PART XIII. ROOFING SLATE. CHAPTER I. CLAY-SLATE. Gemeiner Thonschiefer [Germ) ; schiste argileux commun (Fr.). Clay-slate is a sedimentary argillaceous rock, generally compact and fine-grained, of colours vary ing from grey to purple, green, and even black, and splitting along planes of cleavage which only ac cidentally coincide with those of bedding. These slates are chiefly used for roofing houses and public buildings, and are valuable in proportion to their compactness and durability, the smoothness of the surface which they present, their uniformity of colour, and incapacity for absorbing water. Cleavage. The phenomena of cleavage have en gaged the attention of physical geologists, both in our own country and abroad, and especial light has been shed on its origin by the observations of Mr. D. Sharpe 1 and Mr. Sorby, 2 who, having subjected cleaved 1 Journ. Geol. Soc. Lond. iii. 74, and v. 3. 2 Edinb. New Phil. Journ. iv. 137 (1853).