a 
Igneous Rocks of Pontesford Hill. 667 
upon the land-surface, flowing up the depressions in the undulating 
expanse of marls, and successive overlaps of the several infra-Bone- 
Bed deposits resulted : the greatest overlap apparently taking place 
during the formation of the Bone-Bed. At those localities where 
the distribution of the infra-Bone-Bed deposits indicates elevation 
of the Keuper Marls in immediate pre-Rheetic times, it is notice- 
able that there is also a non-sequence at the base of the Lias. 
June 22nd.—J. E. Marr, Sc.D., F.R.S., President, 
in the Chair. 
The following communications were read :— 
1. «The Igneous Rocks of Pontesford Hill (Shropshire).’ By 
Prof. William 8. Boulton, B.Sc., Assoc.R.C.S., F.G.S. 
This paper is confined to a description of the characters and 
sequence of the rocks within the limits of Pontesford Hill, and no 
attempt is made to correlate them with those of the Uriconian 
areas. The hill is a ‘plagioclinal ridge,’ bounded on all sides by 
faults; 1t is made up entirely of igneous rocks, but some of the fine 
tuffs and volcanic grits show unmistakable signs of deposition in 
water. There are two distinct groups of igneous rocks: a bedded 
group, consisting of rhyolites and acid tuffs, with andesites and ande- 
sitic 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 1000 feet thick, a pale pink and purple rhyolite with 
much epidote, chlorite, and secondary quartz, showing vesicular, 
spherulitic, pyromeridal, and banded structures. Macroscopic 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 crescentiform 
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 central 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 1600 feet. A 
thickness 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 dark red or purple, coarsely- 
vesicular, well-banded rock, often with light-green and white 
