S6G REroKT— 1898. 



beat and tlio talus, more or less water-worn, wliicli fell from them. The continued' 

 sinkinj? cf the land led not only to the Khtetic overspreadiDp: the Trias, and extendinfj^ 

 beyond it, but to the Lias eventuallj' overlapping; all earlier deposits. Each 

 formation, as it overlaps its predecessor and comes into contact with the PaLneozoic- 

 rocks, becomes cont^lomeratic, and it thus happens that a conglomeratic subdivision 

 though actually continuous is of Triassic, Kb,t;tic, and Liassic age in different 

 parts of its outcrop; a state of affairs which is not easily represented by the usuaF 

 methods of colouring a geological map. 



The boundary between the New Red Marl and the Rha3tic has hitherto been 

 taken at the base of some green marls which graduate downwards into the Ked 

 Marls, but during the revision it became evident that the only satisfactory base to 

 the RhiBtic occurred above the Green Marls and at the base of the black shales of 

 the Avicula contoria zone. At this horizoa there is generally a grit or small 

 quartz-conglomerate which taken with the incoming of the Rhictic fauna indi- 

 cates a somewhat sudden change of phj-sical 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 ia the tracing of 

 the various folds and faults through the Coal-field. The main anticlinal and" 

 synclinal axes are of course brought into prominence on the mapby the subdivisioa 

 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 anticlinal axis is itself brought into prominence by 

 the fact that it brings the Lower Coal Series up to the surface at intervals along it^ 

 course. Especially, also, attention may be directed to the contrast presented by 

 the long dip-slopes of the north crop of the Coal-field to the straight and narrow 

 strips along the highly inclined south crop. Of the numerous flexures which hav& 

 been traced in the Palajozic rocks outside the Coal-field 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 Rocks, which in fact pass horizontally across them. 

 These east and west flexures are consequently assumed to be pre-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 largo 

 thickness of Millstone Grit. It will be remembered that the Carboniferous Rocka 

 of Somerset were still more intensely plicated and overthrust 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 persist- 

 ency is a well-known feature of the Coal-field. Some of them can be traced out 

 into the Secondary area, and are there found to dislocate the Secondary Rocks 

 equally with the Carboniferous. While, therefore, they are obviously post-Liassic, 

 they 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 Coal-field, 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 by a blue 

 line. Thus a normal fault completely proved would be represented by four lines, 

 the order in which the lines occur indicating the direction, and their distance 

 apart the angle of the hade. A further difficulty remains, however ; for the plan- 

 position of a fault encountered in any vein in working up to it from the east would 

 not be the same as the plan-position of the same fault in the same vein if worked 

 up to from the west, owing to the hade. In a fault of 100 yards the discrepancy 

 would amount to 35 or 40 yards, and it becomes necessary to record also frotii 

 which side the fault was proved, which is not always easily ascertained in old 

 workings. The coloured lines referred to are used on the G-'inch maps only ; on 



