510 



ME. E. E. L. DIXON AND DR. A. VATTGHAN ON [Nov. I9H, 



dolomitization by the waters of the Avordan sea. (Dolomitization 

 due to clearly-later percolation, it may be noted in passing, has 

 affected both materials equally.) In fact, the difference between 

 the ' ground-mass ' and the ' fragments ' in this respect is so great as 

 to suggest that at the period of dolomitization the former was still 

 unconsolidated and readily permeable, as well as more unstable. 

 In many pseudobreccias and for long distances in them it has 

 been completely altered, though, as a rule, the whole of the enclosed 



Fig. 5. — Diagram showing the relation of the dolomite, in a partly 

 dolomitized pseudobreccia, to unrecrystallized calcite (see p. 511). 

 Natural size. 



[a, a represent ' fragments,' consisting of recrystallized calcite ; b, b, tracts of 

 ' ground-mass,' consisting of unrecrystallized calcite ; and c, c, dolomite. 

 The limit of the dolomite is generally sharp against the ' fragments,' 

 but against the unrecrystallized calcite it is too indefinite, as a rule, to be 

 shown properly in a diagram.] 



< fragments' have been but little affected. It has thus, by deter- 

 mining the paths of dolomitization, led to a marked accentuation 

 of the pseudobrecciated structure. In many dolomitized pseudo- 

 breccias, as in the example figured in the Swansea Memoir, pi. i, 

 fig. 1, the ' ground-mass ' has been so completely replaced by 

 dolomite-mosaic in which its proper structures have been lost, that 

 the original nature of the material replaced by the dolomite would 

 be open to doubt, were it not that in other such pseudobreccias 

 dolomitization has been incomplete, and wholly absent from parts 



