528 ME. E. E. L. DIXON AND DE. A. VATJOHAN ON [NOV. I9II, 



on the supposition that beds below the plane of unconformity are 

 overstepped between the latter place and Pendine and that beds 

 above that plane are overlapped in the same direction (see fig. 7, 

 p. 527). Both of these phenomena are to be expected, though the 

 extent of the overlap above the unconformity is surprising. Further, 

 the presence of lagoon- deposits at the base of the overlapping series 

 at Pendine and some other places along the same outcrop, as well 

 as at West Willi amston, suggests that across the unexposed ground 

 between the latter place and Pendine the unconformity is succeeded, 

 as a rule, by lagoon-deposits. This is also indicated diagram- 

 matically in fig. 7. 



The deeper-water limit of all horizons of the phase is reached in 

 the south : for they are represented by ordinary limestones in the 

 Mendips, South-Western Gower, and Southern Pembrokeshire. 



The lagoon-phase at the top of S 2 has not been followed 

 out in detail northwards ; neither has its complete seaward limit 

 been reached, although in Gower and Pembrokeshire it is poorly 

 developed in the southernmost outcrops, its phasal deposits being 

 there largely replaced by ordinary limestones. 



(6) Earth-Movements accompanying Lagoon-Conditions. 



The deposition of a lagoon-phase involves the fulfilment of certain 

 conditions. Some of these are enumerated in paragraph 1 (below). 

 Again, certain conclusions may be drawn from the development 

 in the South-Western Province of the Modiola phases present in 

 Gower : as the latter are few, these conclusions are provisional ; 

 but as, at the same time, the evidence is consistent, it may be 

 useful to note its trend. It will also be seen that the deposition 

 of the Gower radiolarian phase appears to have followed an earth- 

 movement similar to one which preceded the deposition of the 

 Modiola phase at the base of C„. 



1. The accumulation of extremely shallow-water deposits to any 

 considerable thickness can have been conditioned only by a general 

 depression of the sea-bottom relative to sea-level — positive move- 

 ment — over the area of deposition. Further, the depression has 

 been isostatic, — the term 'isostatic' being used simply in a descrip- 

 tive sense, to denote the state of an area over which the rate of 

 sedimentation is equal to the rate of depression. Lagoon-phases 

 are, by definition, extremely shallow-water deposits; the accu- 

 mulation of each has, therefore, been accompanied by isostatic 

 depression, provided its thickness is greater than the range of depth 

 of the water in which it has been deposited. 



2. The internal constitution of the Gower Modiola phases (see 

 Table III, 4, p. 514) suggests (i) that in some cases (Km and the 

 top of S 2 ) isostasy has been interrupted ; but that in the other 

 (the base of Ca-f-SJ) it has been accurately maintained, practically 

 throughout the duration of the phase. 



3. A consideration of the nature of the beds below and above and 



