Solid blue clay, which generally requires the use of the pick, of the clays is, perhaps, the most stable, being almost impermeable if free from delaceration; but yellow and most other clays are unequal in texture, faults and breaks are frequently numerous, and water penetrating converts the surfaces and the mass into a muddy and semi-fluid condition resting only when horizontal, which has been painfully experienced in the crushing in, during construction, of some tunnels in the London district. The trickling of water down fissures forms a slimy and easy-sliding surface most difficult to treat or prevent, and so long as the natural contour of the ground does not offer resistance to movement, a slip may extend for a long distance, either in deep or shallow cuttings, and there may be considerable hydrostatic pressure. The disruptions, variableness of character, and existence of fissures cause any but the most homogeneous clays to be treacherous and likely to slip, particularly the yellow and any laminated clays, as they allow water to enter by the veins which are usually frequent in the mass. Yellow clay has a greater tendency to crack upon drying than blue clay and does so much more quickly, hence its dangerous nature, and although a mass may be only damp, fissures will enable water to penetrate and reduce it to a state of instability; it is also not infrequently in a plastic state, having fissures and cavities full of water. It should, however, not be forgotten that in the endeavour to prevent the deleterious effects of aqueous action upon clay soils that they may be over-drained, so that they become too dry, as then the clay will shrink, crack, and fissure; the chief aim should be to keep it always in a sufficiently moist state so as to obviate the formation of cracks and fissures, and at the same time cause it to be dry enough to be firm, and never in a pasty or pulpy condition; in other words, to maintain its natural state if one of stability, and prevent any excess of water penetrating it or reaching its surface. The lias clays are treacherous chiefly owing to the presence of much calcareous matter, and therefore approach a marly state; heavy slips have occurred in the has formation in the midland counties of England, notwithstanding that a slope was adopted which experience had shown produced stability, namely 3 to 1. A slight variation in the composition of this soil or an unfavourable position will cause a slip in such treacherous earth.