When the land at the margin of the water consists of unconsolidated material, or of fragmental material but slightly cemented, the impact of the water is sufficient to displace or erode it. If weak rock be associated with resistant rock within the zone of wave-work, the removal of the former may lead to the disruption and fall of the latter, especially when weak rock is washed out from beneath the strong. The impact of the water is competent also to break up and remove rock which was once resistant, but which has been superficially weakened by changes of temperature. Rock affected by numerous open joints is likewise attacked with success, for by the dash of the waves the blocks between the joints may be loosened and literally quarried out. It may, however, be doubted whether the dash of waves of clear water, even when their force is many tons to the square foot, has any appreciable power to wear rock which is thoroughly solid. [Illustration: +Fig.+ 301.—Angular blocks of rock which have fallen from the cliff above, as a result of undercutting by the waves. Grand Island, Lake Champlain. The rock is Black River limestone. Although from the shore of a lake instead of the sea, the principles illustrated are the same. (Perry.)] The impact of the waves is generally reinforced and made effective by the impact of the detritus they carry. The sand, the pebbles, and such stones as the waves can move are used as weapons of attack, being turned against one another and against the shore. Masses of rock too large for the waves to move (Fig. 301) are worn by the detritus driven back and forth over them, and in time reduced to movable dimensions (Fig. 302). They then become the tools of the waves, and in use, are reduced to smaller and smaller size. Thus bowlders are reduced to cobbles, cobbles to pebbles, pebbles to sand, and sand to silt. The silt is readily held in suspension in agitated water, and thus is carried out beyond the range of breakers, and settles in water so deep as not to be effectively agitated to its bottom. Thus one generation of bowlders after another is worn out, and the comminuted products are carried out from the immediate shore and deposited in deeper water.