452 Proceedings of Philosophical Societies. [June, 



exhibited in the basin of Somerset and South Gloucester, the 

 memoir proceeds to notice the similar basin occupying the forest 

 of Dean, interposing, however, some details relating to a district 

 lying in the interxnediate space between these basins, and 

 extending from the village of Tortworth on the north of that 

 which formed the subject of the foregoing observation towards 

 the Severn. 



This district presents, near Tortworth, two parallel bands of 

 transition limestone underlying the old red sandstone, and distin- 

 guished by the usual characteristic organic remains. 



These calcareous strata are intersected by two dykes consist- 

 ing of several varieties of trap, among which an amygdaloidal 

 species, containing grodes of brown spar, predominates ; the 

 lime is much altered at the point where it is cut by the dyke. 

 Thence the old red sandstone extends to the Severn at Pyrton 

 Passage, and, crossing the river, appears on either bank near 

 that point ; the transition limestone also makes its appearance 

 on the* southern shore. 



On cros'sing the Severn, we enter upon the exterior chains of 

 the forest of Dean basin. Th6 lowest rock exhibited in these 

 chains is a compact variety of quartzose greywacke, which forms 

 May Hill, a low mountain half-way between Gloucester and 

 Ross-; this is invested by mantle-shaped strata of the same 

 transition limestone which has been noticed before at Tortworth. 

 This commences on the south, near Hanley, in a line with the 

 same rock at Pyrton Passage. On the north, it extends on 

 many miles, and may be traced as far as Stoke Edith, in Here- 

 fordshire. The same strata which form the basis of the forest 

 of Dean basin, on the north-east border, again emerge towards 

 its opposite boundary in the centre of Monmouthshire, where a 

 considerable tract of transition limestone ranges round the town 

 of Usk. These two points ; viz. May Hill and Usk, correctly 

 mark the extent of this basin. The intermediate country consists 

 of alow mountain groupe varying from 800 to 1,300 feet above the 

 sea. This group is traversed by the abrupt and romantic detile- 

 of the Wye. The highest strata of the basin, including the coal 

 measures, are entirely confined to the portion of this . groupe 

 which is on the east of that river. The subjacent formations are : 

 1, the old red sandstone : this is here beautifully displayed, 

 and presents beds of red sandstone and slaty marl through- 

 out : towards its middle region occur beds abounding in 

 calcareous matter disposed in irregular concretions, and assum- 

 ing at first sight the appearance of a conglomerate : these 

 are often worked as limestone, and usually distinguished by 

 the name of cornstone. Near the top of the sai:idstone form- 

 ation beds of quartzose conglomerate predominate: the total 

 thickness of this formation is estimated at 1,460 yards. It 

 is succeeded by No. 2, the mountain limestone, which exhi- 

 bits its usual characters, and bears a thickness of 165 yards. It 



