Should permeable soil lie between the top stratum and a bed of clay, water will accumulate upon the clay, make it slimy and cause a flow upon the bed; for example, when a thin bed of vegetable earth rests upon gravel, sand, or peat, and that upon clay, water will percolate, and perhaps air, through the top soils, and may cause them to slip upon the clay-bed. Also should a layer of clay overlie permeable strata, as clay upon sand, or clay upon gravel, unless it is sufficiently thick and solid to prevent infiltration, it may slide upon the permeable soil as its lower surface becomes wet. When clay-hills have veins, water may accumulate in them and flow, and if very dark yellow clay overlies light yellow calcareous clay, which may rest upon hard blue clay, it is obvious each stratum is somewhat differently affected by weather and air, and therefore movement is to be expected. The edges of clay-hills are always likely to slip, especially should they be in the form of spurs. Boulder-clay is seldom reliable, because, although it may be hard and stand vertically in dry weather, in wet it swells, weathers quickly, becomes soft and cakes upon drying. The stability of such soil is governed not only by the nature of the clay, but by that of the boulders and their effect upon the earth in which they are embedded, and much depends upon the degree of changeableness upon exposure to air and moisture of all the particles of which they are composed; hence boulder-clay, although hard to excavate, may quickly dissolve. On the contrary, it may occasionally be so hard that it seems to be solid rock, and may even resist erosion and weathering as well as if it were rock; but care must be taken to prevent indurated mud being mistaken for solid clay-rock, and therefore it is advisable to test the soil with water. Seams of silt, soft pasty soil, or soapy earth met with in clay, which have become decomposed by atmospheric and aqueous action, are to be feared, and the brown clay, especially when soft: red, or dark yellow clays that break into laminæ and crumble upon exposure to the air, and although tenacious in the flakes and when fresh-cut are loosely held together in bulk, often have thin veins of sand in them; and when water percolates it remains, and is very difficult to drain. It has also been found that when minute non-adhesive particles of mica are present in clay that it will become disintegrated by water, although it may be hard to excavate. Some of the gault clays, although stable when dry, become soapy when wet and are not easily managed, but the bluish grey gault is usually tenacious and almost impermeable. The gault clays have little sand in them but much calcareous matter, and, as a rule, they do not swell and bulge like the London clays.