Vol. 60.] IGNEOUS ROCKS OF PONTESFORD HILL. -±57 



VI. Matrix of nodular felsite, from the Lleclr Valley, near Conway-Falls 

 Inn. F. H. Hatch, Quart. Journ. Geol. Soc. vol. xxxix(1883) p. 485. 

 VII. Matrix of a nodular rhyolite, Boulay Bay (Jersev). Hyndman & Bonney, 

 Geol. Mag. 1896, p. 367. 

 VIII. Spherulite of same. 



It will be seen that the rhyolites of Pontesford, especially the 

 Northern Rhyolite, are more acid than those of the ' Lea Rock ' 

 (Shropshire) and Brithdir Farm, both of which are of reputed 

 pre-Cambrian age. On the other hand, they are in fair agreement 

 with some of the Ordovician felsites ; while the mean silica- 

 percentage of the spherulites and matrix of the rhyolite from Boulay 

 Bay (Jersey), which is very similar in character to the Northern 

 Rhyolite of Pontesford, agrees almost exactly with that of the 

 South-Eastern Rhyolite. Although every care was taken in selecting 

 varieties as little altered as possible, it may be that the specimens 

 of the Pontesford rhyolites analysed contain more or less secondary 

 silica than some of the others quoted in the foregoing table. In 

 any case, without an analysis of a large number of specimens 

 taken from different parts of the mass in all these localities, it 

 would be unsafe to decide finallv as to the relative acidity of these 

 different felsites. 



(2) Nodular Structure of the Northern Rhyolite. 



A very pronounced feature of the Northern Rhyolite is the 

 abundance of ; nodules,' which include some of the largest hitherto 

 described in Britain. They are nearly all confined to the centre of 

 the rhyolite-mass, the best specimens occurring on the north- 

 western slope, immediately under the Lower Camp (433), and thence 

 along a line to the Old Quarry (561). The diameter varies up to 

 8 inches or more, and there is considerable variation in their 

 internal appearance and structure. In this part of the rock, where 

 the pyromerides occur, there is practically no sign of flow-structure. 



Prof. Bonney l has described the microscopic characters of a 

 specimen of the nodular rhyolite, from the North-End Quarry, 

 probably (561). He speaks of ' hollow spherulitic concretions, 

 subsequently partially or wholly filled by infiltrated minerals,' and 

 says that the rock of Lea Hill is very similar in structure to this 

 one from Pontesford Hill. 



More recently, a reference has been made to the Pontesford 

 pyromerides by Mr. Parkinson, 2 who points out their strong re- 

 semblance to those of Wrockwardine and Boulay Bay. 



Specimen 15 is a pale-pink, finely-nodular rhyolite, which, 

 under the microscope, shows a light-brown matrix with bright- 

 green and reddish-brown patches, highly coloured in places with 

 bright-blue ferrous sulphate. Between crossed nicols, the whole 

 mass is seen to be devitrified. The matrix has a spongy appearance, 



1 Quart. Journ. Geol. Soc. vol. xxxviii (1882) p. 124. 



2 ' The Hollow Spherulites of the Yellowstone & Great Britain ' Quart. 

 Journ. Geol. Soc. vol. lvii (1901) p. 223. 



