The stability of a sandstone for earthwork and purposes of construction is dependent upon the material which cements or holds it together, whether iron rust, lime, free silica, alumina, &c., &c., &c., its quantity and condition, and the degree of hardness imparted when it was formed, and the nature of the agglutinant; therefore the varieties and degrees of fineness and hardness are numerous. Sandstones are generally found to be laminated when hard; and bare of vegetation if pure and free from marls; they contain and part with water in different proportions, and sometimes have watertight bands crossing them, severing water communication, which may cause earthworks to be of unequal stability. The firmest and strongest are close-grained and fine in texture, the weaker are coarse and gritty, and have a sandy appearance. They may be white, yellow, green, black, red, grey, brown, or other colour, and although of the same hue, their character may not be identical; for instance, the red sandstone is hard and also very soft. Sandstone of a greenish hue is generally hard, much fissured and full of water. When firm, greensand may stand at a steep slope, the surface being protected; but it varies considerably, and may at one place be close and yet be gradually deteriorated until it is of the character of fine loose sand. Should it be found upon excavating sandstone that it is not upon its natural bed, but distorted, upheaved, or vertical, it will probably split and become detached under the destructive action of air and water. In tropical climates it has been found that sandstones generally dissolve and become disintegrated when used in damp foundations. Limestone also varies much in character, and is treacherous, whether it is hard or soft, when pockets of clay or sand are present and beds of clay are contiguous. Percolated water having carbonic acid in it may also soften or dissolve it. The softer kinds if in fragments as ballast, or when deposited in an embankment, often become quickly disintegrated by frost and the weather, as do sandstones.