;270 ME. H. H. ARNOLD-BEMROSE ON [Aug. I907, 



Tuffs. 



Until within a few years ago no contemporaneous tuffs had 

 been discovered in the Yoredale rocks of the county. A small 

 patch of toadstone in the Mountain Limestone, marked on the 

 'Geological- Survey map a short distance to the north of Tissington, 

 was the only igneous rock supposed to come to the surface in the 

 neighbourhood of that village. 



During the construction of the Ashbourne & Buxton Railway, 

 which was opened in 1899, some interesting sections of contem- 

 poraneous tuffs were seen in four of the cuttings. That in Tis- 

 sington cutting was described by Dr. Wheelton Hind, 1 and those in 

 the whole of the cuttings from Tissington to Parsley Hay by me. 2 

 With the object of making the present paper complete, I give a 

 short summary of the results. 



In the cuttings from Tissington to Crakelow four exposures of 

 the same bedded tuff were seen. 



In one place only, namely in Tissington cutting, was the whole 

 bed seen. It was 140 feet thick, and consisted of lapilli of a 

 pumiceous rock, ranging from microscopic dimensions up to an 

 vinch or so in diameter, in laminse of varying thickness. Scattered 

 irregularly throughout the mass were ejected blocks, rounded or 

 subangular in shape, and either vesicular or amygdaloidal. In size 

 they varied from several inches up to a foot in diameter, and their 

 microscopic structure was similar to that of some of the blocks in 

 the Woodeaves-Farm Vent described above (p. 267). Among the 

 -thin limestones and shales which lie above the thick tuff-bed 

 were numerous intercalations of tuff varying in thickness from a 

 quarter of an inch to 2 feet. These layers often contained encrinite- 

 stems, Productus, and fragments of fossiliferous limestone. The 

 intermittent eruptions which gave rise to these bands of tuff 

 continued during an accumulation of at least 80 feet of shales and 

 thin limestones. 



From a section seen in Newton-Grange cutting it is probable 

 that the thick tuff is on a horizon about 60 feet above the main 

 mass of Mountain Limestone. The volcanic action in this 

 neighbourhood, therefore, probably continued about 140 feet up 

 into the Yoredale Series. (See vertical section No. Ill, p. 249.) 



Lavas. 



Two bands of vesicular and fine-grained dolerite are found, one 

 on either side of the valley, a short distance south of Kniveton. 

 The Geological- Survey Memoir (op.jamcit. p. 87) explains their 



1 ' Section in Carboniferous-Limestone Shales at Tissington, &c.' North Staffs. 

 Nat. Hist. Fielcl-Club Report, vol. xxxii (1897-98). 



2 'Geology of the Ashbourne & Buxton Railway' Quart. Journ. Greol. Soc. 

 toI. It (1899) pp. 224-37, & vol. lix (1903) pp. 337-47. 



