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 outliers 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 afibrded by the drift and alluvium of Taff 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 blufis 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 

 anticlinals 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 fiUing-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 



