

 from Mocliola phases merely in consisting of the remains of siliceous 

 instead of calcareous organisms, in addition to fine-grained sediment. 

 For contrast it will, occasionally, be convenient to speak of it as 

 a 'radiolarian phase.' 



Although this conclusion rests primarily on the evidence cited 

 above, it is confirmed by other features of the radiolarian cherts on 

 this horizon. The first is the fine, sharply-defined lamination 

 which is conspicuous in these, as in other, radiolarian cherts and im- 

 parts to them a striped appearance. The laminae differ considerably 

 one from the other : for instance, in the proportion and nature of 

 their detrital material. Further, many of the laminae in the Gower 

 cherts are strongly lenticular, that is, wedge-bedded. The sharply- 

 varying composition and the impersistence of the laminae point to 

 the constant play of gentle currents laden with various, but always 

 fine, sediments. These features, therefore, are to be expected in 

 deposits formed under the lagoon-conditions previously described, 

 and, as a fact, are found in many, including the finest, rock-types 

 of Modiola phases, but they do not accord with a deep-sea origin — 

 the only logical alternative to an origin in lagoon-areas in the case 

 of most radiolarian cherts. 



i Badiolaria are found at the present day at all depths, in greatest abundance 

 in the plankton. Though they swarm in open seas, the environment in some 

 lagoon-areas, also, would be favourable to their increase if the salinity were 

 not appreciably higher than that of average sea-water,— a condition which 

 would be realized in the proximity of a river (p. 525). Their accumulation 

 in lagoon-areas would be further promoted by their ease of transport from the 

 open sea outside. 



