536 MR. E. E. L. DIXON AND DR. A. VATJGHAN ON [NOV. IQII, 



of the poorly-exposed sequence above the radiolarian cherts ; the 

 P-shales, which form the lower part of the Millstone Grit Shales of 

 the Geological Survey, pass up insensibly through the higher part 

 into the Coal Measures — the latter being, in part at least, a 'conti- 

 nental' deposit. 



Conclusions. 



(1) The time represented by the Gower succession discussed 

 above may be divided into four periods, namely (i) Km-C 1 ; 

 (ii) C Q -S-oolites; (iii) top of S 2 -D 2 _ 3 ; and (iv) P. During these 

 periods the depth varied as follows : — 



(i) Km-C. The area at first was under lagoon-conditions (Km) ; 

 its depth increased sharply (K & Z), and later diminished, in all 

 probability progressively but intermittently (Z to Laminosa Dolo- 

 mites ; Laminosa Dolomites to Caninia Oolite), ultimately attaining 

 that of lagoon-conditions again by the time that the next period 

 was initiated. 1 



(ii) C 2 -S., oolites. The area, with the exception of the South- 

 western district, at first was under lagoon-conditions (Modiola 

 phase) ; its depth increased abruptly — to an amount during the 

 rest of C, greater, probably, than at any other Avonian horizon, 

 and later diminished, again, it would appear, progressively but 

 intermittently (S x ; S 2 ), to that of lagoon-conditions by the time 

 that the next period was initiated. The stage of oolite-formation 

 was greatly prolonged, and the shallowness which marked it cul- 

 minated in the lagoon-conditions that followed. 



(iii) Top of S -D 2 _ 3 . The area at first was under lagoon- 

 conditions (Modiola phase) ; its depth increased rapidly, though 

 probably to no great amount, and later was at times very slight 2 

 (coal and underclay in D x ). 



(iv) P. The area at first appears to have been under lagoon- 

 conditions (radiolarian phase), but its subsequent history during P 

 is uncertain. The ultimate replacement of marine waters by 

 continental ones (Coal Measures) was probably gradual. 



(2) The foregoing summary of the periods shows that during 

 each of the first three the area has undergone a bathymetrical cycle, 

 complete in the case of the first two and probably in that of the 

 third also ; in each of the three cases initial shallowness, that of 

 lagoon-conditions, has been followed by rapid deepening and, later, 

 by shallowing, which, unlike the deepening, has been prolonged. 

 The shallowing has, however, been more regular in the first two 

 than in the third period. 



Earth -Movements. 



(3) Both the deepening and the shallowing have probably resulted 



1 At some places in the South-Western Province, such as West Williamston, 

 the shallowing culminated in emersion, the lagoon-concJitions being established 

 as the land again sank at the commencement of the next period. 

 g , 2 At some places in the South-Western Province (see pp. 529-30) this 

 period probably ended in emersion. 



