306 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



either one broad belt, or several narrower bands broken by inter- 

 vening beds of slate. 



This igneous zone is overlaid on the west by a thick bed of gravel, 

 which, for a space of about three miles broad and twenty-four miles 

 long, entirely conceals the beds lying next to the mountain chain*. 

 On the coast near Clynnog, the gravel forms the entire cliffs, nearly 

 100 feet high ; on the east of Carnarvon and Bangor it passes in- 

 land, at some distance from the Menai Straits ; it terminates north- 

 wards on the coast between Aberf and Penman-mawr. It consists 

 of rolled fragments of all sizes^ from mere pebbles up to huge boul- 

 ders, all apparently derived from the rocks of the Snowdon chain. 

 The valley which contains this great drift deposit is the more re- 

 markable when contrasted with the valleys on the east of the chain, 

 which, comparatively, are exempt from gravel. There is however 

 a similar accumulation of gravel in some of the lower parts of the 

 south-west end of Carnarvonshire. 



2. Upper Silurian and more recent formations. — It was only at 

 intervals that the author examined the rocks on the Carnarvonshire 

 side of the Menai Straits. The strata on the shore rise for the most 

 part into considerable cliffs. 



{a.) Carnarvon. — The hill to the north-east of Carnarvon consists 

 of a light brown or yellowish felspar porphyry ; and this mass runs 

 southward more than a mile. On the west the porphyry is overlaid 

 by a brown or liver-coloured shale, which, near the igneous rock, is 

 much contorted and altered. Carnarvon Castle stands on this shale, 

 which may be seen in an unaltered state in the railroad cuttings 

 south-west of the town. The usual dip of the bedding, when undis- 

 turbed, is south-east 45°; the cleavage, which is very decided, dips 

 south 70°. Although the author could not discover any fossils in 

 the shale, yet, from its mineral character, he assigns it without he- 

 sitation to the Ludlow series. 



To the north-east of the porphyritic hills above mentioned, for 

 the distance of some miles, the beds are covered up by gravel. 



(3.) Bangor. — At the Menai Bridge, mountain limestone forms 

 the base of the cliff. Above the limestone are beds of calcareous 

 sandstone alternating with carbonaceous shale. The sandstone con- 

 tains large Producti ; the shale contains vegetable impressions. 

 These beds are overlaid by alternations of ferruginous sandstone 

 and dark shale belonging to the coal-measures. At the top of the 

 cliff is an impure sandy limestone, mixed up irregularly with calca- 

 reous conglomerate, and belonging to the magnesian limestone. These 

 three formations are here conformable, and dip S.S.E. 5°. About 

 one-third of a mile from the bridge they are cut off by a fault, 

 which runs north-east, and comes out on the shore about a mile 

 beyond the bridge. 



On the east side of the fault is a narrow ridge, running south-west 

 by south, consisting of thick beds of hard, coarse, siliceous conglo- 



* Mr. A. Aikin described this enormous deposit of gravel in his * Tour in North 

 Wales.' 

 t Three miles south-west of Penman-mawr. 



