In endeavouring to ascertain the probability of a slip occurring, not only should the superficial strata be considered, but also the original formation of the country; for instance, drift-soil, which is generally met with upon the surface of sloping rocks, may consist of various earths intermixed in endless variety, and in every conceivable shape, and is not necessarily produced by a weathering of the rock upon which it lies, for it may have been brought from a distance. In any case, drift-soil is the result of decomposition and disintegration, and from its nature is unreliable and ever subject to change, to slip, and to subside, and so are most of the glacial deposits and moraine found in mountainous countries; and whenever the contour of a district is irregular and has numerous clefts, soft and marshy places, valleys and hills, earthworks will require to be protected against slips; also, should a cutting be at the base of a cliff or hill, it will probably have to be excavated in drift deposit and, perhaps, in silt if below the water-level of adjacent sea or river, and the ground dips towards the natural outfall of the land-waters. Such drift-soil may be alternately dry and charged with water from the rocks above, especially if they are much fissured and water-bearing and permit easy percolation of water, and must always be in a state of mutability. ROCK. With regard to slips in rock, or earth generally classed as rock, the unstratified or igneous rocks, although they are sometimes traversed by mineral veins and dykes, are the less likely to slip; but rocks liable to surface decomposition and disintegration, such as some varieties of basalt and clay-slate, which latter by atmospheric and aqueous action will partly return to its original state of being fine mud, thrown down from the metamorphic rocks, may change their condition and are likely to slip; and also limestone rock, which however resists the eroding action of water better than sandstone, may become separated by frost although its surface soon dries. Simply knowing the general character of a rock without ascertaining the proportion, state of the different particles of which it is composed, and whether any metamorphic action has taken place, is not necessarily a reliable guide to its stability. In districts situated at a high level, rocks are usually less permeable than in low-lying lands, and the surface discharge is greater and quicker because of the increased rainfall, and less absorption and retention of water.