90 SECTIONAL ADDRESSES. 



were first described, they are typically exposed in tlie Avon section, in 

 South Pembrokeshire and many other localities. The following is in 

 the main an abstract of Dixon's account. The rock consists of patches 

 (' fragments ') of dark limestone generally crowded with foraminifera, 

 surrounded by lighter and more argillaceous limestone (' ground mass or 

 matrix '), the two generally occurring in approximately equal proportions. 

 Both ■' fragments ' and ' matrix ' consist essentially of a calcareous mud, 

 and microscopic examination shows that the outline of the ' fragments ' is 

 not sharp and well defined, but that they shade off into the ' matrix.' 

 The material of both ' fragments ' and ' matrix ' is partly recrystallised, 

 but the change has affected the ' fragments ' more than the ' matrix,' 

 tending to slightly clarify them, and has been accompanied by concentra- 

 tion of the argillaceous and ferruginous material in the ' matrix.' This 

 recrystalhsation is believed to have taken place shortly after deposition. 

 The patchy distribution of pseudobreccia, its widespread occurrence on 

 the same level, and the fact that the ' matrix ' has often been dolomitised 

 while still under the influence of the Avonian sea, point to this conclusion. 

 Dolomitisation tends to increase the appearance of brecciation ; indeed, it 

 was the emphasising of their structure by contemporaneous dolomitisation 

 which first drew Tiddeman's attention to pseudobreccias. Pseudobreccia 

 often passes laterally into what has been termed in the Gower area ' clay 

 with rubble,' and in the Avon section ' rubbly limestone.' These bands, 

 which are very discontinuous, consist of rounded masses of limestone often 

 several inches in diameter, embedded in a red shaly matrix ; they probably 

 owe their character to a concretionary or recrystalhsation process whereby 

 the lime gathered in nodules, from which the iron and shaly material 

 became separated. The characters are emphasised by weathering. 



There are other noteworthy features shown by weathered pseudo- 

 breccia. The more coarsely crystalline character of the ' fragments ' 

 renders them relatively resistant to chemical change, so that they stand 

 out prominently on the weathered surface. This is well seen both in 

 Gower and the Avon section. A more remarkable feature is the deep 

 pitting of the bedding planes, best known from Gower, where it was 

 originally described by Tiddeman,''® but also very well seen at Mells in 

 Somerset. These pits are roughly circular, and as described from Gower 

 are 18-30 inches in diameter and a foot deep. The Mells examples are 

 rather wider and shallower. In each case they contain some clay, and 

 it is suggested that they, like the rubbly limestone, may be an expression 

 of a process of recrystalhsation leading to a local concentration of the 

 argillaceous material. Mr. Dixon, however, informs me that after seeing 

 the pot-holed limestones of the Whitehaven district he believes that the 

 Gower pitting is shallow pot-holing. 



Pseudobreccias, though mainly met with in foraminiferal limestones of 

 Di age, may also occur at other horizons and in other kinds of limestone. 

 High up in D.2 in the Avon section'^ is a band of sandy limestone showing 

 pseudobrecciation in which recrystalhsation has led to the concentration 

 of sandy and ferruginous material in the ' matrix,' while in the more 

 usual type of pseudobrecciation it is argillaceous material that is so 



'* Swansea Memoir (1907), p. 10. 



" Q.J.G.S., vol. Ixxvii. (1921), p. 235. 



