Vol. 55.] PATJLTED IXLIEE IN TIDES WELL DALE. 241 



'Ancient Volcanoes of Great Britain/ vol. ii (1897) p. 22, he 

 writes : — ' It appeared to me that the dark, compact, cr3'stalline 

 dolerite, which was formerly quarried in the middle of Tideswell 

 Dale, may be separated from the vesicular toadstone of that valley, 

 which is undoubtedly a true lava-flow, and that it does not always 

 occupy the same horizon there, being sometimes below and sometimes 

 above the amygdaloid. Where it rests on a band of red clay, the 

 latter rock has been made columnar to a depth of 9 feet. Alteration 

 of this kind is very rare among the Carboniferous bedded lavas, but 

 is by no means infrequent in the case of sills.' This expression of 

 opinion led me to investigate the question carefully, and, though 

 I was at first sceptical, farther work has convinced me that 

 Sir A. Geikie's explanation is the only correct one. 



II. Description of the Area and its Geological Structure. 



The area described in the present paper is about a square mile 

 in extent. It will be seen from the accompanying map (PL XIX), 

 which was drawn up on the scale of 6 inches to a mile, that the 

 greater portion of this area consists of a faulted inlier of Carboni- 

 ferous Limestone, with its associated toadstones. North and south 

 of this inlier are one or two lava-flows, intercalated with the later 

 beds of limestone which lie stratigraphically below the Hammerton 

 Hill lava. A distinction has been made on the map between the 

 compact dolerite of the inlier (the sill or intrusive dolerite) and the 

 vesicular lavas above and below it. 



A vesicular toadstone, about 600 feet east of the quarry and 

 separated from it by a limestone-escarpment, has not been marked 

 on the map. I was able to trace it for only a short distance. 



The evidence for the two faults north and south of the inlier is 

 clear. The southern fault crosses the middle road to Tideswell, 

 near the point w^here it makes a sharp bend in order to avoid a 

 small gully which runs down into Tideswell Dale. The fault 

 passes down the gully, crosses the lower road, and runs up a small 

 depression between Hammerton Hill and the marble-quarry. South 

 of the gully the limestone-beds dip down the dale towards the 

 AVye, and intercalated with them are two small lava-flows, each 

 about 15 to 20 feet thick, and separated by 15 feet of limestone. 

 A few yards away, north of the gully, are massive limestones 

 with disturbed bedding. A ridge of limestone marks the southern 

 boundary of the fault, as it is followed in a westerly direction 

 towards the uppermost road. Pollowing the fault in an easterly 

 direction, the toadstone of Hammerton Hill is brought against the 

 beds of limestone, which are on a higher horizon than the toadstone 

 of the quarry. The beds in the immediate neighbourhood show 

 signs of disturbance. 



The northern fault is distant about | mile from the southern. 

 On both sides of the valley the igneous rock abuts against the 

 limestone- beds higher in the series, and is seen within a few feet 

 of them. The limestone north of the fault contains at least one 



a. J. G. S. No. 218. R 



