132 PROCEEDINGS OP THE GEOLOGICAL SOCIETY. 



on one of the field-maps in, I believe, Mr. Selwyn's handwriting 

 the remark occurs : — " The gradual passage from the black shale to 

 the upper green gritty slates of Llanfechell is best seen at Bothedd, 

 on road from Llanfaethlu to Llyn-llygeirian." 



It is no part of my aim to disprove the existence of faults along 

 the line referred to. These may quite well exist ; but there is 

 assuredly no one gigantic displacement, such as the theory I am 

 combating would require ; while any faults which do occur cannot 

 be greatly different from the others of the district, and do not 

 prevent the true relations of the rocks from being discoverable. 



Where the supposed elliptical fault reaches the shore at Carmel's 

 Point, the two groups of rock seem to me to follow each other in 

 unbroken sequence. The black slates, which are admittedly Lower 

 Silurian, dip underneath a volcanic breccia and greenish (Amlwch) 

 slates. Not only so, but bands of similar black slates occur higher 

 up, interstratified with and sha ding-off into the tuffs and greenish 

 slates. Further, bands of coarse volcanic breccia occur among the 

 black slates south of the supposed break. These, in accordance 

 with the exigencies of theory, are represented as separated by a 

 network of faults from the black slates amid which they lie. But 

 good evidence may be found that they are truly interbedded in 

 these slates. In short, a critical examination of the sections shows 

 that the whole of the rocks in that part of Anglesey form one great 

 series, consisting partly of black slates, partly of greenish slates, 

 with abundant intercalations of volcanic detritus. The age of the 

 base of this series is moreover determined by the occurrence of 

 Bala fossils in a band of limestone near Carmel's Point. 



The rocks which extend eastward along the coast from the north- 

 western headland of Anglesey are marked on the Survey map as 

 "green, grey, and purple slates with conglomeratic and siliceous 

 beds." The truly volcanic nature of a considerable proportion of 

 these has been clearly stated by Mr. Blake *. As they dip in a 

 general northerly direction, higher portions of the series present 

 themselves as far as the most northern projections of the island near 

 Perth Wen. They have been greatly crumpled and crushed, so that 

 the slates pass into phyllites. They include some thick seams of 

 blue limestone and white quartzite, also courses of black shale con- 

 taining Lower-Silurian graptolites. Among their uppermost strata 

 several (probably Bala) fossils, including Orthis Baihjana, have been 

 obtained by Professor Hughes. It has been supposed that the higher 



* Quart. Joiirn. Geol. Soc. vol. xliv. (1888) p. 517. 



