ON THE OLDER EOCKS OF ANGLESEY. 375 



have been examined from the crossing of the river Alaw on the road to 

 Llanfachreth (10), and from near Llanfaban (22). I'hey are decidedly 

 of a more basic type. They contain few fragments of quartz, bat abund- 

 ance of felspathic fragments, sometimes of fresh banded plagioclase, 

 sometimes speckled, and sometimes entirely passed over into sericite. 

 There are also fragments of the finer crystalline material, as at Llanfechell, 

 with minute crystals of green-mica in a clear groundmass ; also of quartz 

 vein stuff, and much epidote ; possibly also olivine and magnetite. One 

 is tempted to ask if these can be in any way connected with the eruption 

 of gabbro, which took place not very far to the south. Another rock, 

 whose numerous fragments are mostly quartz, and the matrix the usual 

 fine-grained material, occurs in the hill over Ogo Lowry, near Llanrhyd- 

 dlad (31), which has fragments of volcanic rock similar to that which 

 forms the agglomerates occurring within little more than a mile. In the 

 Central district the rocks with abundant fragments are an important 

 group. They occur associated with the dust rocks at Cerryg-ddwyffordd 

 (99). Here there is very little matrix at all, the fragments of felspar are 

 more numerous than those of quartz, and there are a number of dusty 

 fragments which may be in some cases devitrified lava, in others lapilli 

 of tiiff. This is one of the rocks described by Professor Bonney, who 

 suggests the same origin. Further south the rocks are seen to contain, 

 scattered irregularly through them, nodular hard masses. A microscopical 

 examination of one of these (89) shows they are places of aggregation 

 of the large fragments of quartz and occasionally of felspar, so closely 

 packed as to leave scarcely any room for matrix, which consists of amor- 

 phous chlorite only. At Dinas Llwyd (126) the rocks of this group are 

 similar to that at Cerryg-ddwyfiFordd, but contain also fragments of a 

 dark rock, consisting of lath-shaped felspars in a black matrix, a type 

 found in situ at Gwalchmai. In the Eastern district two rocks with 

 abundant fragments have been examined. In that from Ty Gwyn, near 

 Menai Bridge (188), the fragments are of quartz and plagioclase, with 

 beautiful banding, set in abundant matrix, much modified by pressure. 

 In the other, from Bryn Minceg, near Llandegfan (189), the fragments are 

 smaller and ai-e embedded in abundant chlorite. This also has suffered 

 from pressure. 



The group in which the groundmass is also of coarse elements in- 

 cludes the various quartzites. In that of H.olyhead Mountain (1) the ground- 

 mass is so altered that it is difficult to distinguish the smaller elements 

 as derivative or authigenetic. The larger fragments are of all sizes, 

 varying from such as are scarcely distinguishable from the groundmass 

 up to large pieces -04 inch in diameter, all except the little zircons 

 being of quartz. In the quartzites at Porth-yr-Ogof (2, 4) less change 

 is apparent, and the groundmass consists of tight-fitting fragments, with 

 only room for a little green-mica between, and the large fragments are less 

 numerous and conspicuous. At the junction with the chloritic schists 

 (4) there is an appreciable admixture of felspar fragments. The rock in 

 the centre of the island (3), though it contains more green-mica in the 

 interstices, more resembles the quartzites of the South Stack Series in 

 having Httle differentiation between the sizes of the fragments ; but it is 

 allied to the Holyhead quartzites by its zircons, and it contains also 

 felspar fragments. It is not, therefore, so homogeneous as the rocks of 

 the South Stack Series. The great masses of quartzite at Bodafon (109) 

 and Craig Fryr (111), however different they may now be, were originally 



