Vol. 67.] THE CARBONIFEROUS SUCCESSION IN GOWKR. 513 



(2) The Modiola Phases — Calcareous Lagoon-Phases — 

 of Gower. 



Introduction.— The Carboniferous succession in Gower in- 

 cludes four lagoon-phases, namely, Km at the base of K, the base 

 of (0., + SJ, 1 the top of S 2 , and the radiolarian cherts at the 

 base of P. The chief features of each of the first three, which are 

 calcareous in facies, are set out in Tables III & V (pp. 514 & 526), 

 the deposits characteristic of shallow waters being distinguished 

 from intercalated ordinary sediments where, as is generally the 

 case, the latter exist. The fourth phase, which differs from the 

 others in being eherty, will be discussed separately, as an origin 

 at a totally different depth has been ascribed to rocks elsewhere 

 possessing closely similar characters. All four phases have been 

 recognized at other places in the South -Western Province and 

 some of them elsewhere also ; the information in the tables con- 

 cerning the Bristol area has been extracted from Dr. Yaughan's 

 accounts, 2 and that about Pembrokeshire has been obtained during 

 the progress of the Geological Survey. 3 No attempt, however, 

 will be made to give full particulars of the geographical extent 

 of any phase. 



Distinctive features of the Modiola phases. — AVe may 

 now learn from Table III in what way Modiola phases differ from 

 ordinary marine formations ; from the interpretation of their 

 distinctive features, which will follow, it will appear why they 

 may be styled ' lagoon-phases.' 



On the one hand, all the Gower Modiola phases exhibit, at one 

 place or another, but not necessarily everywhere, unmistakable 

 shallow-water characters which are shown to some extent, also, 

 by ordinary shallow-water deposits (Table III, 1 & 2). In the 

 first and third phases there are, likewise, many intercalations of 

 ordinary rock-types (Table III, 4), due to the interruptions of 

 lagoon-conditions by more normal ones mentioned on p. 518. 



On the other hand, they all include, also, one or more rock-types 

 (Table III, 3) which, in a fairly shallow-water marine series, such 

 as the Avonian, are confined either to Modiola phases or to merely 

 occasional beds in the shallow-water parts of the sequence. Some 

 of these distinctive rock-types (namely, limestones of a type and 

 purple and green shales) appear to point directly to deposition in 

 shallow, isolated, coastal areas, and, therefore, will be considered 

 first ; the other group to be discussed here — mudstones and shales 

 characterized by an exceedingly fine grain, — which is far more 

 important than the first group on account of its more general 

 occurrence, resembles, rather, deep-sea deposits. 



1 By tbis expression is meant the group consisting of subzones C 2 and S,. 



2 Q. J. G. S. vol. lxi (1905) p. 181 (cited thus: 'Bristol paper,' p. ... ); 

 and. in the case of the Avon section, Proc. Bristol Nat. Soc. eer. 4, vol. i (1906) 

 pp. 74-168 (cited thus : ' Avon,' p. ... ). 



3 ' The Country around Haverfordwest ' and ' The Country around Pembroke ' 

 (Mems. Geol. Surv.) ; in preparation. 



