Vol. 60.] IGNEOUS ROCKS OF PONTESFOEB HILL. 483 



Pontesford Hill is a ' plagioclinal ridge ', bounded on all sides by 

 faults, consisting entirely of igneous rocks, though some of the fine 

 tuff's and volcanic grits show unmistakable signs of deposition in 

 water. 



Two distinct groups of igneous rocks are found : — 



1. A Bedded Group, consisting of the Northern and South - 

 Eastern Rhyolite respectively, differing in composition and structural 

 characters. Of these, the Northern Rhyolite, of Uriconian type, is 

 probably bedded, but may be intrusive in the Andesite- Group, while 

 the South-Eastern Rhyolite, which is associated with breccias and 

 grits of the same composition, is certainly bedded. Between these 

 acid rocks intervenes a thick series of basic andesite-tuffs (palagonite, 

 halleninta, crystal, and gritty), interbedded with basic augite- 

 andesite lava. 



Leaving out of account the highly-siliceous Northern Rhyolite, 

 the bedded tuffs and lavas begin with a silica-percentage of about 56 

 and pass gradually into rocks, which form the bulk of the Andesite- 

 Group, with a little over 50 per cent. ; these again become somewhat 

 more acid, and then pass up abruptly into true rhyolite-grits and 

 breccias and rhyolite-lava, with about 75 per cent, of silica. 



The general strike of these bedded rocks is north-north-east and 

 south-south-west, parallel to that of the neighbouring Longmynd 

 rocks, with an average dip of about 80° east-south-eastward ; while, 

 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. 



2. Olivine-Augite Dolerites (granulitic, ophitic, or coarsely 

 amygdaloidal), having a chemical composition and mineralogical 

 characters similar to those of the dolerites of the Carboniferous 

 districts of Staffordshire and Shropshire, are intruded among the 

 bedded rocks in laccolite-like masses. 



I have to express my gratitude to Prof. Lapworth for the en- 

 couragement and assistance which he has so readily accorded to me 

 during the progress of this work, and also to Prof. Sollas for his 

 kindness in allowing the micro-photographs for the plates to be 

 taken in the Geological Laboratory at the University Museum, 

 Oxford. 



EXPLANATION OF PLATES XXXVIII-XLIII. 



Plate XXXVIII. 

 Geological sketch-map of Pontesford Hill, on the scale of 400 feet to the inch. 



Plate XXXIX. 



Generalized section through Pontesford Hill, from north-north-west to 

 south-south-east; and section along the lower western and southern flank of 

 Pontesford Hill. 



