[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 Palæozoic age is, I think, a matter of considerable interest from a petrological point of view, more especially as their mode of occurrence and their relations to the surrounding 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 "greenstone" 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 Wrockwardine 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 greenstone is the most inappropriate that could possibly be applied to them. They all belong to a highly acid type, and have not the slightest
