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Vol. 66~J TKE3IADOC SLATES OF SOUTH-EAST CARNARVONSHIRE. 153 



can be traced with great accuracy. Across the northern end of 

 the district about Gwern-ddwyryd and on to Coed-yr-Eglwys above 

 Penmorfa Church, the outcrop is cut out by a fault; but from 

 St. Beuno's Church it is readily followed through Wern and 

 Bron-y-foel round the southern slopes of Moel-y-gest to Llanerch, 

 and again from the head of Llyu Gareg-wen to the bay of 

 Borth-y-gest. 



The best exposures are the cliff-section by the railway at 

 Ogof-ddu, the road-and-stream section below Penmorfa Church, the 

 old Bron-y-foel slate-quarry, and the field banks between Gareg-wen 

 and Borth. The section at Ogof-ddu is as follows : — 



Thickness in feet, 



Blocky tough blue slate, weathering dark, which clinks when 



struck. Used for buildings. It makes black walls, but rusts 



to a foxy-brown colour on inside joints. It contains small 



pyrito-calcareous nodules, which weather to white elongate 



spots upon the surface 12 



Ill-exposed splintery blue rock, weathering dark and with yellow 

 rust on the inside joints. White pyrito-calcareous spots in 



the lowest 4 feet 20 



Hard banded blue grit, with pin-head particles bedded in ribs, 



each about 4 inches thick 6 



Kusty grey-blue slates with paler bands in the upper part, be- 

 coming darker below 15 



Finely-banded (30 bands to an inch) blue-black slate, breaking 

 into long splinters and weathering to a dark colour, very 



rusty 6 



Dead-black rock, with blue-grey streaks, homogeneous, and 



breaking into splinters. Contains some spangles of mica 10 



Finely banded, splintery, blue-black slates. Very rusty. Minutely 

 jointed and with rust working in from the joints. The rock- 

 cores bleach to a bluish grey. Agnostus Band with Peltura, 

 Dikellocephahts, and numerous other fossils 7 feet from the 

 top and generally throughout ; small specimens of Orthis len- 

 ticularis abundant in the lower part. These slates generally 



break into bands about 2 to 6 inches thick 18 



Granular felspathic ash-band 2 inches to 1 ft. 



Hard, dark blue-grey or black, gritty slate with three rows 

 of large concretions of iron carbonate. The dominant line of 

 concretions seems to have grown round discontinous masses of 

 a 2-inch bed of coarse gritty material, which contains angular 

 pebbles up to I inch in diameter. The largest concretion is 

 an ellipsoid measuring 3 by 4 by 2 feet, which has obviously 

 grown in situ, distorting the slates in contact with it, and 

 making the bands within it swell out to about 10 times their 

 original thickness. The non-concretionary slate is banded 

 with about 20 beds to the inch, and contains abundant 



Sphcerophthalmus 12 



Finely banded (40 to inch) black rock, with a dark streak, be- 

 coming fine-grained below, containing small pyrite crystals 

 throughout. When the rock has become rusty and has lost its 



pyrite in the weathering, the banding is very prominent 12 



Not exposed 6 



Eather hard, splintery black slate, with a dark-blue streak. 

 Banded, and with thin ashy bands up to a quarter of an inch 

 thick at intervals of about 3 inches. Mica everywhere 

 abundant. Small Obolus and still smaller Orthis not rare. 

 A concretion measuring 6 by 4 by 3 feet lies in the midst 12 



130 



