part 3] SUCCESSION of the ayonian at clieton. 287 



difficult to distinguish between true brecciation and pseudo- 

 breccia tion, where the isolation of the ' fragments ' is due to 

 recry stallization . 



(2) Pseudobrecciation, as described from Gower by Mr. 

 E. E. L. Dixon, is most characteristic of the foraminiferal lime- 

 stones of the D beds ; but near the top of D.^, as has been already 

 shown, are sandy and ferruginous bands which exhibit an analogous 

 change, while a similar band near the base of the Brj'-ozoa-Bed 

 (see p. 218) may also be grouped as a pseudobreccia. 



(3) Dolomitization proves to be considerably more wide- 

 spread in the Avon rocks than had previously been ascertained. 

 There is a band of dolomite-mudstone in Horizon a (Bryozoa-Bed) 

 at the base of K^. There has been a little dolomitization at various 

 levels throughout the K beds, but it is not a characteristic feature 

 of this horizon. The matrix of the petit gran it of Z^, Z,^, and y 

 is almost everywhere dolomitized. The fact that the corals in the 

 Zaphrentis Beds resist dolomitization more than the matrix seems 

 to imply that the aragonite was replaced by calcite prior to the 

 dolomitization. 



The ahnost complete dolomitization of Lower C^ {laminosa- 

 dolomite) and of C^ {Caninia Dolomite) has long been familiar. 

 Some of the bands are dolomite-mudstone. There has been 

 considerable dolomitization in the calcite-mudstones of lower S, 

 and at various levels in S,,, and in particular the matrix in the 

 Seminula-Vi?>o\iie bands is much dolomitized. The same is the 

 case with the relatively massive Litlwstrotion Limestones of 

 Upper Sj and at various levels in S^. Oolites tend to resist the 

 change ; but there is some dolomitization of the matrix between 

 the grains at various levels in the C^ and S beds. There is very 

 little dolomitization in the D beds. 



The whole question of the dolomitization of the Carboniferous 

 Limestone has been fully discussed by Mr. E. E. L. Dixon, J- and 

 the conclusions which he reached regarding the Gower rocks seem 

 to be completely applicable to those of the Avon section. Every 

 argument goes to show that the main masses of dolomite are 

 contemporaneous, or resulting from the alteration of the original 

 limestone shortly after its deposition. 



IV. MoDiOLA Phases of the Avon Section. 



The three Modiola phases — that is, * calcareous lagoon -phases ' 

 of Mr. Dixon (Km, C^-S^, top of SJ represented in Gower and 

 tabulated by that author (Gower paper, p. 514) — occur also in 

 the Avon Section, and the only feature of general interest 

 in relation to these phases which the present paper emphasizes is 



1 ' Geology of the South Wales Coalfield : pt. viii — The Country around 

 Swansea ' Mem. Geol. Surv. 1907, pp. 13-20. — Dolomitization, particularly 

 as affecting the Carboniferous Limestone of Leicestershire, has been discussed 

 by Mr. L. M. Parsons (Geol. Mag-. 1918, p. 246). 



