﻿Vol. 6 1.] ARENIG FA WE AND MOEL LLYFNANT. 633 



to explain the rucking-up, local thickening, and pinching-out of 

 such soft beds as the Daerfawr and Bifidus- Shales as due to the 

 adjustments between the rigid masses consequent on local pressure- 

 reliefs produced during the process. 



As to the age of the cleavage, we have already seen that it is later 

 than the intrusion of the andesitic dolerites. It is, therefore, 

 probably post-Silurian; and as, from the relations of ihe Carbo- 

 niferous, it must date from long before the Carboniferous Limestone, 

 we may tentatively refer it to the age of the Old Eed Sandstone or 

 Devonian. 



The Structure. 



The general structure of the district is that of a terraced 

 escarpment, broken both transversely and longitudinally by 

 faults. The scarp as a whole is determined by the Volcanic Series 

 described, and extends more or less continuously right round the 

 Harlech dome, from Tremadoc by the Moelwyns, Manod Mignant, 

 the Arenigs, the Arans, and Cader Idris, to the sea again near 

 Towyn, the Arenigs being the easternmost outpost of the ring. 



As has been indicated, the Massive Ash of the Upper Series 

 forms the culminating ridges of both Arenig Eawr and Moel Llyfnant, 

 which have also their eastern slopes largely determined by its dip. 

 A prominent terrace or secondary scarp is determined by the 

 Andesites and Agglomerates of the Lower Volcanic Series, and runs 

 round high up on the western flanks of both mountains. About 

 Craig-y-Bychan, Maen Grugog, and Craig Henllan this feature so 

 spreads out as almost to determine separate mountains. The 

 Llyfnant Elags, and with them the andesitic dolerites, give rise to 

 broken ground at lower levels, but do not stand out sufficiently to 

 determine striking structural features. 



Of faults, two series are of great importance. The older of 

 these has a general transverse Caledonian, north-westerly to south- 

 easterly direction, and would seem to belong to the series of earth- 

 movements which produced the cleavage of the district. Two of 

 its members, one to the north and the other to the south of the 

 summit of Moel Llyfnant, have produced notable displacements of 

 the resistant rocks of the Volcanic Series, but appear to die out 

 downwards in the soft shaly rocks of the Tremadoc and Dolgelly 

 groups. That which determines the fine crags of Y Castell, south 

 of Llyn Arenig Fawr, appears actually to die out in the Daerfawr 

 Shale, and cannot be proved to cut the Lower Volcanic Series at 

 all. These faults I regard as tear- faults ; they are very like the 

 faults described by Dr. Marr among the Coniston Grits and older 

 rocks of the Lake District. The rocks in their immediate neigh- 

 bourhood are always much shattered ; and it is interesting to note 

 that the pieces of fault-rock are mostly slickensided in a direction 

 which makes only a small angle with the bedding of the adjoining 

 country rock. 



Parallel to these tear-faults are several fine examples of normal 

 faults. The most important of these is the clean-cut fault which 



