Some clays, when dry, appear to be stable and firm, though they are often drift or dried-up mud simply requiring the influence of water and the atmosphere to cause them to return to their original state; the brown and boulder-clays are of this nature and are distinctly perishing, and therefore treacherous clays. As in clay and all retentive soils, water is always present, and as the impervious nature of clay prevents water gravitating and being drained, and as owing to wet seasons the permeable portion of the clay may gradually become overcharged and be reduced to a muddy condition, slips may occur years after a clay cutting has been excavated or an embankment deposited. In clay embankments the greater weight upon the centre may gradually press water towards the slopes and cause them to slip; hence the value of covering the formation in order to lessen percolation. Cracks and fissures may be so produced in clay embankments, and the danger is that they form sliding surfaces and cavities from the presence of water, and gradually soften the interior until it fails from the thrust of the earth and want of uniform support; therefore, clay earths are more difficult to treat than granular soils, as local weakness is the cause of failure, a mass may be sound, but a crack or fissure may soon become large and pass through an embankment to the seat or the slopes. Although the filtration of still water may not cause an embankment to slip as long as the water merely restores it to its normal absorbing power, when that is exceeded the percolating water will be dammed-up and cause hydrostatic pressure and a soddened surface, destroy cohesion, and disintegrate the particles. When clay is contained within the walls of a building, injurious action and unequal settlement may arise, as the earth inside and outside may be in a different condition; consequently in soils that expand and contract there may be external or internal pressure according to the state of dampness or dryness, consequent upon the ground outside being subject to weather influences and the contained earth being comparatively free from such operation. Shale, whether black or brown, may become decomposed by water and be softened by time, and is either loose or firm. If loose, it requires protection, and the surface should not be exposed to the weather. The presence of iron pyrites in shale has been found to be a cause of its becoming treacherous when water has access to it, as the mass becomes decomposed.