Vol. 63.] THE TOADSTONES OP DERBYSHIRE. 253 



Contemporaneous Tuffs. 



Several beds of tuff, contemporaneous with the Mountain Lime- 

 stone, are found in the northern area. 



Dove-Holes Tuff. — North-east of the village of Dove Holes a 

 thin bed of tuff can be traced for a short distance. In a quarry 

 south-east of Bull Ring it is represented by a bed of clay several feet 

 thick, which contains rounded lumps of limestone. The officers of 

 the Geological Survey described it as 



' a crumbly bed, pale-grey with green specks, and contains pebbles of lime- 

 stone, one of wbich was seen as big as a man's fist.' x 



In a previous paper 2 I stated that the deposit contained no traces 

 of lapilli ; and that, if it represented a tuff, it was so much altered 

 that no sign of the original structure remained. In the year 1900 

 a good section of this tuff was exposed farther east in an old quarry 

 whieh was being re-worked. A bed of tuff 4 feet thick, containing 

 pyrites and numerous rounded pieces of limestone, was seen by me 

 resting upon the limestone below it. The limestone-beds below the 

 tuff exhibited the pothole structure which is characteristic of the 

 limestones below beds of clay, lava, and tuff in various parts of 

 Derbyshire. Sir Archibald Geikie describes this structure as 

 'limestone with a surface dissolved into cauldron-like hollows.' 3 

 The tuff weathers into a brown material, and is very similar to those 

 of the Tissington district. This tuff lies above the lower lava, which 

 is seen in Holderness Quarry a short distance to the south. I saw 

 the lava faulted against the limestone in the quarry. The quarry- 

 men told me that a fault runs through the quarry, and that the 

 limestones to the north of the lava are sunk or lowered. There 

 will, therefore, intervene at least 60 feet of limestone between the 

 tuff and the lower lava. The Geological-Survey Memoir (loc. supra 

 cit.) estimates the thickness at 180 feet. 



Brook-Bottom Tuff. — In Brook Bottom, a valley north of 

 Tideswell, a bed of tuff is exposed on the western side of the valley 

 for a distance of about 50 feet. It dips with the limestone at 

 an angle of 18°, in a direction about 10° west of south, and is 

 evidently contemporaneous with the limestone. The eastern side 

 of the valley is covered with limestone-scree and angular gravel, 

 cemented with carbonate of lime ; I have been unable to find 

 any signs of the tuff on this side. The tuff is hard, shows 

 distinct traces of lamination, and contains blocks of embedded 

 igneous rock. The bedded tuff consists of lapilli with olivine- 

 pseudomorphs, and is cemented by calcite. A full description of 

 the microscopic structure was given in my previous paper. 4 



1 Geol. Surv. Mem. ' North Derbyshire ' 2nd ed. (1887) p. 21. 



2 Quart. Journ. Geol. Soc. vol. 1 (1894) p. 629. 



3 ' Ancient Volcanoes of Great Britain ' vol. ii (1897) p. 19 & fig. 181 : Peak- 

 Forest Limeworks. 



* Quart. Journ. Geol. Soc. vol. 1 (1894) pp. 626-27. 



