724 



DESCRIPTION OF THE. SECTIONS. 



sion of trap. An elevated basin of Old Red Sandstone is seen to be covered by Carboniferous 

 Limestone with broken and unconformable patches of lias and bands of Dolomitic Conglome- 

 rate. The Silurian Rocks are partially seen only in the eastern line of elevation, but are clearly 

 exhibited at Whitfields on the west. 



The Old Red Sandstone is overlaid by Dolomitic Conglomerate near Thornbury, and between 

 that place and the Severn by Carboniferous Limestone, New Red Sandstone, and Lias. The 

 " bone beds" or base of the lias cap the low cliffs of Aust, which are composed of red and green 

 gypseous marls, &c. See pp. 447 et seq. and map. 



Fig. 19. Descending order of the strata exposed by the road descending from Tortworth Green 

 to Falfield. A thin zone of Upper Silurian Rocks dips under the Old Red Sandstone, and 

 small arches of Lower Silurian Rocks, rising in the vale, are flanked on the west by Old Red 

 Sandstone. See pp. 447, 455. 



Fig. 20. Traverse from Thornbury to Wickwar across the two axes exhibited in f. 18., but in a more 

 southern parallel. The eastern line of elevation is marked by the highly inclined position of 

 the Old Red Sandstone, flanked on the east by unconformable strata of New Red Sandstone, 

 which are surmounted by lias and inferior oolite. The western elevation is seen in Milbury 

 Heath, where the Old Red Sandstone is thrown up " en dome." The Carboniferous Lime- 

 stone and coal of Cromhall occupy a trough between these ridges of Old Red Sandstone. The 

 Dolomitic Conglomerate forms an irregular fringe, adherent to the edges of the inferior strata, 

 though at Thornbury it is very slightly unconformable to the underlying Old Red Sandstone, 

 the conglomerate of the one appearing almost to graduate into that of the other. See pp. 447 

 et seq. 



Fig. 21. Section of certain alternations of grit and impure limestone, which occupy the intermediate 

 space between the chief mass of Carboniferous Limestone and the productive coal-field of 

 Cromhall (Lower Limestone Shale). See p. 452. 



Fig. 22. From Oldbury and Kington on the North to Old Down on the South, showing the fault on 

 the south-eastern face of the western branch of the Tortworth anticlinal, near its termination. 

 The Old Red Sandstone is thrown up on one side against the Carboniferous Limestone, and 

 dips under it on the other. See p. 461. 



Fig. 23. Silurian Group of Usk. The transverse section across this tract exhibits Silurian Rocks on 

 both banks of the river Usk. In the centre is a dome of Caradoc Sandstone overlaid by Wen- 

 lock Limestone. The Ludlow formation is pretty fully developed, and the whole is surrounded 

 by Old Red Sandstone, which in this figure is represented as dipping under the Carboniferous 

 Limestone and Millstone Grit of the South Welsh coal basin, near Pontypool, &c. Numerous 

 large boulders encumber the surface on the exterior of this Silurian elevation, but none are 

 found within its inner area. 



The Castle of Usk stands on a point of Upper Ludlow Rock, at its junction with the Old Red 

 Sandstone. See pp. 438 to 441. 



Fig. 24. Section in descending order from Chepstow to Usk, exposing in succession the Carboni- 

 ferous Limestone more fully expanded than in any other part of this region, with its subordinate 

 divisions of Upper and Lower Limestone Shale. The latter is seen to graduate into the Old 

 Red Sandstone, which is also fully developed } the Upper Sandstone and Conglomerate being 

 underlaid by cornstones, and the latter by marls, flagstones, and tiles, which graduate, in de- 

 scending order, into the Ludlow Rock. A small patch of Dolomitic Conglomerate overlies the 

 Carboniferous Limestone. See pp. 159, 438 to 441, and 453. 



