TRANSACTIONS OF SECTIOX C. 8G.J 



with rugged sides. At its base occurs the seam variously known as the Red Ash, 

 Tillery, Brithdir, or No. 2 Rhondda. 



3. The Lower Coal Series (Steam Coal Series of Glamorganshire), which con- 

 sists principally of shales and thin beds of quartz-grit. This series contains the 

 thickest seams of coal and the bands or nodules of clay-ironstone which were 

 formerly worked in South Wales. It crops out all along the margin of the Coal- 

 field, but is exposed only in the deepest valleys or along the crests of the anticlines 

 in the more central parts. 



.Through the eastern end of the field these three subdivisions are readily 

 distinguished, but they expand rapidly westwards, and at the same time sandstones 

 not to be distinguished from Pennant appear in the upper part of the lower series, 

 Avhile measures of the supra-Pennant type replace the upper grits of the Pennant 

 group. They continue, however, to form the most suitable broad divisions that 

 fould have been selected, though a further subdivision may become necessary in 

 view of their increasing thickness. 



The other rock-groups have been treated on similar principles. The Old Red 

 Sandstone of Monmouthshire at once lends itself to division into an upper series of 

 grits and quartz-conglomerates, a thick mass of red sandstones, and a great under- 

 lying deposit of red marls with thin limestones. Special attention has been paid to 

 the relations of these subdivisions to one another in view of the possibility of an 

 unconformity having remained undetected in the middle of the red strata ; but 

 though the grits and quartz-conglomerates disappear in Brecknock, no break of 

 any significance in the sequence has yet been discovered. The conformity of the 

 Old Red Sandstone to the Upper Silurian rocks of Usk, however, may prove to be 

 more apparent than real, and must remain an open question for the present. 



The Carboniferous Limestone also expands westwards and southwards, for, while 

 only 100 feet thick at Abergavenny, it is 500 to 700 feet in northern Glamoraranshire, 

 and attains still greater dimensions in the southern part of that county. The lower 

 portion consists of shales with a more or less persistent limestone below, which 

 constitute the Lower Limestone Shales. In the main mass no subdivision has 

 been made, except that certain light-coloured oolitic bands have been picked out. 



The mapping of the Millstone Grit is founded on purely lithological distinctions. 

 Over a large part of the north-eastern crop it consists of a grit (the Farewell Rock 

 of old miners) in the upper part ; shales, and sandstones, occasionally with some 

 coal and ironstone, in the middle ; and a massive grit, usually crammed with quartz- 

 pebbles, in the lower part. This order, however, does not hold good everywhere, 

 and shales and sandstones are traced as far as practicable, and merely coloured on 

 the map as such. Though perfectly conformable to the limestone, the oncoming of 

 the quartz-conglomerates seems to have been accompanied by some ero.sion, for 

 they fill small hollows in the topmost limestone, and are even suspected of cutting 

 across some of the beds, so as to simulate an unconformity. Matters are further 

 complicated by the fact that the upper surface of the limestone has undergone 

 extensive dissolution during later ages. 



Some fossils which occur in calcareous shales and thin impure limestones in 

 the lower and middle parts of the Millstone Grit are all marine, but in the upper 

 part Aiithracomya becomes the abundant shell, and indicates an approach to Coal 

 Measure conditions. Marine forms, however, recur at intervals high up in the Lower 

 Coal Series. It will be noticed that there is nothing corresponding to the ' Yoredale 

 Rocks,' or upper part of the Carboniferous Limestone Series of the North of England, 

 nor to the alternating series of sandstones and limestones which border the Flint 

 and Denbigh Coal-fields. 



The Secondary Rocks whicli fall within the revised area include Trias (Keuper 

 or New Red Marl), Rhjetic and Lower Lias. These strata were deposited along a 

 land which was undergoing gradual submergence after prolonged exposure to 

 subaerial denudation. The New Red Marl, consequently, was irregularly distri- 

 buted in what must have been bays diversified by numberless islands, and the old 

 shore-lines, though subsequently buried, have "been revealed by denudation, so 

 that it is often possible to examine the cliffs against which the Triassic waves 



1898. 3 K 



