A. Strahan — Geological Survey/ of South Wales, etc. 491 



The Carboniferous Limestone also expands westwards and south- 

 wards, for, while only 100 feet thick at Abergavenny, it is 600 to 

 700 feet in northern Glamorganshire, 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 litho- 

 logical 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 iron- 

 stone, 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 oncomiug of the quartz- 

 conglomerates seems to have been accompanied by some erosion, for 

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

 sus^Dected 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 Anthracomya 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 ' Yore- 

 dale 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 Coalfields. 



The Secondary Rocks which fall within the revised area include 

 Trias (Keuper or New Red Marl), Rhastic, 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 distributed 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 beat and the talus, more or less water- 

 worn, which fell from them. The continued sinking of the land led 

 not only to the Rhsetic overspreading the Trias, and extending 

 beyond it, but to the Lias eventually overlapping all earlier deposits. 

 Each formation, as it overlaps its predecessor and comes into contact 

 with the Palgeozoic rocks, becomes conglomeratic, and it thus 

 happens that a conglomeratic subdivision though actually continuous 

 is of Triassic, Rhsetic, and Liassic age in different parts of its 

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

 usual methods of colouring a geological map. 



