Yol. 57.] GEOLOGY OF MTNTDD-Y-GARN. 27 



Llanfair-y^nghornwy Beds. 



Between the Green Series of the JSTorthern District and the 

 Black Slates of the northern and eastern slopes of Mynydd-y- 

 Garn intervenes a group of rocks consisting of green, blue, and 

 brown gritty slates and phyllites, with layers and nodules of grit, 

 and beds of grit, quartzite, and limestone. For convenience of 

 description I style them the Llanfair-y'nghornwy Beds, as 

 the village of that name is built upon them. In a quarry near the 

 church they are exposed as green slaty rocks (phyllites), and they 

 there lie upon the Black Slates and appear to dip conformably with 

 the latter, but there is no passage between the two kinds of rock, 

 which are crushed at the junction. The junction, in fact, forms 

 part of the great thrust-fault which sweeps in a curve through 

 Llaufflewin in one direction to the sea at Forth y Corwgl, and in 

 the opposite direction in a sinuous line to the sea at Perth yr Ebol.^ 



Near the thrust the rocks are often broken, crushed, and sheared ; 

 the structure is usually phacoidal, and coarser and harder portions 

 of rock are broken up and involved in the finer and more slat}' — the 

 structure, in short, is frequently that of a crush-conglomerate. 

 Good examples of these effects of pressure-action are to be found 

 on and around the farm of Ty'n-y-maen, where the weathered 

 surfaces of the beds have a conglomeratic aspect. In these broken 

 beds occurs a thick band of limestone, which has no visible out- 

 crop but was met with, close to the thrust-plane, in a 'level' 

 driven through these rocks ; and irregular masses of quartzite of 

 the ' quartz-knob ' type also lie in them. A thick bed of the same 

 kind of quartzite can be traced at intervals from Llanfair- 

 y'nghornwy Church in a north-westerly direction to Hendre-fawr; 

 and similar quartzites are again exposed near Mynachdy, where 

 there is also an intrusion of granite. 



The Llanfair-y'nghornwy Beds are cut off on the west by the 

 Forth yr Ebol thrust, and on the east by the fault which brings the 

 rocks of the Green Series against them. For some distance these 

 dislocations have a parallel course, but they converge southward 

 and meet south-east of Llanfair-y'nghornwy Church. 



From the beds of quartzite and limestone that they contain, the 

 Llanfair-y'nghornwy Beds may be correlated with the rocks of the 

 Llanbadrig Series of the northern coast ; and they probably lie at-or 

 near the base of that Series, for as a whole they resemble more 

 particularly the rocks along the southern border of the ' JSTorthern 

 Complex,' where the present writer has sometimes felt that the 

 boundary between the Green and Llanbadrig Series has been rather 

 arbitrarily drawn. 



Some of the Llanfair-y'nghornwy Beds also resemble the rocks 

 under the Conglomerate very closely, both in the field and under 

 the microscope. 



1 See description in Quart. Journ. Geol. See. vol. Iv. (1899) p. 645. The 



E resent map shows that the fault does not fork at Caerau, as I there stated, 

 ut at a point south-east ol" Llanfair-y'nghornwy Church. 



