MONIAN SISTESI OF ROCKS. 509 



to and meeting the qiiartz-knob, and bent into a small synclinal 

 between its two branches. The rock therefore has the characteristic 

 behaviour of all these quartz-knobs, and has the special value that it 

 shows more clearly than any other its relation to surrounding rocks. 

 Besides these limestones and quartzes, there are seen some intrusive 

 rocks of diabase-aspect, and others whose structure is of a pepper- 

 and-salt pattern, and which I do not understand. 



It is on these higher members of the series that the Cambrians 

 here rest, instead of on the lower ones, as they do near Ked-Wharf 

 Bay ; and we thus see how by tracing the sequence in these lower 

 rocks we can best appreciate the unconformity of the overlying 

 series. 



The Aeea. south-wtjst or Mtnydd Llwtddiaet — This area is 

 probably an isolated portion of the Central District, as it is on 

 the other side of the great bounding fault which brings in the 

 Ordovician between it and Mynydd Llwyddiart, and is intermediate 

 in vertical position between the two districts. I do not think it is 

 itself divided by a fault of any consequence, though doubtless it is 

 broken. Commencing on the west, we find at Bryngwallen, near 

 Llanffinnan, at Ty-hen and as far as Hendre, near Pentreath, the 

 same kind of irregular chloritic slates that may be seen near the 

 Mona Inn and also near Menai Bridge, that is, just below the 

 commencement of the distinctly volcanic group, and these are 

 followed to the east by the slates and breccias so well described by 

 Dr. Callaway. He also refers to the limestones : these occur at 

 Wugan-bach and at Pentreath, on the same line of strike, and 

 jasper accompanies them, as at Cerrig Ceinwen. We are thus led to 

 see that we have an undisturbed succession. The importance of 

 this is the indirect evidence it affords of the essential unity of the 

 whole series, since the appearance of one portion belonging to the 

 lower part is always followed, if there be room, by a portion be- 

 longing to the upper part. The mode of occurrence of the limestone 

 is also instructive here, running, as it does, amidst the purple slate 

 in a curious pattern, and catching up fragments of the latter into its 

 substance. I think there must be some fault also in connexion with 

 the diorite-band, because there are associated with it on the western 

 side some rocks which have the appearance of marbled slate, but are 

 rich in the debris of the diorite, whereas on the eastern side the 

 grey gneiss is close at hand. It may be, however, that the marbled 

 rock is only a reasserted endoclast or, in other words, a fine fault- 

 breccia. 



YoLCANic Geotjp OP Caee& Gwladts. — At the other end of the 

 district there is a remarkable development of rocks, which must, 

 indeed, have been seen by Sir A. Eamsay as surveyor, but which 

 does not seem to have impressed him, or to have attracted the 

 attention of any other geologist. To my mind they are the most 

 extraordinary and interesting in the whole island. 



As we pass southwards from the railway at Llangaffo, over the 



