242 
THE GEOLOGIST. 
they take their final north-east dip and disappear beneath the coal- 
measures. Bedded in the corresponding synclinal troughs to which this 
arrangement gives rise, are the nearly horizontal but much denuded 
strata of dolomitic conglomerate, which help to constitute a series of 
parallel basins, the deeper depressions in the magnesian rock being 
occupied by outhers of triassic marl, capped by lower lias lime- 
stone, while the more moderately denuded portions of the surface are 
covered up with beds of bouldered drift, which mostly exhibit imper- 
fect stratification, with false bedding, and are often of great thickness. 
South-east of the belt of limestone, between Llantrissant and Llau- 
harry, at Tregwylum, there is a protrusion of old red sandstone, 
showing the transition-beds with their yellow sands and the overlying 
lower carboniferous shales ; but the real " old red " base of the South 
Wales coal-field commences south of Pentyrch, and, with the exception 
of the interruption afforded by the drift and alluvium of Taflf Vale, 
extends thence continuously into Monmouthshire and Herefordshire. 
As a consequence of the geological structure, the surface of the country 
presents a number of narrow and shallow valleys, running mostly to 
the sea on the south-west ; and if we look north , we may see in the 
foreground, trending from east to west like low crested waves on 
a lazy ocean, the limestone-ridges and conglomerate -bottoms, while 
beyond, rising in a series of dwarfed bluff's one over the other, and 
only to be separately distinguished in the view by the alternations of 
lights and shadows caused by the hills and valleys, are the basset- 
edges of the lower and middle groups of coal-measures with the 
pennant-sandstone. Turn to the south, and the same undulating 
country lies stretched out before us, but the crests of the waves or 
antichnals now fall lower, while the swell of the strata spreads 
wider, until, over and beyond the last rising land, a dim misty streak 
terminates the scene and marks the line of the sea-shore. 
The accompanying diagram represents the rocks in ideal section near 
Llantrissant, and illustrates the prevailing geology of the Glamorgan- 
shire sea-board. It represents one of the many undulations of the 
limestone, and the unconformable filling-up of the troughs by the 
dolomitic conglomerate. It also shows, what is here characteristic, 
the comparatively rapid inclination of the northern dips, as compared 
to the more gradual curves of the strata where the limestone rises into 
