LONGMTND AND AECHiEAN KOCKS OF SHROPSHIRE. 483 



The Uriconian Series partly derived from pre-existing rock-groups. 



Conglomeratic beds containing rounded fragments of granitoid 

 rock are found in the very core of the Wrekin axis, for example, in 

 the large quarry at Lawrence Hill ; but the conglomerates of 

 Charlton Hill furnish a great variety of well-worn pebbles of pre- 

 existing rocks, chiefly metamorphic. It will be well, in this place, 

 to review the evidence for the Archaean age of these interesting 

 strata. 



On the top of Charlton Hill the conglomerate appears in several 

 small outcrops which lie approximatel}^ in a line running east and 

 west ; but no clear bedding is visible at this spot. A few feet to 

 the north, we find ordinary gritty and slaty rocks of the Wrekin 

 series, and in the adjoining field and lane there are numerous 

 exposures of typical volcanic grit. A few yards to the east of the 

 conglomerate ordinary Wrekin rock also occurs ; but the decisive 

 section is in the hollow lane to the south. In the northern bank is a 

 weU-marked band of pebbly grit, with a clear dip to the north at 80°. 

 This is the usual dip of the Uriconian series as regards direction, 

 but the angle is exceptionally high. The contained fragments 

 (No. 200, infra) of this bed are similar to those in the chief conglome- 

 rate, but smaller. The ordinary volcanic rocks overlie and underlie, 

 and a little lower down the road the underlying beds are overlain in a 

 very clear section by quartzite, dipping south-easterly at about 50°. 

 The rocks between the grit and the quartzite are exposed in 

 unbroken series, so that evidence of unconformity between the two 

 groups is complete. The relations between these volcanic rocks and 

 the quartzite are the same as at the Wrekin, and as the former are 

 lithologically similar to the ordinary Wrekin grits and dip in the 

 same direction, there is no reasonable doubt that the Charlton 

 conglomerates are of Uriconian age. 



The following descriptions are of slides from the chief conglo- 

 merate at the top of the hill, except No. 200, which is from the 

 pebbly grit in the hollow lane. Fragments of rhyolite are found 

 in all the specimens ; but as these result from contemporaneous 

 denudation, they will not be further noticed. Most of the pebbles 

 show very clear rounded outlines under the microscope : — 



195. Fragments of granitoidite, quartz-sehist, and a crystalline 

 rock consisting of decomposed felspar, quartz, and chloritic minerals. 



196. Granitoid rock, quartzite, and vein qnnrtz. Prof, Bonney 

 considers that the fragments of the first-named variety have " under- 

 gone crushing and been recemented with secondary quartz previous 

 to their conversion into pebbles." 



197. In addition to some of the above varieties, this slide contains 

 an angular fragment of argillite and one of grit, composed of bits 

 of quartz and quartzite in a matrix of micaceous material. This 

 grit strongly suggests the " mineralized grit," which I described in 

 my paper on Donegal * as characteristic of the Lough-Foyle series. 



198. Fragments of the altered grit, quartzite, quartz-schist, and 



^ Quart. Journ. Geol. See. 1885, toI. xli. p. 231. 



