tected, over on to
the harbour slope, and in recoiling down the outer slope, draw down
the stones on the face, so that the top and sea slope of the mound
need replenishing with a fresh deposit of stones after severe storms.

[Illustration: FIG. 1.--Table Bay Breakwater]

Under the action of the breaking and recoiling waves, the mound
assumes a very flat slope on the sea side, from a few feet above
high-water down to several feet below low-wafer level (fig. 1). The
flatness of the sea slope depends on the exposure of the site, and the
limited size of the stones covering the outer portion of the mound;
and its extent increases with the range of tide, as a large tidal rise
exposes a greater length of slope to the action of the waves. This
flattening of the sea slope greatly increases the amount of stone
required for a rubble-mound breakwater, in proportion to the exposure
and the range of tide; and the amount is also affected, but in a
proportionately minor degree, by the depth in which the breakwater is
situated. In order to avoid the injuries to which an ordinary rubble
mound is subjected by waves, certain methods have been devised for
protecting the top and sea slope of the mound. For instance, the upper
portion of Plymouth breakwater has been covered over by granite paving
set in cement, to diminish the displacement of the stones by the
waves. Frequently, on the continent of Europe, rubble mounds have been
formed of materials so sorted that the smallest stones are placed in
the centre of the lower part of the mound, and covered over along the
slopes and top by layers of larger stones, increasing in size towards
the outer part of the mound, so that the largest stones obtainable are
deposited on the outside, and especially on the top and sea slope of
the mound. This is, no doubt, theoretically the correct method of
construction of rubble mounds exposed to the sea; but it involves a
considerable amount of trouble and expense.

Concrete blocks with rubble mound.

Practically the chief point of importance is to cover the outer slope
and the top of the mound with the largest stones that can be procured,
and where large stones are not readily obtainable concrete blocks
furnish a very convenient substitute. These blocks are generally
deposited as the outer covering on the top and sea slope of a rubble
mound, as for example at the mound breakwaters i