South-western Coal District of England. 217 



souths the sandstone may be traced accompanying the limestone escarpment 

 through the valley of Llandaffand the river Ely (though here partially con- 

 cealed by the horizontal deposits) to the neighbourhood of Cowbridge ; but 

 being covered over by an investment of mountain limestone at Penline Castle, 

 near Cowbridge, it appears no more on the east side of the bay of Swansea. 

 From Penline Castle a collateral branch of the calcareous chain, dipping west- 

 south-west, is sent off towards the east-south-east, which skirts the south side 

 of the valley of the Ely from Cowbridge through St. Lythian's Down to Wen- 

 voe and Dinas-Powis, 3 miles west of Cardiff*. Thus the vale of the Ely is 

 flanked on both sides by an escarpment of bands of limestone, dipping in con- 

 trary directions : on the north, by the exterior chain of the coal-basin, dipping 

 towards that basin, and extending from Penline Castle through the New Forest 

 to Pentirch, the lowest part of the chain being in the vale of the Ely, 3 miles 

 to the south of Llantrissant : on the south, by the collateral branch of St. Ly- 

 thian's Down, dipping towards the Bristol Channel. The old red sandstone 

 forms an arch on which the two calcareous chains repose ; the keystone of the 

 arch being in the middle of the vale of the Ely, along which the anticlinal line 

 extends. 



On the Mode of Distribution and local Extent of the Rock Formations of the 



Second Series. 



In that part of the great escarpment of the horizontal strata which lies 

 wholly to the east of the coal-district, the overlying series of rocks, to reckon 

 from the newer red sandstone upwards, is usually found entire, the lowest 

 members of the series being seldom or never wanting. It might therefore 

 perhaps be expected that the same law should prevail throughout the whole 

 coal-district, and wherever within its area the overlying formations occur, 

 that there should be found invariably the lowest member of those formations 

 incumbent on the older rocks. Such, however, is not invariably the case ; 

 but a rock, which is usually intermediate in the system, will frequently be 

 found to be the lowest. The newer red sandstone, or its subordinate conglo- 

 merate, is not always present as the lowest of the horizontal strata ; nor, the 

 sandstone or its conglomerate being wanting, is the lias then always the 

 lowest ; but the oolite is sometimes found reposing immediately on the in- 

 clined beds ; and it will appear from examples in the sequel, that either red 

 marl, lias, or oolite may be placed immediately in contact with the older rocks. 

 The whole structure of this district, it is to be remembered, presents the in- 



* We beg to refer to Mr, Greenough's Map of England for a clear geological representation 

 of this part of the basin. See also the Section at PI. XXXIV. fig. 2 and 3 of this Memoir. 



2f2 



