THE BASAL ROCK-GROUPS OF SHllOPSHTRE. 117 



road, and examine the re-entering angles in the vertical rock-surface, 

 we can make out the true dip, bed after bed, and measure a thick- 

 ness of from 20 to 30 feet. The upper strata are strictly typical of 

 the volcanic series, as it is exposed in Lawrence-Hill Quarry, at the 

 foot of the Wrekin. Associated with these grits and hornstones are 

 bands which are even more distinctly sedimentary, consisting of a 

 sort of claystone, with thin seams of small pebbles composed of 

 materials substantially identical with the included fragments of the 

 main conglomerate. At a recent visit, I was accompanied by my 

 friend Dr. Geo. Deane, F.G.S., who, with compass and clinometer, 

 ascertained that these beds dipped to the north at 70^-75°, a result 

 which differs very slightly from my observation already recorded in 

 the Quarterly Journal *. 



At nearly the bottom of the section, just before we reach the 

 Cambrian quartzite, we come to another conglomerate, with large 

 well-rounded pebbles of quartz, quartzite, granite, and various 

 schists. In hand-specimens, the matrix looks like a slightly por- 

 phyritic felsite. Prof. Bonnej' finds this matrix very difficult of 

 determination : but he is disposed to regard most of it as clastic 

 volcanic material. I confess I am not quite satisfied with this 

 opinion,, but the point is not very important. 



Further proof of the Uriconiau age of the conglomerate is found 

 in the small elevation to the S. of Charlton Hill. Typical Uriconian 

 ashes and grits, with an E. and W. strike, are exposed on the 

 plateau. Towards its southern margin, forming a small crag, there 

 is an outcrop of the ash containing an irregular band of conglomerate, 

 8 or 9 feet from E. to W., by 3 or 4 feet in breadth. Some of 

 the pebbles are of unusual size, reaching a diameter of 5 or 6 inches. 

 Hand-specimens are procurable, in which one or more rounded 

 fragments of granite and other rocks are embedded in as typical an 

 ash as any that can be found in the Wrekin area. 



I may add that in the Lawrence-Hill Quarry, and on the Wrekin 

 itself, grits and conglomerates occur which differ only from those 

 just described in the larger proportion of felsite-fragments. South 

 of the Wrekin and Wrockwardine masses, the signs of purely vol- 

 canic action decrease. They are less conspicuous even at Charlton 

 Hill ; but, in the Church-Stretton area, rhyolites are much less pro- 

 minent and felsite fragments decrease in number, while distinctively 

 sedimentary material increases in proportion. 



It is almost superfluous to point out that the proof of the 

 Uriconian age of the conglomerates involves the pre-IJriconian age 

 of the granite and metamorphic land-masses from which so many of 

 the fragments were derived. Some of the granite-fragments, as 

 Prof. Bouncy and myself have shown f, are similar to the rock which 

 occurs at the two opposite ends of the Wrekin chain. This state- 

 ment has an obvious bearing upon Prof. Blake's opinion that the 

 granite of the Wrekin is intrusive in the volcanic series. 



* Vol. xlii. (1886) p. 483. 



t Quart. Journ. Geol. Soc. vol. xxxv. (1870) p. 0.^)4 ; vol. xlii. (1886) 

 pp. 4^^3-485. 



