492 A. Sfrahan — Geological Survey of South Wales, etc. 



The boundaiy between the New Eed Marl and the Rhfetic has 

 hitherto been taken at the base of some green marls which graduate 

 downwards into the Eed Marls, but during the revision it became 

 evident that the only satisfactory base to the Rb^tic occurred above 

 the Green Marls and at the base of the black shales of the Avicula 

 contorta zone. At this horizon there is generally a grit or small 

 quartz-conglomerate which taken with the incoming of the Eheetic 

 fauna indicates a somewhat sudden change of physical conditions. 

 It marks, in fact, the first complete invasion of this area by the sea. 



One of the most important parts of the revision has consisted in 

 the tracing of the various folds and faults through the Coalfield. 

 The main anticlinal and synclinal axes ai'e of course brought into 

 prominence on the map by the subdivision of the measures before 

 referred to. Thus the difference of tint shows the positions of the 

 two deep synclines which introduce the Upper Coal Series at 

 Caerphilly and Llantwit on the south side, and at Blackwood and 

 Gelligaer on the north side of the main anticline ; while the anti- 

 clinal axis is itself brought into prominence by the fact that it brings 

 the Lower Coal Series up to the surface at intervals along its course. 

 Especially, also, attention may be directed to the contrast presented 

 by the long dip-slopes of the north crop of the Coalfield to the 

 straight and narrow strips along the highly inclined south crop. 

 Of the numerous flexures which have been traced in the Palseozoic 

 rocks outside the Coalfield it is sufficient to state that they run in 

 about the same direction as those mentioned above, and that they do 

 not affect the Secondary Eocks, which in fact pass horizontally 

 across them. These east and west flexures are consequently assumed 

 to be pi'e-Triassic. 



To this series also we believe the great Vale of Neath disturbance 

 and some other kindred folds to belong. This great faulted fold 

 seems to have attracted but little notice hitherto, though it displays 

 many remarkable features, among others a thrust by which 

 Carboniferous Limestone has been pushed over a large thickness 

 of Millstone Grit. It will be remembered that the Carboniferous 

 Eocks of Somerset were still more intensely plicated and overthi'ust 

 in pre-Triassic times, and that there also the disturbances run in 

 a general east-and-west direction. 



The set of faults which run about north-north-west with such 

 remarkable persistency is a well-known feature of the Coalfield. 

 Some of them can be traced out into the Secondary area, and are 

 there found to dislocate the Secondary Eocks equally with the 

 Carboniferous. While, therefore, they are obviously post-Liassic, 

 thej'- may be of very much later date. 



The exact representation of the faults, of whatever age, upon the 

 map is of the greatest importance in the Coalfield, and is managed 

 as follows : — The surface-position of the fault is indicated by a white 

 line, or by a broken white line where the exact position is uncertain. 

 If the fault has been proved in the workings of the Mynyddislwyn 

 Vein, its underground position in that vein is shown by a red line, 

 if in the Tillery Vein by a yellow line, and if in the Lower Coals 



