MONIAN SYSTEM OF BOCKS. 487 



belt. This must be wrong in the north. The only possible line of 

 division that is not immediately contradicted on the ground would 

 run along the western branch of the river Gwna, and thence across 

 the old Holyhead road along the N.E. branch of the river Cefni. 

 On one side of this line most of the dips recorded are towards the 

 east, on the other side they are mostly to the west. Since the rocks 

 on the two sides are not the same, if any reliance is to be placed on 

 the dips, we have the choice between an overturn to the east and a 

 faulted synclinal. Dr. Callaway chooses the latter. The former 

 commends itself to me— first, because no sudden change of rock, 

 except where the grey gneiss comes on, indicates any fault ; and, 

 secondly, because, unless there is an inversion, the order of the upper 

 part in this district would be different from that which obtains in 

 the others. Moreover, of the many bosses of rock exposed, only a 

 few show any recognizable dip, and hence it is not certain how far 

 those that are observed are reliable. 



I take, then, the rocks on the east of the fault to show a pretty 

 continuous upward succession. We have seen that the throw of the 

 fault may not be great, and the lowest rocks exposed may not be 

 far above the grey gneiss, and they are so chloritic that they do not 

 differ much from the lower schists to which Dr. Callaway refers 

 them. The rock at Careg-engan-fawr, which he refers to the slaty 

 series, is quite of the same type. Some part of these may be best 

 placed as chloritic schist ; but the point of the whole matter is that 

 they so gradually change upwards by the introduction of scattered 

 angular fragments that no dividing line can be drawn. Thus, though 

 the material of the railway-cutting is very chloritic, so is the 

 material by the Mona Inn. A green grit in the cutting is just like 

 one at Nant-yr-lowddy, on the east of the Ordovician of the north. 

 There are breccias on the shore close to Aberffraw, and finer 

 breccias on the east of Llyn Coron. As a whole the rocks are more 

 allied to the lenticular pelites than to the chloritic schists. 



Passing to the east they become rapidly more ashy and irregular, 

 especially about Llangadwaladr and to the south, while to the north 

 slates become more abundant, and great agglomerates set in on either 

 side of the Llangefni railway. The whole set become more and more 

 irregular, as though produced by volcanic agencies, and in so far 

 agree with the north-west. But the materials are coarser and more 

 mixed with stratified deposits. As I look upon the beds as inverted, 

 I place the grits and porcellanites of Ceryg ddwyffbrdd, near 

 Llangefni, at the top of the series. After the admirable description 

 of this portion by Dr. Callaway, under the head of the " Llangefni 

 synclinal," I need not enter into any further details on the general 

 development, but only call attention to points of special interest. 



Amongst them are : — 1. The great agglomerates, so well seen in the 

 railway north of Llangefni, and on the coast on the south. These 

 contain huge masses of quartzose and igneous rocks, in the wildest 

 confusion : we cannot call them conglomerates and look to more 

 ancient land for their source ; they are not disposed after the manner 

 :X such rocks, but they contain ejected blocks from a volcano, con- 



