ON PITCHSTONES AND PERLITES FROM SHROPSHIRE. 449 



22. On certain Ancient Devitrieied Pitchstones and Perlites from 

 the Lower Silurian District of Shropshire. By S. Allport, 

 Esq., P.G.S. (Bead May 23, 1877.) 



[Plate XX.] 



I. 



The principal object of the present communication is twofold : — in 

 the first place, to bring under the notice of the Society the occurrence 

 in Shropshire of an extremely interesting series of ancient vitreous 

 and semivitreous lavas, with their associated agglomerates and 

 ashes ; and in the second, to show, from an examination of their 

 structure and composition, that originally they were absolutely 

 identical with some of the glassy volcanic rocks ejected during the 

 most recent geological periods. 



In a previous paper (published in vol. xxx. pp. 529-567, of the 

 Quarterly Journal) I arrived at the same result as to the identity of 

 ancient and recent volcanic products, from an investigation restricted 

 to the basic group of rocks ; and I am now enabled to show that 

 those of the acid type afford equally strong evidence in the same 

 direction. 



The discovery of several highly characteristic varieties of glassy 

 rocks and volcanic ashes of Palseozoic age is, I think, a matter of 

 considerable interest from a petrological point of view, more espe- 

 cially as their mode of occurrence and their relations to the sur- 

 rounding strata afford the clearest evidence that the geological 

 structure of a part of the district in which they occur has hitherto 

 been misunderstood. 



On Sheet 61 of the Geological-Survey map a band of " green- 

 stone " is represented as constituting the axis of the somewhat 

 irregular narrow ridge of which Ercal Hill and the Wrekin form the 

 greater portion. It commences half a mile south of Wellington, and 

 extends in a south-westerly direction to a distance of two miles and 

 three quarters. At two points not far from each other the ridge is 

 cut through by deep and narrow gorges, which traverse it from 

 south-east to north-west, or at right angles to its general direction. 

 The isolated hill thus formed by the two ravines is locally known as 

 Lawrence Hill, and is indicated though not named on the map. 



At a'short distance to the west there is laid down a still larger 

 mass of " greenstone," which extends from the vicinity of Wrock- 

 wardine as far as Uppington, and forms a low ridge parallel to that 

 of the Wrekin. 



With the exception of some thick beds of indurated ashes which 

 occur in the Wrekin and Lawrence Hill, the principal rock-masses 

 are of similar character in both ridges ; and a slight examination 

 with a lens would suffice to show any petrologist that the term green- 

 stone is the most inappropriate that could possibly be applied to them. 

 They all belong to a highly acid type, and have not the slightest 



