508 ME. E. E. L. DIXON AND DE. A. VATTGHAN ON [Nov. I9I I, 



really fragmental. Ultimately, doubtless, they owe their structure 

 in part to their composition, 1 but the proximate cause of it alone 

 concerns us here. 



Before passing to this subject, however, it should be remarked 

 that Dr. Hume connects the nodular structure of parts of the Chalk 

 with current- action. 2 A complete comparison of nodular chalks 

 with pseudobreccias has not yet been made, but it is known that 

 some of the former closely resemble the latter. It is possible that 

 in the clay with limestone-rubble, into which occasional pseudo- 

 breccias pass laterally (p. 490), the rubble, which resembles the 

 4 fragments ' of pseudobreccias proper, but is sharply separable from 

 its clay-groundmass, has been more or less rearranged by currents, 

 after originating in a patchy recrystallization (and segregation) of 

 calcareous mud in the manner presently to be described. There is 

 no reason, however, to believe that the ' fragments ' in the pseudo- 

 breccias proper have also been washed up and redeposited ; and it 

 remains to be seen whether all nodular chalks show evidence of 

 current- action, or whether some are not pseudobreccias. 



As contradicting the view that the ' fragments ' in the Gower 

 pseudobreccias are derived, and have been incorporated in their 

 present condition, may be adduced the fact that occasionally 

 some are continuous one with the other and partly enclose ' ground- 

 mass/ thus losing their resemblance to fragments. It may be 

 suggested, however, that the ' fragments ' in these rocks are true 

 fragments of contemporaneous sediments, which, unrecrystallized 

 at the time of their incorporation, have lost all trace of their 

 limiting surfaces through later recrystallization of themselves and 

 some of the surrounding ground-mass. Apart from the absence 

 from the pseudobreccias of any sign of current- action in either a 

 sorting or a banding of their constituents, no evidence of a truly 

 fragmental origin has been detected in any of the numerous 'frag- 

 ments' that have been examined, and the suggestion takes no 

 account of the fact that, so far as known, the pseudobrecciated 

 structure is confined to foraminiferal limestones. 



To turn to the proximate cause of the pseudobrecciated structure, 

 the features whereby the matrix in the * fragments ' differs from 

 that in the ' ground-mass ' are, in addition to its less argillaceous 

 character, mentioned above : (1) that it is invariably rather more 

 crystalline, though still fine-grained and obviously a slightly re- 

 crystallized mud ; and (2) that it is often, possibly always, more 

 foraminiferal. Both features are illustrated by PL XXXIX, in 

 which, indeed, hardly any foraminifera are seen in the dense 

 1 ground-mass.' (The ' fragments ' are the light areas with irregular 

 boundaries along the top, bottom and left-hand margins of the field.) 



1 Rocks similar in structure to the pseudobreccias of Gower are known in 

 other regions, in the same and in other formations, but all appear to be 

 foraminiferal. Many foraminiferal limestones, however, are not pseudo- 

 breccias, whence it is obvious that other conditions, besides a suitable com- 

 position, have been necessary for the production of the distinctive structure. 



2 Proc. Geol. Assoc, vol. xiii (1894-95) p. 230. 



