ON THE OLDER ROCKS OF ANGLESEY. 373 



to the tinstratified portion, to which a more or less volcanic origin is 

 assigned ; in fact, they are indurated tuffs. These are the green rock ab 

 Peniel (11), the unstratified yet feebly false-bedded tuff of Church Bay 

 (29), and the dust rock at Pant-yr-Eglwys (34). The remaining fine- 

 grained rocks in this district show more or less metamorphism, the 

 greatest amount occurring nearest to Holyhead Island. Of these may be 

 cited the rocks at Caer Ceiliog (9) and Bodedern (23), also from near 

 Llanddeussant (20, 25), Llanfachreth (18), Llanfaethlu (27), and Llan- 

 rhyddlad (30). These are in parts of the series which are far separated 

 from each other on stratigraphical grounds ; but there is little to choose 

 between the rocks. Another, dynamically altered, occurs at Forth Dryw 

 (12). Only one of this type has been examined from the South Stack 

 Series at Porth-y-crug (C2). 



In the Central district the fine-grained rocks occupy two distinct 

 positions. In the western portion the mass between the granite areas at 

 Llanfaelog (82, 83) is of this type (though much metamorphosed), and so 

 are almost all the rocks examined between Bodafon and Llanerchymedd. 

 One near Chlorach Bach (103) is highly calcareous; the others, from 

 Trewyn (104), Clegyr (108), and the west side of Bodafon (110), are 

 more crystalline. In the eastern portion there is a longitudinal band of 

 such rocks from Cerryg-ddwyffordd (98) to Llangefni (100), and so on, to 

 the banks of the Cefni, half-way to Llangwyllog (102). 



In the Eastei-n district a fine tuff occurs in association with the 

 igneous rocks at Gwladys (208), but the remainder belong to the strati- 

 fied series, and are chiefly found along the eastern edges, as at Bodlew 

 (147), Cadnant Vale, near Menai Bridge (187), Clawdd-y-parc, near 

 Llandegfan (196), Ty-garw, near Beaumaris (195), and Tyddyn, north of 

 Beaumaris (199). In all these places they have become finely crystalline. 

 The only place in which the dust may be suspected of being produced by 

 local crushing is on the north-eastern edge of Malldraeth Marsh (190). 

 Dust rocks of doubtful character, which appear to have undergone only a 

 slight chemical or infiltrational metamorphosis, occur at Llyn Bodgolched 

 (198), in the road between Garth Ferry and Beaumaris (192), and in the 

 country behind this road (191) ; in the latter case the rock is full of 

 derivative epidote. In the Northern district so many of the rocks have a 

 elaty aspect that few have been examined. Most of them, however, con- 

 tain larger fragments, and two only have been found free from them — viz., 

 at Mynydd Mechell (212) and Bodewryd (220). Both of these have 

 become minutely crystalline. 



It will thus be seen that only six out of the thirty-four examined can 

 be said to be in any sense unaltered, and many of them are completely 

 crystalline. This is entirely in conformity with the observations of Dr. 

 A. Geikie on the rocks of St. Davids, that ' certain layers or par- 

 ticular kinds of fine detritus, more especially some of the finely com- 

 minuted volcanic dust, have been specially susceptible of change.' 



BocJcs of Coarser Grain may arise either from the running together of 

 the materials of finer rocks in the process of crystallisation — and such 

 must be omitted for the present— or from the original elements being 

 larger. The interest of these latter arises from the fact of their asso- 

 ciation with the finer rocks, after the manner of ordinary sediments, and 

 from the greater diflBculty in their metamorphism leaving them more 

 unaltered. Thus, on the west side of Cemmacs (216) we find a rock of 

 which one half is fine-grained and the other half coarser, composed of 



