﻿618 ME. W. G. EEAENSIDES ON THE GEOLOGY OE [Aug. I905, 



cordanee marking the uprise, wane, and degradation of some huge 

 continental mountain-chain, but an unconformity and surface of 

 overlap produced by the passing of some early Caledonian earth- 

 wave, which has left but little permanent impress upon the dip and 

 strike of the rocks which it affected. So slight was the disturbance, 

 that the district mapped is almost too small to permit of proving it ; 

 and one might hammer and map for many days on end, without 

 discovering any discordance at all. It was only by following the 

 line of Mr. Williams's Garth Grit for several miles to the west, 

 that I was able to make up my mind as to the importance of the 

 unconformity. In that direction, the grit rests successively upon all 

 members of the Tremadoc Series, and at the Nant-y-Derbiniad slate- 

 quarry comes into the Black Eand or Peltura-Be&s of the Dolgelly, 

 or Upper Lingula-'Fl&gs. Further excursions along the same line 

 suggest that the various members of the Lower Ordovician 

 Series overlap each other westward against the surface 

 of unconformity; and the knowledge that the similar grit at 

 Ty-obry, Minffordd (Portmadoc), is directly overlain by shales 

 belonging to the zone of Didymogvaptus MurcMsoni, Beck, lends 

 credence to such a supposition. 



The Basal Grit of the Arenig Series [13]. 



Upon the unconformity rests a grit, notable for its inconstancy 

 of thickness and variability in character. Sometimes it may be as 

 much as 50 to 100 feet thick, but usually it thins off rapidly, and 

 may entirely die out in something less than 150 yards. Occasionally 

 it is clean and white, and from the abundance of disseminated 

 vein-quartz may become almost a quartzite. More commonly it is 

 rather impure, and has much ashy material among it. Usually it is 

 a fairly-coarse, even-grained grit ; but sometimes, especially when 

 thickest, its middle part becomes almost conglomeratic. In all 

 cases, the constituent grains are rounded rather than angular. 

 Grains of hyaline quartz are fairly common, but pebbles directly 

 derivable from the underlying Tremadoc or Dolgelly Beds are 

 conspicuous by their absence. Under the microscope, it is seen that 

 the grit is a good deal sheared, and that grains usually show well- 

 developed strain-shadows and suture-cracks. The quartz-cement is 

 in crystalline continuity with the adjoining grains, the original out- 

 lines of which are often almost obliterated (4640). A certain amount 

 of carbonate- cement (chalybite ?) is also present, and patches of 

 rather fresh andesine or oligoclase are not rare (4641). No newly- 

 formed minerals were observed. 



The best exposures are the patches mapped on the northern 

 flanks of Moel Llyfnant and adjoining the dolerite-intrusion south 

 of Llechwedd Erwent ; but the grit is more easily visited at the 

 patch east of Waen-goch on the way to Amnodd-wen, or that along 

 the Ffestiniog road at Llechwedd Deiliog. 



