A crude test to indicate the probable character of a clay as regards its stability in earthwork is to burn a piece of it and notice the colour. If it becomes white or of a whitish tint, the clay is generally less likely to slip than when it is of a reddish or yellowish tinge. Another rough experiment can also be made. Get a piece of clay, place it in water, and note the time taken and the depth to which the surface has become saturated, and whether it is very slimy and will easily slide down a slightly inclined plane; its tenacity may then be approximately judged. Also by weighing, an idea of the amount of sand may be imagined; the more sand there is in clay the lighter it will be, all other conditions being identical. A comparison between two clays will enable some opinion to be formed of their relative stability in earthwork, though, of course, there are many other features to be considered. All impure clays, such as shaly clay, sandy clay, loamy clay, and marly clay require to be carefully treated, although they may be easier to manage than yellow or brown clay. When two retentive clay beds overlap or overlie, and have no intermediate permeable stratum, they must be in a humid state, as is the case in the Fen country, unless they are constantly drained; but serious slips are not so likely to occur in them as when two masses of clay have an interposing seam of sand or silt liable to be eroded by water falling down fissures in the clay, which probably extend to considerable depths, with the result that two slimy surfaces are formed and the clay slides. Clay underlying gravel often contains numerous pockets and seams filled with running sand, and should there be a permanent head of water the discharge will be in large quantities and at a considerable velocity. A cutting in wet sandy clay is generally treacherous and difficult to manage. As a clay bed near the surface of the ground is sometimes upheaved, if a permeable stratum such as gravel or sand overlies it, the drainage of water through or down the slopes will be arrested, and the earth at the back of the slope will be constantly wet and may ultimately become saturated through the damming back of the water; then a slip may be expected. All upheaved, dislocated, and twisted superficial beds of clay, which will generally be of varying consistency and therefore settle unequally; over or underlying seams of sand or gravel, are likely to slip and subside, and their stability much depends upon whether or not the lie of the beds obstructs the permeation of water. If the dip of the clay-beds is towards the natural outfall, most probably an adjacent river, slips are probable because of the creation of sliding surfaces and the continuity of the beds being destroyed by a cutting and the consequent loss of support.