Welsh Journals

THE NATURAL RESOURCES OF WALES VIII.— SLATE AND MATERIALS OF MINOR IMPORTANCE by F. J. North, D.Sc., F.G.S. (National Museum of Wales) SLATE is, after coal, the most important ot the useful materials obtained from the rocks of Wales, and it is responsible for more than half the total value of the quarry products of the Principality. The quarrying of slate in Wales and its pre- paration for the market gives employment to more than 8,000 people, and although the num- ber of quarries in work is variable, it is usually in the neighbourhood of sixty; of these, about a dozen are very large, the remainder being for the most part comparatively small undertakings. The nature of slate and the means by which it acquired its cleavage (that is, the property ot splitting, to which its commercial value is due) are fully discussed in a publication of the National Museum of Wales [4],* and for present purposes reference need only be made to its dis- tribution. Of the total quantity produced in Wales, Caer- narvonshire provides about two-thirds, Merioneth- shire one-fourth, and Denbighshire, Cardigan- shire, Pembrokeshire, Carmarthenshire and Mont- gomeryshire, in order of importance, the re- mainder. In a recent year the Welsh quarries and mines produced nearly 240,000 tons of slate, valued at over £ 1,900,000. Slate, which results from the alteration of rocks like clay, occurs in regions where such rocks were at one time deeply buried, and were subjected to extreme pressure and considerable heat during periods of earth movement, and it now appears at the surface in places where prolonged denuda- tion has exposed rocks that once formed part of the deeper layers of the earth's crust. Since the older the rock the more likely it is to have been deeply buried and to have been affected by earth movements, it follows that slate is more prevalent among the older rock forma- tions. In Wales there are five principal regions in which slate of economic value is found. They are as follows :­(1) Central Caernarvonshire, where slates occur in beds of Cambrian age; (2) the country around Blaenau Ffestiniog, where the slates are of Ordovician age; Ordovician slates also occur in Snowdonia (3) the country between Towyn and Corris, where both Ordovician and Silurian strata yield slates; (4) the country be- tween Llangollen and Corwen, where they occur in the Wenlock and Ludlow divisions of the *Numbers in brackets refer to papers mentioned in the list of references with which this paper concludes. Silurian formation; (5) the Prescelly district of Pembrokeshire and parts of the adjacent counties, where the slates are of Ordovician age. It should be noted that 'Cambrian,' 'Ordovician' and 'Silur- ian' are names applied to three of the divisions of the older rocks of Wales; the Cambrian is the oldest of the three, and the other names relate to successively newer rocks. In Caernarvonshire, commercially important slates (the oldest in Wales) occur in a narrow region extending for about eleven miles in a south- westerly direction from Bethesda, by way of Llan- beris to Nantlle and Penygroes. Most of the slates are reddish purple or blue in colour, al- though some are green. The two largest quarries (the Penrhyn Quarry of Bethesda and the Dinor- wic Quarry of Llanberis) are at the north-eastern end of the tract. Here the quarries have been opened out on the sides of a mountain, but in the Nantlle district the rock occupies lower ground, and the quarries take the form of deep chasm-like pits. A tract of slate, formed from rocks geologically somewhat younger than those of Bethesda, ex- tends south-westward from Bettws-y-Coed to Llanfrothen; the most valuable slate occurs near the middle of the region, that is, around Blaenau Ffestiniog. The rock is mostly grey, and is, on the whole, finer in grain than the other Welsh slates; some varieties are capable of being split into unusually thin sheets. In this district, the inclination of the beds is such that the useful slates pass rapidly beneath the surface, and they are worked in great underground chambers reached by means of inclined shafts. Slates, also of Ordovician age, occur in the neighbourhood of Abergynolwyn, Corris and Aberllefenni; these, too, extend in a north- easterly and south-westerly direction, and are of some commercial importance. They are blue or grey in colour, and are worked partly in mines and partly in open quarries. In North Pembrokeshire and parts of the ad- jacent counties, the Ordovician rocks have suffered compression, and slaty cleavage has been developed in some of the beds. Near Clydey there are blue slates, and in the neigh- bourhood of the Prescelly range, the slates, which range from olive green to silvery grey, were derived from a fine grained volcanic dust, rather than from an ordinary marine clay. Some of the Prescelly slates are irregularly stained and speckled with shades of red and brown, and are much esteemed where a 'rustic' effect is desired;