450 PROP. W. S. EOULTON ON THE [Nov. 1904, 



31. The Igneous Rocks of Pontesford Hile (Shropshiee). By 



William S. Boulton, Esq., B.Sc, A.K.C.S., F.G.S., Professor 



of Geology in University College. Cardiff. (Read June 22nd, 



1904.) 



[Plates XXXVIII-XLIIL] 



Contents. 



Page 



I. Introduction and Previous Literature 450 



II. General Structure of Pontesford Hill 452 



III. Detailed Description of the Rocks : — 



(1) The Northern Rhyolite 455 



(2) Nodular Structure of the Northern Rhyolite 457 



(3) The Andesite-Group : 



(a) The more Acid Grits and Tuffs 463 



(//) Palagonite-Tuffs, Grits, and Hallenintas 4( >4 



(c) Andesite-Lavas 470 



(d) Summary of the Andesite-Group 47- 



(4) Rhyolite-Breccias and Grits associated with the 



South-Eastern Rhyolite 474 



(5) The South-Eastern Rhyolite 470 



(6) Summary of the Bedded Rocks 477 



(7) The Intrusive Basic Rocks 478 



(8) Relation of the Intrusive Basic Rocks to the Bedded 



Rocks 4,^2 



IV. General Summary of Conclusions 482 



I. Introduction and Previous Literature. 



Pontesford Hill is situated on the north-western flank of the 

 Longmynd range of Shropshire, about 7 miles south-west of 

 Shrewsbury. With a length of about a mile, and a breadth of 

 half a mile, it rises to a height of just over 1000 feet, and from 

 its summit, which, is the site of a well-preserved Roman camp, a 

 fine view of the surrounding country is obtained. A mile to the 

 west, and running through the village of Pontesbury, the Stiper- 

 Stones Quartzite, the local base of the Ordovician System, crops 

 out ; while to the south-west stretches nearly the whole sweep of 

 the Ordovician district of Shelve and the Corndon. Immediately 

 to the east and south rise abruptly the conglomerates and purple 

 grits of the Western Longmynd, making up the conspicuous woody 

 ridges of Radlith and Oakswood. Between these and Pontesford 

 Hill is a deep and picturesque wooded gorge, cut by the Habberley 

 Brook. Here, about a third of a mile east of the northern end 

 of Pontesford Hill, is the Lyd Hole, a big, circular pot-hole, at 

 and near which are some conspicuous exposures of rocks referred 

 to by Mr. Blake and Dr. Callaway in their papers dealing with 

 the geology of the district. Northward stretches the great plain of 

 Shrewsbury and Chester. 



The hill, which is nearly severed into two roughly-equal portions 

 by a north-eastern and south-western gulley, rises from the valley 



