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CLAY-SLATE. 289 Uses of Slate. It is scarcely necessary to attempt an enumeration o the uses to which slate, such as that of North Wales, is applicable. The superiority both in durability, and especially in regard to light ness (or less weight), of Welsh slate for roofing pur poses, has caused it to supersede, since the introduc tion of canals and railways, those bedded fissile tile- stones (sometimes erroneously called ‘slates’) which were in use in many parts of the country, and which were derived from the Upper Silurian, the Carbonife rous, and the Oolitic formations. 1 But while in some respects this has been an advantage, it has been otherwise as regards architectural taste; for while the heavy tilestones were employed, high pitched roofs were a necessary consequence, but the introduc tion of the Welsh slates has been generally accom panied by the construction of flattened roofs; an alteration certainly not in the direction of architec tural beauty. Besides its use as a roofing material, slate is very properly applied to the construction of cisterns for holding water, thus doing away with the objectionable use of lead for such purposes. It is also used for ornamental structures, and as a substitute for marble; but although it is capable of receiving a fair polish, 1 Viz., the tilestones of the Upper Ludlow, the thin flags, or tiles, of the Millstone or Yoredale series, and the Stonesfield and Collyweston slates of Gloucestershire, Oxfordshire, &c. U