SOME NEW POINTS IN THE GEOLOGY OF THE USK 

 DISTRICT. 



By J. W. SALTER, Esq., F.G.S., &c. 



Unlike the neighbouring Woolhope elevated valley, miscalled a raUey 

 of elevation, the Silurian district of Usk presents an ii-regular set of hills and 

 vales, forming it is true, on the whole an elevated district, but without any of 

 the picturesque symmetry of the "WooUiope amphitheatre. The cause of the- 

 difference Ues wholly in the different nature of the rocks, for the outburst is. 

 essentially of the same kind, and has many features in common. 



In the "Woolhope district, hard May HLU sandstone, capped by a thick 

 band of limestone, is followed by soft Wenlock Shale, Limestone, soft Ludlow 

 Shale, Limestone again, tough Upper Ludlow rock, surmounted by a band of 

 typical Downton Sandstone, and this again by the softer Old Eed. The effect 

 is to give successive bands of hard and soft rock, causing alternate ridges and 

 furrows in concentric order, and the elevation is on a North-west Ime from 

 Linton to Dormington, parallel in a rough way to the main coui'se of the Wye. 



The Usk Mils range in a north and south Une, parallel to the eastern 

 border of the coalfield of Pontypool, &c. The denudation has only exposed the 

 Wenlock Shale. A low valley opening to let the Usk river through, in the 

 northern part, but high ground toward Cilfigan Park and Prescoed, where the 

 Wenlock Shale changes to hard Sandstone, so like the Blay Hill rock as to have 

 originally deceiveil the Geological Survey, who called it Caradoc, It is also 

 so called in the Silmian System, but not in SUmia. 



Outside the ring of Wenlock Limestone, again, the Ludlow rock consists 

 in part of hard Sandstone, and there being no Aymestry to tlivide this from 

 the Upjier Ludlow the latter tough rock combines with the Wenlock to form a 

 ring of solid liills around the AVenlock shale. The concentric character, there- 

 fore, of the Woolhope elevation is lost here entirely, and models of the two 

 lUstricts would have hardly a feature in common. The difference is the more 

 remarkable, inasmucli as the chief faults of the district arc parallel and similar. 

 The course of the Usk through this district is marked by a line of fault in a 



