resistance; clay, less; and in loose ashes the power is further reduced. When a clay stratum is thin and overlies gravel or sand, provided the necessary precautions are taken that the sand does not “boil,” it is better to have the foundations upon the underlying gravel or sand, as the layer of clay may bulge and slide upon the gravel and thrust out the wall. Should a stratum of clay be superimposed upon another of similar character, it will always be in a damp state conducive to lateral instability, as there will be two sliding surfaces, and the upper may be forced forward and carry the wall with it, and should a seam of sand be interposed it may become “quick” and flow away. In such cases the foundations should extend to a safe depth in the lower clay stratum or be below the unstable seam, and when the depth of silt or unstable soil overlying a firm bed, as rock, is considerable, the well system can be adopted, provided the bottom is levelled. As seams, weak veins, and fissures are so frequently met with the examples might be continued almost _ad infinitum_. To ascertain whether they exist and their location is one of the primary precautions to be observed, as dock-walls usually fail from bulging or slipping forward, causing fracture or overturning. Additional weight or increase of the thickness of a wall may not suffice to arrest movement of the base or forward motion of the earth. To place the foundations at a greater depth may be impracticable, although it may be the best method of restoration; it then becomes necessary to insert an invert or strut between the walls in a narrow dock, or in a large dock to attempt to remove the unstable seam in front of the wall and prevent movement of the vein by a curtain-wall, additional weight, or other means of consolidation; or by removing the solid backing, draining the back, and the erection of a timber platform instead of the earth, so that it reposes at its natural slope without creating a thrust upon the wall; or by having packed rubble filling instead of ordinary solid backing so as to remove or lighten the lateral pressure upon it, by draining the ground at the back; or by counterforts from the foundation to the ground level, at the front of the wall, especial care being taken to prevent their parting from the main wall.