East of Gelli Hill, Radnorshire. 503 



embedded in chlorite are possibly the result of the further alteration 

 of this mineral, Acicular apatite is present in small quantity. 



The amount of metaraorphism effected by the diabase is never 

 great, the most striking result being a rock resembling a spilosite, 

 produced in consequence of the alteration of the shales which almost 

 invariably occur at the contact. At one point, however, in the 

 brook just south of Pye Corner the contact rock is a limestone 

 containing fossils which Dr. Morley Davies recognizes as Strick- 

 landi7iia lirata (Sowerby), S. lens (Sowerby), Strophonella euglypha 

 (Sowerby), Atrypa sp., Encrinurus sp., and Ralysites sp. This fauna 

 he considers to be sufficient to prove the Upper Llandovery age 

 of the sediment, and it therefore becomes a matter of extreme 

 importance to determine the relative ages of intrusion and limestone. 



A specimen of the diabase from near the contact includes a patch 

 of calcite, which mineral is seen in thin section to be interstitial with 

 regard to the felspars. The latter are not more basic than in the 

 diabase remote from the limestone, but apatite is distinctly more 

 abundant. 



The limestone varies from pale grey to black. In places it 

 exhibits some resemblance to a conglomerate, containing subangular 

 fragments of quartz. Occasionally on the bedding plane is seen 

 a spheroidal projection which, broken across, is not to be dis- 

 tinguished from the adjacent igneous rock, and similar igneous 

 material is interbedded with the limestone, sometimes with a layer 

 of crystalline^ calcite at the contact. Lenticles of calcite occur 

 within the igneous material and patches of the igneous rock within 

 the limestone. Some pyrite occurs at and near the junction. A thin 

 section through one of the igneous spheres shows a diabase exactly 

 comparable with that of the adjacent sill ; the felspars interlock in the 

 usual way and enclose angular patches of chlorite, and idiomorpliic 

 crystals of apatite occur in the felspar. The margin of the sphere is 

 sharply defined ; there is no transition into the limestone and, at this 

 contact, there is no evidence of re crystallization of the latter. 



The part of the rock not obviously igneous in origin exhibits 

 features of considerable interest. Some portions of the sections are 

 made up of a network of felspar crystals related just as in the 

 igneous rock, but instead of interstitial augite or chlorite there is 

 crystalline calcite. Here again the felspars are not more basic than 

 in the diabase, and there is no support for the idea that calcareous 

 material has been incorporated by the felspars. Other crystals of 

 felspar appear to be isolated; they are sometimes idiomorphic, but 

 elsewhere are moulded on the calcite. Quartz, either as simple 

 individuals or as aggregates, occurs scattered through the rock as 

 subangular equidiraensional grains, as extremely angular individuals 

 of various shapes, and as idiomorphic crystals. In one particularly 

 interesting case the felspar and quartz are intergrown to form 

 excellent micrographic structure. This example is sufficient to 

 demonstrate the igneous origin of some of the quartz in the lime- 

 stone and to render it extremely probable that no detrital quartz is 

 present in the rock. 



There is no question as to the relative ages of the two rocks ; the 



