Vol. 67.] THE CARBONIFEROUS SUCCESSION IN GOWER. 485 



as gasteropods and orthocerates, but occurs at many Avonian 

 horizons throughout the South- Western Province. In view of 

 the instability of aragonite, this phenomenon is readily explic- 

 able, but implies that the organisms were still aragonitic when 

 dolomitized. Further, the wide occurrence of the phenomenon, 

 taken in conjunction with the short life of organic aragonite 

 under the conditions accompanying vein-dolomitization, sup- 

 ports the conclusion, based on other considerations, that the 

 dolomitization was ' contemporaneous.' Where there is no 

 independent evidence of 'contemporaneous' dolomitization, it 

 is found that organic aragonite has been replaced by coarse 

 calcite-mosaic. 



In places, the lowest bed contains fragments of the highest 

 of the deposits of the underlying Modiola phase. 



1. Modiola phase. A variable group of limestones, dolomites, 

 breccias, and more or less calcareous shales or clay-mudstones, 

 thinly bedded to finely laminated or papery ; grey, dull buff, 

 or greenish in colour (where free from Triassic reddening). 

 The limestones, some of the argillaceous varieties of which 

 are nodular and concretionary, 1 are either fine-grained to 

 compact, or very finely oolitic (these resembling parts of the 

 Caninia Oolite) or, occasionally, ' pisolitic' Crinoidal lime- 

 stones are rare, and are merely thin laminae. Many of the 

 fine or compact limestones and dolomites fracture conchoidally, 

 and resemble ' chinastone-limestones ' ; but some are finely- 

 laminated : they are calcite- or dolomite-mudstones (see p. 516)*. 

 Either kind, but, it appears, more frequently the dolomitic, 

 has recrystallized in places to calcite- or dolomite-mosaic 

 of fine to coarse grain. Some finely-crystalline dolomites, 

 however, may be 'contemporaneous' replacements of originally- 

 calcitic rocks. 



Ordinary detrital matter is frequent. The mudstones con- 

 tain fine clayey material, which may exceed the calcareous, 

 or, in some cases, fine quartz-sand. Breccias, of limestone 

 and clay-mudstone fragments, measuring up to several inches 

 in length, set in an argillaceous, calcareous, or fine-sandy 

 matrix, may occur at any horizon. The fragments are similar 

 in structure and fossils, either to some of the intercalated beds, 

 or, in the case of some in the basement-bed, to the Caninia 

 Oolite below, and evidently have been derived from both 

 sources by contemporaneous erosion. Undoubtedly far- 

 travelled material, other than clay and sand, appears to be 

 absent, and the fragments show little, if any, rounding. 



Some beds, however, have a brecciated structure, due in part 

 to the penetration of an alien matrix into calcite-mudstone, 

 apparently along irregular cracks. The origin of the structure 



1 The concretionary structure here mentioned is the type usual in argilla- 

 ceous formations, not the peculiar structure of the 'concretionary beds' of the 

 Avon. 



