Vol. 6 7. J THE CARBONIFEROUS SUCCESSION IN GOWER. 50' 



III. Note ox the Pseubobreccias of the D-Zone 

 [E. E. L. D.]. 



The following observations relate to the pseudobreceias of Gower, 

 where these rocks are peculiar to the Dibunophyllum Zone and 

 at that level are extensively developed. While to some extent 

 supplementing' Mr. Tiddeman's description, 1 this note deals more 

 particularly with the question of origin, which is of interest on 

 account of the resemblance of the rocks to true breccias, and also 

 because other nodular, foraminiferal limestones, such as some of the 

 bands in the Chalk, may owe their structure to the same cause. It 

 may be mentioned that the explanation of the structure of pseudo- 

 breccias, here advanced, has suggested itself to Mr. J. A. Howe 

 also, in the course of a comprehensive study that he is making of 

 these and other limestones. 



The structure in question, the outward appearance of which may 

 be gathered from PI. XXXVIII, fig. 2, appears at first sight to be 

 due to the incorporation of angular fragments of rather dark lime- 

 stone, of various sizes up to a diameter of several inches, in a 

 ground-mass which is limestone in some places, and dolomite in 

 others, but is always lighter in colour and more argillaceous than 

 the ' fragments,' 2 sometimes markedly so. The greatest contrast is 

 presented where the ' ground-mass ' is dolomite, as the latter forms 

 a rather coarsely-crystalline, pale-grey mosaic. 3 We may first, 

 however, consider the undolomitized examples. 



; Fragments ' and ' ground-mass ' vary in their relative proportions, 

 but as a rule are approximately equal, and the latter occurs 

 characteristically as meandrine tracts enclosing the former. In no 

 pseudobreccia has it been possible to distinguish bands differing 

 one from the other in the size or abundance of their included 

 ' fragments.' The same fauna, a standard marine assemblage, 

 is found in the ' fragments ' as in the 'ground-mass.' 



Microscopic examination shows that the matrix of the fossils in 

 both 'fragments' and 'ground-mass' is a consolidated 'mud,' con- 

 sisting essentially of finely-divided calcite and containing, as a 

 rule, a notable quantity of foramiuifera. 4 Furthermore, such 

 examination shows that the outlines of the ' fragments ' are not 

 sharp and well-defined -like those of true fragments, but that 

 their matrix shades off into the matrix of the 'ground-mass.' 

 On this account largely, and also in view of other considerations 

 which will be adduced, it is inferred that the rocks are not 



i Swansea Memoir, p. 10. 



- See analysis by Mr. E. Gr. Eadley, Swansea Memoir, p. 13. 



3 A doloinitizecl example is figured in the Swansea Memoir, pi. i, fig. 1. 



4 The following have been identified in microscope-sections by Dr. E. L. 

 Sherlock: — ' Callencroft Quarry, Mumbles [E 5000] : very rich in foramiuifera, 

 chiefly Endothyra?, such as Endothyra bowmani Phillips, Endothyra sp.. and 

 Textularia sp. (either gibbosa d'Orb. or eximia Eichw.). — Oxwich [E 5001] : 

 Endothyra sp. (small form).' 



Q.J.G.S. No. 268. 2n 



