T. M 'KENNY HUGHES ON THE GEOLOGY OF ANGLESEY. 239 



an immense series of similar black shales succeeds the fossiliferous 

 brown sandstones, passing up into slates with subordinate sand- 

 stones and, in the upper part, containing a few irregular beds of 

 felspathic slates and breccias. 



The black-shale group succeeds the brown sandstones everywhere 

 with a general northerly or northwesterly dip from west of Llan- 

 erchymedd to the coast east of Amlwch, where they dip as if they 

 passed under the green gnarled beds. 



A difficulty occurs west of Llanerchymedd : What becomes of the 

 black slates as we follow them S.W. ? They must have thinned 

 out or been faulted out or overlapped. I think we have faults and 

 an overlap of the Treiorwerth beds. 



In Henslow's collection in the AYoodwardian Museum there are a 

 few fossils labelled from Treiorwerth. I have not yet verified the 

 locality by finding the fossils myself in place ; but the rock looks 

 like the grey calcareous sandy part of the felspathic brecciated con- 

 glomerate of Treiorwerth, and very different from the quartz-jasper 

 conglomerate of the base of the Cambrian. The fossils collected by 

 Henslow in the shale of Treiorwerth are mostly mere casts ; but I 

 think the following genera and species occur amongst them : — 



Nebulipora, a form like N, lens and Orthis, various sp., ? O. testudinaria, 



a smaller var. Balm., ?0. crispa, M'Coy. 



Petraia elongata ? Strophomena rhomboidalis, WilcJc. 



Atrj-pa marginalis, Dalra. Euomphalus, sp. 



Meristella angustifrons, M^Coy. And one specimen of a curious fossil, 



, sp. which, for the present, we can only 



Orthis calligramma (?), Balm. suggest may be 



, var. plicata, Sow. Acantholepis Jamesii, M'Coy. 



protensa, Sow. 



These fossils refer the beds with considerable probability to the 

 May-Hill group, under which I include the Lower Llandovery ; but 

 they are not conclusive, and I hope on another occasion to offer 

 further evidence on this head. 



The great black-shale group appears to dip under the gnarled 

 series ; and it seems improbable that ^they are everywhere thrown 

 down into this position by faults. 



Another group of fossiliferous sandstones and shales overlies the 

 gnarled series, as at Glanffynon and near Llanpadrig. 



It seems therefore worth considering whether the gnarled shales 

 may not be the equivalent of the Bala volcanic series deposited be- 

 yond the region of principal volcanic activity. 



Discussion. 



Prof. Eamsay said that in the area of Llanberis and Xant- 

 ffrancon the Tremadoc slates had not yet been proved beneath the 

 other Lower Silurian rocks. He had himself, with Mr. Etheridge, 

 observed Arenig slates in Anglesey, between the Cambrian rocks 

 and the Carboniferous Limestone north of Beaumaris. That had 

 already been published. He could not agree that the grits and 

 schists of the northern part of Anglesey could represent the Bala 

 volcanic series of the mainland. These, in places, contained fossils. 



