part 1] OX THE GEOLOGY OF THE OLD RADNOli DISTRICT. 13 



with in both cases ; but on Lyth Hill basalt-pebbles appear to be more plentiful 

 and of slightly greater dimensions. Again, the pink conglomerate of Castle 

 Nimble may be compared with the conglomerate exposed near the summit 

 of the Longmynd, beside the Eatlinghope road. In the latter exposure the 

 pebbles are. for the most part, of types similar to those described above : even 

 the pink quartzite with the characteristic rhombs of ehalybite is represented ; 

 but the pebbles of green devitrified rhyolite, sparingly met with at Castle 

 Nimble, occur more plentifully in the Ratlinghope exposure, as also frag- 

 ments of a superficially ironstained quartz-schist. 



It is interesting to note that in the conglomerate passed through in the 

 boring for coal at F0II3- Farm, near Presteign. the same assemblage of rock- 

 types was encountered, including the pink quartzite with charybite-rhombs. 1 

 It is thus more than probable that this conglomerate may be correlated with 

 the pink conglomerate of the district to the south-west. 



The grey conglomerate, so well shown on Old Radnor Hill, has many points- 

 in common with the uppermost conglomerate of Lyth Hill. 



It appears, therefore, from the foregoing statements, that these arenaceous 

 sedimentary rocks, both as a whole and individually, may be assigned with 

 certainty to the Lpngmyndian, and correlated more particularly with the 

 Bayston Group of that region. 



It is interesting, in passing, to reflect upon the fact that these rocks indicate 

 the widespread denudation of an area of still more ancient sedimentary rocks. 

 rocks that had in many instances suffered considerable alteration before their 

 disintegration began. Further, that these sediments were associated with a 

 remarkable series of contemporaneous lavas of both an acid and a basic com- 

 position, more especially the former. In all probability, they were invaded by 

 plutonic and hypabyssal rocks characterized by the dominance of soda-felspar. 

 These igneous rocks were brought within the influence of denuding agencies, 

 and thus it is clear that unconformable relationship of great magnitude must 

 be inferred to exist between the newer and the older sedimentary deposit-. 



(c) The Woolhope Limestone. 



The basement conglomerate. — The junction between the 

 Pre-Cambrian rocks and the limestone is. in general, a faulted one; 

 several exposures, however, do occur which show the original rela- 

 tions of the two formations. Callaway mentions one locality 

 where this may be seen : namely, in an old quarry on the north- 

 eastern margin of the mam limestone outcrop, about 300 yards 

 south-west of Yat Farm (Quarry K, in the map, PI. VII). This 

 section shows several feet of conglomerate at the base of the 

 limestone, dipping gently north-westwards. The beds here rest 

 upon a fine-grained green greywacke, which can be seen cropping 

 out in the wood on the other side of the fence, a few yards farther 

 east. 



Fragments of this basement-bed have also been met with in the 

 elliptical block of limestone which has been faulted down to the 

 north of the Harp Inn. showing that the limestone here also origin- 

 ally rested unconformably on Pre-Cambrian rocks on the western 

 flank of Old Radnor Hill ; the former line of junction, however, is 

 now complicated by faulting. Similar evidence of the original 



pi 



esence of a basement conglomerate is seen at the southern end of 



.- 



1 Geol. Mag. dec. 6, vol. iv (1917) p. 489. 



