116 ME. F. DIXEY A:ND PEOF. T. F. SIBLT OjS THE [vol. Ixxiu^ 



Limestone, consisting of hard crystalline dolomites, and the upper 

 half, composed mainly of dolomite-mudstone, is clearly reflected in 

 the form of the gromid (PI. XII, fig. 3). 



The prevalent northerly to north-westerh^ dip of the strata is 

 seldom interrupted. At Machen, however, a sharp roll brings up 

 an inlier of Old Red Sandstone, and at Tongwynlais anticlinal and 

 synclinal rolling of the Lower Limestone Shales results from a 

 similar disturbance. North of Ystradowen, another anticline in- 

 volves the Lower Limestone Shales and the lower beds of the Main 

 Limestone : this is one of several minor folds developed in the 

 westward-pitching ' nose ' of the Cardift'-Cowbridge anticline. On 

 Mr. T. C. Cantrill's interpretation of the structure,^ the main axis 

 of the Cardift'-Cowbridge anticline coincides with the faulted anti- 

 cline of Penlline, and thence passes eastwards a little to the south 

 of Llansannor, thus traversing the south-western corner of the 

 area described in this paper. 



Faulting of the Carboniferous Limestone is practically confined 

 to dip-faults, which belong to the well-marked north-north-westerly 

 fault-system 2 of the coalfleld. These vary from small local frac- 

 tures to displacements of considerable magnitude, the largest being 

 the Taff^s-Well Fault, the Creigiau Fault, and the Mislan Fault. 

 Both the Taff's-Well Fault, which is regarded by Dr. Strahan as a 

 continuation of the Darenddu Fault of the coalfield,-^ and the 

 Creigiau Fault, which involves the Keuper, Khietic, and Li;is,^ are 

 attended by much shattering of the Carboniferous Limestone. 



In the area east of the Taff valley, no vestige of Mesozoic 

 deposits remains on the Carboniferous Limestone, and the outcrop 

 of the latter is almost wholl}' free from Glacial drift. West of 

 the Taff the case is A'ery different : a cover of Trias and Lias 

 conceals a large part of the Carboniferous Limestone Series imme- 

 diately west of the Creigiau Fault, and the Keuper oversteps the 

 northern limit of the Main Limestone almost continuously in the 

 ground west of Brofiscin. Grlacial drift is widespread : it conceals 

 the Lower Limestone Shales effectually over large tracts, and 

 covers a great part of the Main Limestone Avest of the Ely valley. 



III. Note ox the Dolomitizatiox. 



Dolomites bulk very largely in the Carboniferous Limestone of 

 this district, and in the area east of the Taff valley they com- 

 pose the Main Limestone almost to the exclusion of non-dolomitic 

 limestones. Both contemporaneous dolomites and vein-dolomites 

 (subsequent dolomites) are developed extensively, but the former 

 in much larger amount than the latter.-^ 



^ Bridgend Memoir, i^p. 7-8, with sketch-map (fig. 1, p. 8). 



- Newport Memoir. 2nd ed. p. 4. ■^ Ibid. pp. 25, 71. 



■* Cardiff Memoir. 2nd ed. pp. 47, 72. 



'" Methods of distinguishing contemporaneous and subsequent dolomitization 

 are discussed hj Mr. E. E. L. Dixon, in ' The Country around Swansea ' Mem. 

 Geol. Siirv. 1907,[pp. 13 20 & pis. i-ii. 



