476 Reports and Proceedings — Geological Society of London. 



water. There are two distinct groups of igneous rocks : a bedded 

 group, consisting of rhyolites and acid tuffs, with andesites and 

 andesitic tuffs; and an intrusive group of olivine-dolerites. The 

 general strike of the bedded rocks is north-north-easterly and south- 

 south-westerly, parallel to that of the neighbouring Longmyndian 

 rocks ; the average dip is about 80° east-south-eastward, but at the 

 extreme south-east of the hill the rhyolite and associated breccias 

 dip in the opposite direction (west-north-westward) at about the 

 same angle. The northern end of the hill consists of rhyolite (the 

 'northern rhyolite'), about 1,000 feet thick, a pale pink and purple 

 rhyolite with much epidote, chlorite, and secondary quartz, showing 

 vesicular, spherulitic, pyromeridal, and banded structures. Macro- 

 scopic and microscopic descriptions of the rocks are given, and the 

 origin of the spherulitic and nodular structures is fully discussed. 

 In many cases, though certainly not in all, the nodules appear to 

 have begun as a vesicle, often irregular in shape, and sometimes 

 with ci'escentiform spaces round the main cavity, and separated from 

 it by similarly-shaped portions of the glass. The spherulitic fibres 

 appear to develop, not from a centi-al point outward, but locally 

 from vesicles or other cavities, crystals, etc., coalescing finally to form 

 larger and longer growths. The spherulitic type of devitrification 

 is not all of the same age, for fibrous growths traverse small and 

 earlier -formed spherulites, which have been dissolved out and 

 replaced by quartz. The andesitic group is made up of felsitic- 

 looking, gritty pink and green tuffs, passing up into and inter- 

 bedded with andesitic glassy (palagonitic) and crystal tuffs, 

 halleflintas, and lavas ; the thickness is about 1,600 feet. A thick- 

 ness of about 150 feet of rhyolite-breccias (glassy and crystal tuffs) 

 and grits succeeds ; and this is followed by the south-eastern 

 rhyolite, about 250 feet thick, a dai-k red or purple, coarsely 

 vesicular, well -banded rock, often with light -green and white 

 amygdules. The andesites consist of oligoclase and malacolite, 

 embedded in a hyalopilitic groundmass containing palagonite, in 

 which ilmenite, leucoxene, and magnetite are embedded. A table 

 of the silica-percentages and specific gravities of the bedded rocks 

 shows that a gap occurs between the ' northern rhyolite ' and the 

 more acid of the andesite- tuffs that immediately follow ; this, 

 together with a discordance in strike, may indicate a break in 

 volcanic history, a disturbed junction, or that this rhyolite is 

 intrusive. From this point onward, the tuffs and lavas form 

 a continuous series, despite the difference in the average silica- 

 percentage of the andesite group and the rhyolite-breccias. The tuffs 

 thin out to the north-eastward, their lapilli diminish in size, and 

 they become more gritty and washed in aspect in the same direction; 

 facts which all point to the inference that the volcanic vent may 

 have been to the west of the hill. The intrusive rocks are basic, 

 and often amygdaloidal ; they are granular or ophitic, and compare 

 in composition with such olivine-dolerites as those of Eowley, the 

 Clee Hills, and Little Wenlock, while they differ considerably from 

 the intrusive dolerites of North "Wales. 



