534 



ME. E. E. L. DIXON AND DR. A. VATJGHAN ON [Nov. I9II. 



most varied and most typical of the South-Western Province 

 generally. In the following table the members of each sequence 

 are placed in order of superposition, and lettered like the parallel 

 member of the other sequence : — 



(1) The sequence Z, Ci and the 

 base of C2 :— 



(2) The sequence C2 and S :— 



d. Modiola phase at the base of C 2 . 



d. Modiola phase at the top of S 2 . 



c. Caninia Oolite. 



c. S 2 oolites. 



b. Laminosa Dolomites (dolomitized 

 crinoidal limestones) ; in East- 

 ern & North-Western Grower 

 with similar dolomites of Z-age. 



b. Partly dolomitized gasteropod- 

 limestones, most conspicuous 

 inS x . 



a. Crinoidal limestones, some dolo- 

 mitized ; lower part or all of Z. 



a. Various limestones, some dolo- 

 mitized ; C 2 with part of S 1 . 



The chief differences between the two sequences are (1) dolo- 

 mitization has been much less complete in S than in the Laminosa 

 Dolomites ; (2) the S 2 oolites, on the contrary, are much thicker 

 and include dolomites more frequently than the Caninia Oolite ; 

 (3) the S 2 oolites pass gradually into the overlying Modiola phase, 

 whereas the Caninia Oolite is separated from the parallel phase by 

 a sharp line and, in places, contemporaneous erosion. (In fig. 8, 

 p. 533, the passage of the S 2 oolites into the Modiola phase is 

 indicated by the overlapping of unclosed brackets.) 



As regards interpretation, it is clear that in each sequence the 

 establishment of the Modiola phase (d), that is, of lagoon-conditions, 

 has been preceded by a considerable period during which shallowness 

 of the sea, though less extreme than during the phase itself, has had 

 a marked effect on the facies of sedimentation : this is shown by 

 the presence of thick dolomites (b) 1 and oolites (c) below each 

 phase. Put it is uncertain what cause has led to the replacement, 

 in each sequence, of dolomite- by oolite-formation, for the two 

 sequences are sufficiently parallel to suggest that the cause has been 

 similar in both. Though, in view of the frequency of contempo- 

 raneously-eroded fragments in the oolites (c), we may well suppose 

 that the waters in which these rocks have been formed have been 

 shallower than during the preceding dolomitic stages (6) — just as 

 the latter have been shallower than the preceding stages (a), — it 

 seems probable that oolite-formation has depended on some condition 

 additional to shallowness. For, in the Avon, the incoming of S 2 is 

 marked, as at many places in the South-Western Province, by the 



1 The view of Prof. E. W. Skeats, Q. J. a. S. vol. lxi (1905) pp. 133-38, 

 and others, that within certain limits shallowness favours dolomitization, is 

 adopted; it is supported by the relations observed (see below, p. 537) between 

 dolomitization and the probable relative depths of the Avonian sea. Although 

 S 1 thus bears evidence of deposition in shallower waters than those of C 2 in 

 Gower, in the Avon it includes some limestones of deeper-water origin than 



