154 ME. F. DIXEY AXD PROF. T. F. SIBLY OX THE [vol. Ixxill, 



On the dip-slope of Cefn-On, about 200 yards west of the line of the 

 Caerphilly Tunnel, the soixthern quarry in some old workings (Glam. 37 NW 

 A 1) affords a good exposure of the uppermost beds. These are dolomite- 

 mudstones, with some dolomitic shale in the lower part and a regular inter- 

 calation of finely- crystalline dolomites in the upper part. Some bands of 

 nodular dolomite-raudstone in the lower beds contain calcitic patches. The 

 overlying crystalline dolomite comes on in the northern part of these workings 

 (see below). 



In the Cefn-On Farm traverse (horizontal section, fig. 4, p. 120). the lower 

 beds of the group are exposed in a cutting on the cart-road, 120 yards north- 

 north-west of the farm. These are dolomite -mudstones with a little purple- 

 and-green shale and a few bands of crystalline dolomite. Another cutting, 

 70 to 80 yards farther north on the cart-road, exposes dark-grey calcite- 

 mudstones, with platy bedding and shale-partings, which lie about the middle 

 of the group. 



Many small exposures occur in the neighbourhood of Maenllwyd, east of 

 Rudry. A well-developed dry valley marks the outcrop of the group north 

 and north-east of the Maenllwyd Inn. An old quarry on the northern side of 

 this valley, situated 130 yards north-north-west of Tir-Sion-Philip-Morgan, 

 exposes beds near the top of the group, compact limestones and fine-grained 

 dolomites with some shale and marl. 



The band of cr^^stalliiie dolomite and dolomitic 

 limestone (C.-SJ, \Yhich can be traced from Cefn-carnan-fawr 

 to Pen -how, gives rise to a well-marked feature along much of this 

 distance. Its thickness amounts to 120 feet or more at Thornhill, 

 and to about 100 feet on Cefn-On. It includes rocks which vsny 

 much, as regards both size of grain and degree of dolomitization . 

 Fairly-pure dolomites predominate, but a considerable amount of 

 partl^^-dolomitized limestone is also found. Dolomitized oolite 

 occurs in small amount on Cefn-On. Fossils, other than crinoid- 

 ossicles, have been found at one locality only, on Cefn-On. 



On Cefn-On. in the old workings (Glam. 37 NW A 1) which lie 200 yards 

 west of the line of the Caerphilly Tunnel, the northern quarry, much over- 

 grown, exposes the lower beds of this group. The rocks are grey saccharoidal 

 dolomites, coarse-grained for the most part, but including some fine-grained 

 bands. On the cart-track which skirts this quarry, some bands of fiiie- 

 grained dolomite have yielded Bellerophon, Scliellicienella cf. crenistria, 

 tit^minnla sp., and crinoid-ossicles. all preserved as casts or moulds. 



The upper Modiola phase (S^) is a very variable group 

 averaging roughly 50 feet in thickness. The rocks are mainly 

 calcite-mudstones and dolomite - mudstones, or closely - similar 

 compact limestones and dolomites i; but they also comprise some 

 medium-grained oolites and some persistent beds of crj^stalline 

 dolomite. In all these, ver}^ fine quai-tz-sand, in small or moderate 

 amount, is ubiquitous. Thin shale-bands are frequent. 



Unmistakable vein-dolomitization has everywhere affected the 

 limestones to some extent, and in places it has produced an 

 extensive conversion of limestone, amorphous-looking in its original 

 state, into coarsely-crystalline dolomite. This is well seen in 

 stream- sections on both sides of the Khymney valley : on the 



' Types covered by Mr. E. E. L. Dixon's description of ' Calcite-mudstones 

 and similar rock-types,' Gower paper, pp. 516-17. 



