Vol. 63.] THE TOADSTONES OP DERBYSHIRE. 265 



and that it was near the top of the Carboniferous Limestone Series 

 or abutted on the Yoredale Series, and he threw some doubts on 

 any necessity for the faults as mapped by the Geological Survey. 1 

 Mr. Pocock considers that the thickness of the beds about Wirksworth 

 has been exaggerated. 2 In the Geological-Survey map two faults are 

 introduced, to separate the agglomerate from the Yoredale rocks to the 

 south of it. But a careful examination of the ground shows that the 

 agglomerate is found south of the supposed faults, also that it cuts 

 across the strike of the limestone-beds, and pierces not only the lime- 

 stones but the shales above them. Near its south-eastern boundary 

 it forms a slight feature in the shales. For these reasons, although 

 in other cases I have, in the maps illustrating this paper, taken the 

 boundaries of the members of the Carboniferous Group from the 

 Geological-Survey maps, I have ventured to alter slightly the boundary 

 between the Carboniferous Limestone and the Yoredale Shales in the 

 immediate neighbourhood of this vent. There appears to be insuf- 

 ficient reason for the existence of a fault cutting off the Mountain 

 Limestone from the shales near the vent. There is plenty of room 

 between the sandstone of Stainsbro' Quarry, a short distance to the 

 south, and the limestones near the vent for the intermediate beds 

 of the Yoredale Series. 



The igneous rock is a breccia, composed of small lapilli and more 

 or less subangular blocks of dolerite and basalt. The fragments of 

 basalt contain augite and felspar often in a fresh condition, olivine 

 generally altered, in a black or sometimes in a glassy base. ? 



Tuffs. 



The Shothouse-Spring Tuff has been described above in connexion 

 with the Grangemill Vents (p. 263). 



Low Farm. — North-west of Low Farm near Bonsall is a small 

 patch of tufaceous limestone and tuff which are seen in situ, close 

 to the north-eastern boundary of the Bonsall Sill. The walls in the 

 neighbourhood contain numerous boulders of the rock. The relations 

 of the tufaceous rock to the limestones are uncertain, and there is no 

 proof that it cuts across the limestone-beds. I have, therefore, mapped 

 it as a tuff. 



As hover Tuff. — A bed of tuff occurs in the inlier of Mountain 

 Limestone at Ashover, and is exposed in the valley through which 

 the small stream runs. It is contemporaneous with the limestone, 

 and the beds of the latter are seen to rest conformably upon it. 

 The thickness of the tuff has not been accurately determined, 

 although a shaft was sunk into it for a distance of more than 



1 ' Ancient Volcanoes of Great Britain ' vol. ii (1897) p. 16. 



2 ' Summary of Progress of the Geological Survey for 1903 ' 1904, p. 7. 



3 Quart. Journ. Geol. Soc. vol. 1 (1894) pp. 635-36. 



