part 4] 



UPPEK ORDOYICIAN OP THE BEHWTN HILLS. 



490 



*fe 



5*5 



South-west of Llanfyllin, on Allt Goch, a peculiar grit with 

 basal calcareous conglomerate is found, lying upon Ashgillian beds 

 which contain Galyinene quadrata, etc. This grit is also fossili- 

 ferous, yielding corals and a few brachiopods. It may be noted 

 that in Salter's Catalogue l fossils are recorded from this locality, 

 / the lithology of such species 



as are preserved showing 

 that they come from this 

 conglomerate. The follow- 

 ing forms are noted : — He- 

 liolites tubulata (Lonsdale), 

 Favosites sp., and Penta- 

 merns rotundus Sowerby. 

 The last-named is, however, 

 an extremely poor specimen, 

 and even the generic deter- 

 mination is uncertain. 



That the sandstone (with 

 Meristina crassa) and the 

 grit lithologically belong to 

 the overlying beds is shown 

 by their passage upwards 

 into the higher beds : blue- 

 shale wisps becoming in- 

 creasingly more abundant, 

 until the whole has passed 

 into peculiar wavy-bedded 

 sandy shales, with partings 

 of dark-blue shale. Unfor- 

 tunately, these beds have 

 yielded no fossils. 



The sudden change in li- 

 thology from the Ashgillian 

 mudstones to the sandstone, 

 and the gradual passage of 

 the sandstone into the over- 

 lying beds, together with 

 the presence of such large 

 numbers of M. crassa and 

 other Meristince and the ap- 

 parent absence of Phacops- 

 mucronatiis Beds, would 

 seem to corroborate the sug- 

 gestion that there is a slight 

 unconformity between the sandstone and the Ashgillian, and to in- 

 dicate a Silurian (Lower Aleutian) age for the sandstone, although 

 some of the fossils appear to occur in the higher Ashgillian beds 

 elsewhere. 



1 J. W. Salter, ' Catalog-ue of the Collection of Cambrian & Silurian 

 Fossils, &c.' Cambridge, 1873, pp. 73-79. 



