DEEIYED FEAGMENTS IN EOCKS OF SHEOPSHIEE. 481 



38. On some Derived Feagments in the Longmtnd and jS"ewee 

 AECHiEAN EocKs of Sheopshiee. By Chaeles Callaway, Esq., 

 D.Sc, E.G.S. (Kead June 23, 1886.) 



In" my papers on the Arclisean rocks of Shropshire I have touched 

 upon the evidence of age furnished by inckided fragments. I stated* 

 that the Longmynd conglomerates of Haughmond Hill contained 

 pebbles of purple rhy elite similar to the typical rhyolites (Uriconian) 

 of the Wrekin. In the Uriconian t series itself I had found con- 

 glomerates X full of rounded pieces of granitoid and gneissic rocks, 

 evidently derived from a crystalline area of which Primrose Hill, at 

 the south-western end of the Wrekin, was an undenuded fragment. 

 I have since been able to enlarge this evidence, and, in order to give 

 precision to previous conclusions, a series of specimens have been 

 submitted to microscopic tests. 



The Longmynd Rocks of Shropshire largely derived from the 

 Uriconian Series {Newer Archcean). 



The plum-coloured conglomerates and grits of the Longmynd 

 hills are well known. With the exception of a band of pale-green 

 slate fringing the area along its eastern margin, they form the chief 

 mass of the formation. Whatever estimate of their thickness may 

 ultimately be obtained, there is no doubt that these sediments 

 represent the washings from a very considerable extent of land. 



The conglomerates are sometimes crowded with pebbles of purple 

 rhyolites absolutely undistinguishable under the microscope from 

 well-known Wrekin types, and many of the fragments display the 

 spherulitic and perlitic structures originally described by Mr. 

 Allport §. Well-rounded pieces of a pale-green felsite are less 

 common. 



The prevailing grits are of a purple colour. Quartz is the most 

 abundant constituent ; the purple rhyolite comes next. Fragments 

 of felspar and mica are also seen, and, occasionally, bits of mica- 

 schist. The proportions of these constituents vary with the locality. 

 South of Church Stretton the grits contain a large admixture of 

 materials derived from a granitoid district. In Haughmond Hill, 

 at the northern end of the Longmynd mass, there is a green grit 

 (No. 209, m/ra), which would seem to have originated from the 

 denudation of a region of mica-schist ; but within a hundred yards 

 is an outcrop of conglomerate packed full of rhyolite pebbles. 



One of the typical green slates (No. 211) has also been micro- 

 scopically examined. The rock is apparently of mixed origin ; it 

 contains minute fragments of quartz and mica ; but Prof. Bonney, 

 who has been kind enough to look through my slides, writes : — " It 



* Q.uart. Journ. Geol. Soc. 1879, vol. xxxv. p. 661. 



t These rocks may be Pebidian, but, in the absence of positive proof, a local 

 term is convenient. 



\ Quart. Journ. Geol, Soc. 1879, vol. xxxv. p, 654, 

 § Ibid. 1877, vol. xxxiii. p. 449. 



