Humus, or vegetable soil, is a dark-coloured loam containing the results of the decay of vegetable matter. Sometimes, as in the black earth of Russia, it may contain six or ten per cent. of organic matter, and it may cover thousands of square miles, as in the American prairies. It is very valuablefor agricultural purposes, and in India is known as cotton soil. Geologically, humus is interesting as furnishing a group of little-known but most active humus acids, to which the names humic, ulmic, crenic, and apoerenic acid have been applied, though it is doubtful whether they have been isolated. These acids have (1) a powerful solvent effect upon alkalies, alkaline earths, and even silica: they have (2) a great affinity for oxygen, and thus reduce peroxides to protoxides or to native metal, and sulphates to sulphides; and (3), possibly by neutralising alkaline solutions, they bring about the precipitation of silica, especially in or around organic structures, as in the formation of silicified wood, arid possibly of flint. Much of the solvent action attributed to carbonic acid, because it results in the formation of carbonates, is probably due to them. The red, brown, or yellow peroxide of iron colouring many sandstones, is by them often converted into ferrous oxide and then removed ae carbonate, bleaching the rock or, in intermediate stages, forming layers of lilac sand, such as are often seen under heathy moorland. Copper-ores in contact with decaying wood, or decomposing fish, have been reduced to native copper: native silver has been similarly formed; and gypsum has been converted first into calcium-sulphide, then into calcium-carbonate and sulphuretted hydrogen, and finally into layers of limestone and native sulphur.