of the Parallelism of Contemporaneous Lines of Elevation. 407 



two parallel undulations of carboniferous limestone and old 

 red sandstone. From Pembroke this line crosses Milford 

 Haven, still proceeding westwards. Between the haven and 

 the Irish Channel intrusive masses of trap rocks penetrate the 

 strata, which are here very remarkably contorted : these may, 

 perhaps, be considered as indicating the probable volcanic 

 origin of these disturbances. The whole of these phaenomena 

 are fully described in Mr. De la Beche's memoir in the Geo- 

 logical Transactions, and most accurately represented in the 

 accompanying plates. The age of all this line is clearly defined 

 by the contact of the disturbed coal and undisturbed dolomitic 

 conglomerate in Glamorganshire. To trace it more particu- 

 larly : its southern extremity forms a double line of elevation, 

 including the Nailsea coal-field; between them the southern 

 of these anticlinal lines ranges from the Island of Flatholm in 

 the Bristol Channel through Broadfield down to Leigh Down, 

 where it is continued by the great fault of the carboniferous 

 limestone crossing the Avon at St. Vincent's Rocks, nearly in 

 the same line with which we trace an anticlinal line crossing 

 the centre of the coal-field and throwing up its lowest grits 

 on the north of Kingswood. The range of this southern por- 

 tion of the double line is S.W.and N.E. The northern portion 

 at first seems to have its axis beneath the Severn, being con- 

 nected with a tremendous fault, which on the north of Cleve- 

 don has thrown down the coal-measures more than 1000 feet, 

 so as to bring them into contact with the old red sandstone 

 range at the foot of Leigh Down, causing the carboniferous 

 limestone and old red sandstone again to crop out in one place 

 at a superficial distance of 3 miles from this natural line. 

 Beyond this subsided tract the anticlinal line is continued 

 from Portishead Fort across the Avon, near Sneyd Park; 

 thence northerly to Thornbury and Berkeley, between which 

 places the transition limestone is elevated, apparently by the 

 intrusive trap dykes of Tortworth : here it crosses the Severn, 

 from hence forming the eastern boundary of the Forest of 

 Dean coal-basin, as it has before formed the western boundary 

 of that of Bristol. It ranges by Nuneham, throws up the 

 transition quartz rocks of May Hill, and the transition lime- 

 stone chain of hills which ranges between the river Wye. 



The interior of the coal-basin in Glamorganshire is affected 

 by another anticlinal line, nearly parallel to the former : this 

 commences near Bedwas in Monmouthshire, ranges by Cefn 

 Eglwysilon, crosses the Taafe near Newbridge, and thence 

 ranges west to the iron-works at Duffryn Llanry, and is cut 

 off' by the estuary of the Neath at Britton Ferry. 



The strata on the north edge of the South Welsh coal-basin 



