MONIAN SYSTEM OF ROCKS. 467 



Western, Central, Eastern, and Northern districts. Of the other 

 two, one which lies south of Traeth Dulas and near Bodafon Moun- 

 tain is naturally in association with the Central district, and the 

 other, which lies east of Parys Mountain and north of Llanwenllwyfo, 

 is most connected with the Northern district. It will be found 

 most convenient to arrange the descriptions in the order thus giveo. 



The Western District. 



Holyhead Island. — This island is divided geologically into two 

 parts by a master-fault which runs in a N.W. and S.E. direction, 

 and separates a newer series on the S.W. from the older portion on the 

 N.E. Confining our attention at present to the latter portion, we 

 find an order of succession very clearly made out, the well-knoAvn 

 quartzite of Holyhead Mountain being lowest in the sequence. In 

 this there is very clear cleavage, and on the seaward slopes every 

 yard is shivered into a thousand fragments. Along the direction 

 of cleavage, congregating as best they may in the intervals between 

 the larger grains of quartz, are innumerable flakes of mica, or feebly 

 coloured chlorite, producing a foliated rock ; but there is no obvious 

 connexion here between the foliation and bedding. The actual direc- 

 tion of any minor beds in this great mass is not easy to determine ; 

 but standing on one of the eminences in the wild country to the 

 S.W., and seeing scarp after scarp descend in giant steps from the 

 summit to the sea, one fancies a bedding on a large scale with a 

 dip to the S.E. If these are really beds, the dip is a gentle one, 

 whereas the cleavage is vertical. In any case it is in the direction 

 of this dip that we must proceed to find any higher beds, and when 

 we do find them the dip is confirmed. On the S.E. slopes of the 

 mountain the quartzite becomes dirty and of finer grain, and gives 

 place, where the road is reached, to solid micaceous foliated grits. 

 These and alternating more quartzose beds rise into the summits of 

 the low hills in the same direction, and then somewhat suddenly 

 change into the chloritic schists, so that a clear line can be drawn 

 after which no quartzites appear. This is rendered more evident 

 by the occurrence of two minor faults, somewhat parallel to the 

 master one and to the running of the greenstone-dykes. The smaller 

 of these, on the S.W. side, brings the chloritic schists with their wavy 

 lines close beside the massive quartzose rocks and emphasizes the 

 contrast. The larger fault on the N.E, limits the mountain in that 

 direction, and substitutes for it the low ground that terminates in 

 Penryn Garw. This headland consists of the upper part of the 

 quartzitic group, with its bands of micaceous grit, and is succeeded 

 immediately by the chloritic schists, which are seen lying on its sur- 

 face to the west of Porth-yr-ogof, thereby proving that there is no 

 fault in that inlet, as supposed by Dr. Callaway. All things are here 

 in regular succession, and the fault which separates them from the 

 mountain may be clearly traced to the neighbourhood of the town. 



The chloritic schists of Holyhead are very remarkable rocks. They 

 were originally laminated, as is evidenced by the parallel lines of dust 



