524 EEV. J. r. BLAZE ON THE 



shales. The lowest beds are true chloritic schists, not far removed 

 in character from the nearest rocks of the same kind in the western 

 district. They become coarser, more quartzose, and more contorted 

 in the heights of Mynj^dd Mechell, but ultimately obtain a pretty 

 uniform E. and W. strike. They are followed by ashy grits and 

 sericitic shales, and then by a broad band of green slates, which are 

 undisturbed and younger-looking in the west; but in the east, 

 where the Parys Mouutain has interfered with the motion, they 

 have become greatly contorted. Towards the far east they become 

 gritty again. They contain sporadic nests of limestone, and are 

 scored by dykes, which were intruded previous to the faulting. 

 The higher part of the series consists of laminated and, often, cleaved 

 ashy rocks, which become agglomerates in the west, and contain 

 large deposits of precipitated limestone and pure quartz-knobs, 

 irregularly placed and crossing the bedding. Above these knobs, 

 and derived from them, there are large conglomerates ; but they are 

 succeeded in most places by ashy rocks similar to those below, 

 which, in one place, have yielded fossils. The whole series is 

 unconformably overlain by another set of conglomerates leading up 

 into black shales, which are, in other places, let down between faults 

 and contain Ordovician fossils. 



The Disteict East op Pakys Motwtain. 



Peoops of the Pee-Cambeian Age oe the Rocks. — This dis- 

 trict is entirely isolated from the northern, though by a very 

 narrow band. It is coloured on the Survey Map as "altered 

 Silurian." It is therefore necessary to prove that it is rightly 

 included in the description of Pre-Cambrian rocks. This is not so 

 easy a matter as in the case of the neighbouring district to the 

 south of Traeth Dulas, since here there are no obviously overlying 

 basal Ordovicians, but the main boundaries are faults. These faults, 

 as seen on the sea-shore, have been well determined by Sir A. Eamsay, 

 and have been again more recently described by Prof. Hughes*. 

 Tracing the northern fault inland, we find it pretty correctly laid 

 down on the Survey map, and the rocks on the two sides remain 

 everywhere most distinct, as is well seen on the road between Ehos- 

 manarch-mawr and Rhos-manarch-ganol, and by the cottages of 

 Pen-rallt. Further on we find a strip of dark Ordovician shale 

 between this belt and the Parys Mountain. The boundary on 

 the other side of the belt is obscure, but black Ordovician shale is 

 seen quite close to the granite in Nebo Street. Tracing the southern 

 fault inland, it is not so clear, but the beds are disturbed and 

 broken, and no conglomerate i's seen. There is thus only left the 

 boundary between Plas Ucha and JS'ebo, which is in a direction at 

 right angles to the other boundaries, and is not therefore their 

 natural continuation, and need not be faulted. It is along this fine 

 that the quarries quoted by Dr. Callaway, at Nebo, are worked 

 in the basal Ordovician conglomerate, which is immediately followed 



* Quart. Journ. Geol. Soc. vol. xxxviii. 



