500 EEV. J. p. BLAKE ON THE 



Bodafon mountain and Craig Pryr, considered by Dr. Callaway 

 to belong to Ms slaty series. In this, I think, he is right ; they are 

 the continuations northward of the halleflintas and the pelites, 

 while Bodafon mountain and Craig Pryr are modifications of the grey 

 gneiss. The two most remarkable features in these pelites are the 

 comparative fineness of the materials, and the unusual amount of 

 sericite or chlorite developed in the interstices. With regard to 

 the upper group the same kind of fine orientated dust which occurs 

 in the extreme south here again produced dull sericitic slates with 

 well-developed cleavage, such as are best seen at Man-addwyn and 

 Trewyn, while to the north the great shoulder of Bodafon called 

 Clegyr is a more compact and halleflintoid variety of the same. The 

 slopes on the west side of Bodafon are coarse and more quartzose, and 

 recall the halleflinta of Tycroes and Gualchmai, but they are very 

 chloritic and also foliated. These are so comparable under the micro- 

 scope with the finer-grained chloritic schists (though the latter are 

 less altered) that one is tempted to include them in the lower series ; 

 but their microscopic aspect is not schistose, and their relations to 

 the Bodafon quartzite is not one of conformable stratification. 

 There is an interesting variation at the fork of the roads near 

 Clorach bach, in the form of a pink massive limestone, now brecciated, 

 and the fine dust-rock in the neighbourhood is largely impregnated 

 with calcite. This feature reminds one of Cerrig Coin wen and other 

 sporadic limestones, but I have found no quartz-knobs here. A coarse 

 grit-band occurs near the line of junction at Man-addwyn, and there 

 is reason to believe that there are, or have been, other grit-bands in 

 the series still coarser and more felspathic. In other areas the rocks 

 most similar to these in general character are the dust-rock at 

 Pant-yr-Eglwys, and the slates on the railway north of Llangefni. 



The relations of these rocks to those of Bodafon mountain seem 

 to be everywhere those brought about by a fault. This has been 

 shown by Dr. Callaway for the south-west end, and it is equalh* true 

 at Clegyr, where the fine-grained slate and the grey quartzite are 

 seen side by side with not a foot's breadth between them. But I am 

 not quite certain of the character and age of this fault. The Bodafon 

 quartzite is a highly foliated rock with a large proportion of other 

 ingredients than quartz, and the grains of the latter are of small 

 size. It thus approaches in character some varieties of the grey 

 gneiss, but, lacking any recognizable felspar, may properly be referred 

 to the lower part, on the horizon of the Holyhead quartzite. It in 

 no way resembles even the most stratified in appearance of the 

 quartz-knobs. Seen from certain aspects, the bedding with a low 

 dip to the S.E. seems evident. Thus it ought to be followed by 

 chloritic schist, and so pass up to the finer shales ; but it does not ; 

 on the eastern side fragments of the slate lie on it, which, though 

 broken as now found, must have been the next succeeding rock. It 

 is possible therefore that the fault may have taken place in con- 

 nexion with the eruptive outbreak, and the finer dust-rocks have 

 been deposited against the cliff thus formed. iN'o doubt it has been 

 faulted again — Craig Pryr has been cut off, Bodafon has been 



