Sketch of the Structure of the country extending from Cader Idris to Moel Siabod, North Wales

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Abstract
The rocks are arranged into the following groups, of which the names are only provisional, and the thicknesses assigned from rough estimation. In the ascending order:— A. The Barmouth and Harlech sandstones, 3000 feet. A mass of quartzose sandstone and conglomerate, with some beds of blue and purple slates, and occasionally trap rocks. No organic remains discovered in this group. B. The Trappæan group, 15,000 feet: subdivided into— B 1. Blue and grey schistose slates and flagstones, interstratified with many beds of a grey calcareo-feldspathic “ash” often crystalline, together with feldspathic trap and greenstone; the slaty rocks predominating. B 2. Great masses of feldspar-porphyry and some greenstone, with feldspathic trappsean “ash” arranged in beds a few inches or many feet in thickness. Interstratified with these, and passing into them (especially into the ash) by almost imperceptible gradations, are many beds of black slate, forming often irregular and apparently lenticular-shaped masses. In some parts of B 1, Lingulæ are found in great abundance, and a few other fossils. In B 2 Lingulæ and Graptolites also occur, but not very abundantly. C. The Bala group, 9000 feet: subdivided into— C 1. Black slates of variable thickness, very fine-grained, brittle, and frequently having their true lamination or stratification entirely obscured by cleavage and numerous joints. C 2. Grey, fine-grained, arenaceous slate-rock , often passing into a hard, compact, splintery gritstone; in its lower portion are one or two beds of trappæan ash, sometimes crystalline, sometimes flaky and sometimes brecciated, often highly calcareous. About its centre is one thin bed of impure limestone (Bala limestone), and sometimes, but very rarely, another little band in its upper portion (Hirnant limestone). Organic remains are very rare in C 1; but in C 2, as is well known, are very abundant and in great variety.

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