E. Greenly — Arenig Shales at Menai Straits. 



561 



about 25° to 30°, and the line of junction is exposed, obscured only 

 in places by a little stalagmite. The base of the pebbly sandstone, 

 which is a little uneven, overhangs in a little shelf, underneath 

 which the upturned edges of the reddened shales upon which it rests 

 can be clearly seen. 



Further, the shales, in their black, unstained portions, have 

 yielded the following fossils:^ Didpnograptus patulus, Hall; Did. 

 nitidas, Hall ; Did. extensus?, Hall ; Climacograptiis?; and Cari/ocarts 

 — an unmistakable Arenig assemblage. One stipe of Did. nitidus 

 measured If inch and is incomplete, and. another of probably 

 Did. extensus is two inches in length. 



It is clear, therefore, that the base of the Carboniferous rocks is 

 exposed in this shore section, and that they rest unconformably upon 

 a series of black shales of Arenig age. 



I had for some time hoped to find an exposure of such rocks in 

 the vicinity of the Bridge, for the following reason : — At Tyn y Caeau 

 on the Holyhead Eoad, about a quarter of a mile west of the Bridge 

 Village, there is a large section in glacial gravels, which are composed 

 chiefly of well-rounded pebbles of a soft black shale. The shale is 

 of. Ordovician type, and the pebbles have yielded Didymograptus 

 Murchisoni ; Did. bijidus, lledl ; Did. extensus?, Hall ; Did. patidus, 

 Hall ; Caryocaris Wrightii, Salter ; Caryocaris ; and Lingula or 

 Lingulella. I know of no such shale in the neighbourhood on the 

 Anglesey side. The gravels appear to rest upon and to be completely 

 surrounded by crystalline schists, and, as the general direction of 

 transport of the blocks in the drift is from the north-east, the beds 

 from which the fossiliferous pebbles have been derived are probably 

 concealed beneath the waters of the Straits a little to the north- 

 east of the Suspension Bridge. 



There is some reason to suppose, indeed, that the floor of the 

 Straits east of the Tubular Bridge is in great part composed of 

 these Ordovician shales. For, as the Carboniferous rocks, though 

 lying at low angles, dip on the whole to the south-east, and as we 

 have seen that their base rises above high-water mark for about 

 250 yards of the shore on the Carnarvonshire side, it is improbable 

 that they cover much of the floor of the Straits, which descends to 



Fig. 2. — Hypothetical section through the Menai Straits between the bridges. 

 a, Crystalline Schists, b, Arenig Shales, c, Carboniferous Sandstone. 



some 40 feet below that level. The evidence yielded by the gravels 

 above mentioned points to an extension of the Ordovician shales 



^ The graptolites and other fossils mentioned in this paper have been examined, 

 with his usual kindness, by my friend and former colleague, Mr. B. N. Peach, F.K.S. 



DECADE IT. VOL. T. NO. XII. 36 



