188. _MARL is a combination of clay, silex (76), and lime: and is denominated calcareous, argillaceous, or siliceous, as the lime, clay, or silex, is most abundant._ The calcareous part of marl is frequently composed of shells, whence it frequently has the name of _shell marl_; and where these are predominant, it affords an excellent manure for sandy, dry, gravelly, or light lands. Marl likewise produces very beneficial effects on mossy and clayey soils; and these effects, where it has been properly applied, have been observable for twelve or fourteen years. Some kinds of marl that contain but a small portion of lime have been successfully used in the manufacture of earthenware. This mineral is usually found at the depth of from five to nine feet beneath the surface of the ground, and deposited between beds of clay and sand. It is dug out with spades; and, in the digging of it, in Ireland, the workmen not unfrequently meet with the horns of deer and other curious fossils. The usual mode by which persons, generally unacquainted with minerals, distinguish this from other clayey substances, is, to break a small piece of dry marl into a glass of vinegar. If it be marl it will immediately dissolve with considerable effervescence; and the briskness of the effervescence will be in proportion to the quantity of lime which it contains. 189. _FLORENCE MARBLE is a kind of indurated or hardened marl, and is remarkable for presenting, when polished, the appearance of ruined edifices or rocks._ This kind of marble is never used in architecture. Little slabs of it are cut for Mosaic work, and to be framed like pictures; and the latter, when of considerable dimensions, are sometimes purchased at a high price. If held at a distance from the eye, an inexperienced observer might mistake a slab of Florence marble for a drawing in bistre. Here, observes a French writer, we remark a shattered Gothic castle, there the mouldering fragments of a cathedral; in one part ruined walls, and in another shattered bastions and towers. But, when we approach the picture, the illusion vanishes, and those imaginary figures which, at a distance, appeared to be so correctly drawn, become changed into irregular spots, lines, and shades, which present nothing distinct to the view. 190. COTTAM MARBLE, which, when cut and polished, also exhibits the appearance of a landscape, is a kind of compact marl. It has its name from being found at Cottam, near Bristol. 191. _LIAS, or CALP is a kind of limestone of bluish black, or greyish blue colour, and composed chiefly of lime, silex (76), clay, and oxide of iron (21)._