The chief danger is from variation of the earth in the foundations and the existence of thin seams of an unstable character, or fissures in rocky soil allowing percolation of water, most difficult, if not bordering upon the impossible, to discover unless by trial pits and borings over the whole area of the site of a dock, which would be a most unwise proceeding, as has been previously named in this chapter. The marvellous labours of geologists and others have enabled an accurate opinion to be formed of the locality and the depth at which water and certain minerals are to be found and also to indicate the earths in which fissures and seams are frequent; but it must ever be impossible for man to absolutely state that no unstable seams or veins exist over a considerable area of ground, more especially in the superficial beds upon which a dock-wall has to be constructed, although the probability of their presence may be determined with some degree of precision. The importance of a complete examination of the ground of the site and neighbourhood is imperative, and particularly of any cuttings and embankments in the locality. In Chapter II. many conditions of earth are referred to in which slips and movement are probable, but that now under consideration is usually caused by a wall resting upon seams or weak veins of unstable soil; therefore, there is danger when a comparatively thin seam of soft slimy earth interposes between the hard bed upon which the foundations may rest and one below the vein; for, but a little additional moisture may sufficiently lubricate the surface to enable the firm bed to slide upon it, the frictional resistance being thereby so reduced that movement results. Similarly, in the case of a seam of gravel overlying clay, the firm gravel bed may slide upon the clay: the foundations should then be carried down to the clay. Piles at the base driven into the gravel will be of little use; in fact, for permanent work, they are now almost abandoned except for jetties, cills, aprons, dolphins, &c., for uniformity of support is very difficult to obtain in pilework and cannot be proved to exist; but piles may be useful to help to sustain a structure until it has taken its permanent bearing and is in possession of its full strength. For purposes of permanent support they are somewhat unreliable, for failures have happened because of their weakness, and walls have consequently been overturned; their chief use is as auxiliary temporary aids to lateral stability, such as confining concrete or lessening the percolation of water. However, the earth may be in such a condition that it cannot retain them in position when they are strained, then they are useless for lateral support. Their resistance to horizontal strain varies much with the nature of the soil and not necessarily according to its cohesive power; for instance, experiments have shown sand to afford the greatest horizontal