Vol. 52O 



eastern corner oe Anglesey. 



627 



' Llanddona '), it is clearly not dying out in that direction, it is 

 probably cut out by the fault which throws down the Carboniferous 

 rocks, and if so, the beds with the trilobite and Obolella must be 

 below it. The obscure fossils hitherto found may quite well be 

 Arenig forms, and therefore only the upper part of the shale-series 

 can be upon any higher horizon. 



An interesting point remains to be noted concerning the rocks 

 among which the pisolitic ironstone lies at Bryn Poeth (fig. 11). 

 These are not ordinary 



Fig. 11. — Bryn Poeth Quarry. 

 Section at the northern end. 



E. 





grits and shales, but 



bedded felspar - tuffs of 



varying texture, mixed 



with some sandy and 



muddy sediment. They 



are quite uncleaved. The 



finer bands approach very 



nearly to shales, but the 



coarser (fig. 12) are chiefly 



composed of broken lath-shaped felspars, most of them quite angular, 



and many even with re-entering angles, about T \y inch in length. 



They are often wonderfully fresh and beautifully twinned. The 



| Fig. 12. — Felspar-tuff heloiv pisolitic, ironstone, Bryn Poeth. 



f jit. V-ii 



x 35. 



matrix is dark, partly green by transmitted light, like that of the 

 pisolitic ironstone, and contains also angular fragments of dark 

 shale and andesitic lava, with generally a few pisolitic grains. 



