2 9 2 VOLCANIC PHENOMENA OF THE WREKIN. 



Sreen matrix ; and under the microscope each central red spot and its 

 border exhibit a radiated fibrous structure. Frequently the spheru- 

 litic structure is replaced by perlite structure, and when the rock is 

 examined by the lens, it then shows small concave or convex sur- 

 faces. The original glassy base of these rocks has undergone the 

 change of devitrification. Quartz veins are not uncommon in the 

 rock, and nodules of chalcedony are abundant. The old lava contains 

 72 per cent, of silica, 14^- per cent, of alumina, 6 per cent, of potash, 

 and 2 per cent, of soda. 1 



The volcanic agglomerates consist of ash of varying size, fre- 

 quently somewhat vesicular, having the cavities filled with chlorite 

 and crystalline quartz. Epidote is frequent in the coarser ash. 



The Ercal is a hill overlooking "Wellington, quarried for a bright 

 red felspathic rock of a granitoid character, which suddenly changes 

 to compact felstone, which is apparently the Wrekin rhyolite intrusive 

 in the red rock. The north-east end of Lawrence Hill consists of 

 felstones similar to those of Ercal, with felspathic tuff, which contains 

 fragments of pitchstone and felstone. 



The tuff which forms the Wrekin is continued and appears on 

 the northern slope of Primrose Hill, and there intrusive diorite is 

 found, which is similar to the diorite of the Malvern district. 



The Lea rock to the N.W. is a great mass of rhyolite, remarkable 

 for its spherulitic and perlitic structure. 



In the pre-Cambrian rocks of Lillieshall Hill there are two asli 

 bands, each about 10 feet thick, soft and ferruginous, and excavated 

 by weathering. According to Dr. Galloway, the minimum thickness 

 of the alternations of hornstone, ashy slates, shales, and felspathic 

 agglomerates there seen is 1500 feet. 



Other examples of the same alternations of pre-Cambrian felstones 

 and tuffs occur at Rock ward en, where the felstones are spherulitic, 

 at Charlton Hill, at both ends of Lawley Hill, the centre of which 

 is an intrusive mass of greenstone. At Caer Caradoe there are also 

 pre-Cambrian rocks with felstones and ashy shales, and the volcanic 

 series is traced onward by Cardington Hill and Ragleth Hill. 

 Hence the chain of hills which forms the axis of South Shropshire is 

 of pre-Cambrian age, and largely formed of volcanic materials of a 

 rhyolitic type, though sometimes broken through by dolerite and 

 gabbro. 2 



Pre-Cambrian Volcano of Caernarvonshire. In north-west Caer- 

 narvonshire, quartz-felsite or rhyolite is well exhibited in the vicinity of 

 Bangor, Caernarvon, and Llyn Padarn. These rocks were regarded by 

 Professor Ramsay as metamorphic, and as stretching for thirteen 

 miles in the Cambrian strata. Professor Bonney 3 believes that the 



1 Allport : " Ancient Devitrified Pitchstones and Perlite," Q. J. G. S., vol. 

 xxxiii. p. 449. 



2 Galloway : " Pre-Cambrian Rocks of Shropshire," Q. J. G. S., vol. xxxv. 

 p. 643. 



3 Bonney: " Quartz Felsite in North-West Caenarvonshire," Q. J. G. S.,May 

 1879. 



