626 



ME. E. GREENLY ON THE GEOLOGY OE THE [Nov. 1 896, 



complicated structures, now concealed beneath the Carboniferous 

 Limestone, may underlie the old rocks of Bangor and Caernarvon. 1 



Pig. 10. — Mylonitic seam in Tairfynnon Breccia. 



X35. 



IY. Oedovician Seeies. 



The rocks of this age which fall within the district under con- 

 sideration arc almost entirely covered by Boulder Clay, and, towards 

 the northern end, nearly, 2 though not quite, overlapped by the Car- 

 boniferous Limestone. 



The best section is that on the coast at Careg Onen, where about 

 700 feet of black shale, with a few red bands, are seen (fig. 1, p. 61D). 



These have yielded a shell like Obolella, and a fragrrent of a 

 trilobite (undetermined), as vrell as the fossils found by Prof. 

 Hughes (Proc. Camb. Phil. Soc. vol. iii. pt. viii. p. 347) ; and inland, 

 in some mudstones west of Bryn Poeth, I found some fragments 

 apparently of phyllopoda. 



The occurrence of a bed of pisolitic ironstone is alluded to by 

 Pamsay (' Geology of North Wales/ p. 223). It does not occur at 

 Careg Onen, 3 but as, from evidence to be given below (see p. 628, 



1 I am unable to confirm the occurrence of another outlier of this group 

 near Llanddona (Quart. Journ. Geol. Soc. vol. xliv. 1888, p. 465, and appended 

 map) : the rocks belonging, in my opinion, to the schistose complex. 



2 Their western boundary, which has been moved about ^ mile westward in 

 a recent map, was correctly drawn by the Geological Survey. 



3 I do not think that the ironstone which there overlies the schists can be the 

 same. It has not the same character, is only 6 inches thick, and its associated 

 rocks are quite different. 



