Vol. 67.] THE CAEBONIFEROCS SUCCESSIOX IN GO WEE. 483 



of its oolitic structure l — other material, such as crinoid-ossicles 

 aud smaller organisms, being largely confined to subordinate 

 bands, while larger fossils amount to little more than a few 

 beds of brachiopods or Belhrophon and an occasional Syringo- 

 pora. Some bands, generally current-bedded, show evidence 

 of contemporaneous erosion in included, sometimes rounded, 

 fragments of oolite. Minute to microscopic ooliths are fre- 

 quent, in places to the exclusion of others, especially towards 

 the top. There, also, more often than elsewhere, the rock 

 is dark in colour. Chert is unknown, unless represented by 

 big, light-grey, concentrically-zoned siliceous nodules (Three- 

 cliff Bay) ; these, however, have not been critically compared 

 with the siliceous aggregates found later in Xorth- Western 

 Gower. The rock passes up from the dolomite below, through 

 intermediate phases in which the ooliths are less dolomitized 

 than the matrix ; and in places dolomite recurs in bands in the 

 lower part. 



Thickness.— 160 feet (Threecliff Bay); 125 and 70 feet 

 (Caswell Bay anticline) ; 150 feet (Longland Bay). 



1. L aminos a Dolomites. Dark -grey or black, finely-crys- 

 talline dolomites, 2 chiefly representing fairly thin-bedded, 

 crinoidal limestones, with here and there a few such beds 

 still unaltered ; towards the top, where the dolomite is rather 

 paler and more thickly-bedded than below, it may be seen in 

 places that it represents oolite. As a rule, dolomitization has 

 obscured or obliterated most of the original constituents, 

 including, among the fossils, some of the crinoid-ossicles, 

 highly-resistant though they be. A feature of the crinoidal 

 dolomites and others of the same type (as, for instance, those 

 in Z) is the sporadic occurrence, throughout their mass, of 

 small nests, up to several inches in diameter, of a coarsely- 

 crystalline mosaic of glassy calcite and milky-white dolomite ; 

 on wave-worn surfaces these nests contrast strongly with the 

 surrounding black rock. Many of them appear, from traces 

 of organic structure still preserved, to have originated, partly 

 through the alteration and recrystallization of calcareous mud, 

 inside fossils with internal cavities, such as gasteropods and 

 certain corals. 



Thickness indeterminable (see Z ). 



Pa una. — Little known (p. 546), the Laminosa Dolomites having 

 lost most of their fossils, and the Caninia Oolite never having 

 had many. 



1 Under the microscope the grains, like those of most of the oolites of the 

 Gower Avonian, show a radial-fibrous as well as a concentric structure, and 

 differ thus from the S 2 -pisolites. They are approximately spherical, with a 

 diameter which in many is about 0*4 mm. and is seldom greater than 06 mm., 

 but in the smallest measures 0*13 mm. or less. 



2 The completeness of the dolomitization may be gauged from the fact that 

 an average sample, analysed for commercial purposes, was found to contain 

 2035 per cent. MgO, the percentage in CaC0 3 . MgC0 3 being 21 -9. 



