Vol. 63.~] THE TOADSTONES OF DERBYSHIRE. 269 



glassy lapilli, often vesicular and seldom containing felspars or altered 

 olivine. The blocks are vesicular or amygdaloidal, with altered 

 olivine and felspar scattered often in a more or less isotropic base. 1 



The only bedded tuff that I have discovered near this vent is 

 found some little distance to the south, in the stream formed by 

 the union of the two northern streams. Two thin beds of tuff, 

 with a bed of limestone 1 foot thick between them, are seen in 

 the bank and brook-course. Two other exposures of agglomerate 

 in the Yoredale rocks are seen, the one in the banks of a stream 

 a short distance north-west of the larger vent described, the other 

 some distance to the north and to the south-west of Lea Farm. 

 They probably are volcanic vents and not bedded tuff, although 

 (especially in the latter case) their relations to the surrounding 

 rocks cannot be clearly made out. The limestones immediately 

 to the south of the former exposure are vertical, and strike at 

 the agglomerate. 



Wibben-Hill Yen t.— East and north-east of the village of 

 Tissington, a bed of tuff was seen during the construction of the 

 London & North- Western Eailway. The sections were described 

 by me in 1899. 2 A bed of tuff with included blocks was exposed 

 in the cutting immediately to the east of Tissington. Since the 

 paper just mentioned was published, I have been able to trace this 

 outcrop of tuff over a larger area. Wibben Hill is nearly in the 

 centre of that area, and has the contour of a vent. The deposit 

 of tuff surrounds it on all sides, and on the hill itself the ejected 

 blocks are bigger and more numerous than those in the railway- 

 cutting. Its relations to the surrounding rocks are not so clear 

 as in other cases, because the limestones and shales are contorted 

 and the ground in the neighbourhood is often covered with Drift. 



In two small quarries, respectively north and south of the hill, 

 the limestone and shale are seen below the tuff. In the southern 

 quarry the dip is 20° south-westward, showing that the agglomerate 

 cuts across the strike. The boundary between the tuff and the 

 agglomerate is uncertain ; I have therefore denoted it on the map 

 (PI. XXII) by a broken line. 



The contour of the hill, its relations to the bedded tuff, and 

 the numerous ejected blocks on its slope and summit, may be 

 adduced as an argument in favour of its being a vent or the lower 

 part of the volcanic cone directly connected with the thick deposit 

 of tuff. The only alternative supposition appears to be that 

 the hill consists of a small, sharp, and very symmetrical pericline 

 of limestone and shales covered by a layer of tuff. In that case 

 it is difficult to understand why the softer covering of tuff has 

 not been removed by denudation, leaving the beds underneath it 

 exposed. 3 



1 Quart. Journ. Geol. Soc. vol. 1 (1894) pp. 638-39. 



2 'Geology of the Ashbourne & Buxton Eailway' Quart. Journ. r G-eol. Soc. 

 vol. lv (1899) pp. 224-37 & pis. xvii-xviii. ; " 



3 ' Sketch of the Geology of the Lower Carboniferous Eocks of Derbyshire ' 

 Proe. Geol. Assoc, vol. xvi (1899-1900) p. 212. 



