Pure clay shrinks some 5 per cent. in drying, the contraction being less as sand is present in it, for when it is mixed with twice its weight of sand it is reduced to 3 per cent., and as impurities increase in clays the less impervious they become. Most clays have silicious earth in them, but if sand is present the clay is then more open, and water will permeate and drain more freely; but mixtures of clay and sand may assume a pulpy condition when impregnated with water, consequently it is always advisable to test such earth. The varieties of sandy clay are many, and all are usually more or less unstable. Among them may be named red clay with sand and mica, blue sandy clay, sandy green clay, stiff red sandy clay, the loamy clays of various hues, dark grey, red sandy, and black clayey loams. If clay could be kept in a moist state fissures would seldom occur. The constant alternation of wetness and dryness creates the fissures, and water completes the disintegration. There are a few clays which are stable when kept in solid masses, as then only a small surface is affected by air and water, but if they are loosened and broken up, as in the process of excavation and deposition, they readily become in a muddy condition. Any mud or silt which may be soft and readily pressed when wet, but cakes and shrinks in to detached lumps when dry or upon being exposed to the atmosphere, is a treacherous soil, as it will return to its original state upon becoming wet. Clays or any soils that cake should always be regarded with suspicion, as although having the appearance of solidity and the possession of weather-resisting qualities in their natural position, when disturbed, quickly become worthless for earthwork purposes, and may stand in one situation almost as a soft clay, and when disturbed and wet assume a horizontal surface. Such ground may repose at a 4 to 1 TO 8 to 1 slope, because its crust has become caked or case-hardened, yet when it is broken it may become, upon being exposed, simply fluid mud. To prevent clay soil weathering upon the surface, a layer of gravel 1 foot or so in thickness has been placed upon it, the idea being that it is not only a protection, but the weight of the covering upon the clay will cause the water to be pressed out from the soil into the gravel through which it can percolate to the drains.