518 ME. E. E. L. DIXON AND DE. A. VAUGHAN ON [NOV. ^H, 



condition a steep slope of the sea-bottom down from the shallow 

 area would be favourable — , and that the shallowness of the sea 

 was extreme. 



In other words, each phase was deposited in a shallow area of 

 sufficient extent to constitute, under conditions of minimal tides and 

 excessive shallowness, a great series of lagoons as denned in the 

 footnote on p. 511. In such a lagoon-area the pools would catch 

 the finest material, both that of local origin and the part that was 

 brought into the area from various sources outside, 1 and there 

 wafted about by gentle currents. Slightly stronger currents would 

 lead to the washing-up and incorporation of fragments of con- 

 temporaneous sediments ; and the area would be liable to invasion 

 at times by waters depositing standard rock-types. Such invasions 

 might result either from some deepening of the whole lagoon-area, 

 or from the temporary demolition of a bar — whether subaerial or 

 merely submarine. At other times, local emersion would probably 

 occur and be followed by cracking of the mud. In this way the 

 brecciation in situ may have arisen. 



Further, as some such conditions of deposition as are here outlined 

 alone explain satisfactorily the whole of those lithological features 

 of the Grower Modiola phases that are discussed above, we con- 

 clude that the strata of these phases have been deposited under such 

 conditions and are, accordingly, representative of 'lagoon-phases.' 



Landscape-marbles and 'pisolites.' — Some remarkable 

 rock-types, however, which appear to be confined to Modiola phases, 

 do not receive complete explanation, although their features are 

 not inconsistent with genesis in a lagoon-area ; consequently 

 their discussion is deferred. Of these rock-types the chief may be 

 called ' landscape-marbles/ as they reproduce the essential features 

 of the Rhaetic bed of that name, itself part of a Modiola phase. 

 Landscape-marble is conspicuous in the top of S 2 , the so-called 

 ' concretionary beds,' in the neighbourhood of Bristol and elsewhere ; 

 and one of its features is developed on a minute scale at the base 

 of C 2 in North-Western Gower. The so-called ' pisolites ' of C 2 and 

 S must have also required for their formation special conditions 

 as well as shallowness. 



Faunal characteristics of the Modiola phases. — The 

 lagoon-area concept bears the same relation to the faunal peculiari- 

 ties of the phases as it does to the lithological features; it explains 

 most of them, and accords with the rest. The mudstones, which must 

 be regarded as deposited under conditions of greatest isolation, are 

 also most restricted in their faunal contents, being in many eases 

 unfossiliferous, and in most of the others yielding, whatever be their 

 horizon, only a special, phasal fauna. Of this fauna the members 

 of most general occurrence are certain lamellibranchs, especially 

 Modiola and Sanguinolites, .JSpirorbis-like annelids, as also ostracods 



1 The colouring-material of the purple and green shales and of the lime- 

 stones of a type almost certainly came directly from the land, but much other 

 raud, including part of the calcite, may have entered from the seaward margin. 



