osed of rubble or
concrete blocks, or generally a combination of the two, carried up from
the sea-bottom, on the top of which some form of solid superstructure
is erected. This superstructure reduces considerably the amount of
materials required (which, on account of the slopes of the mound,
increases rapidly with the height) in proportion to the depth at which
the superstructure is founded; and the solid capping on the mound
serves also to protect the top of the mound from the action of the
waves. In the case, however, of a mound breakwater, portions of the
highest waves generally pass over the top of the mound, and also to
some extent expend their force in passing through the interstices
between the blocks; whereas a superstructure presents a solid face to
the impact of the waves. A superstructure, accordingly, must be very
strongly built in proportion to the exposure, and also to the size of
the waves liable to reach it, which depends upon the height and
flatness of the slope of the mound just in front of it on the sea side.
Special care, moreover, has to be taken to prevent the superstructure
from being undermined; for the waves in storms, dashing up against this
nearly vertical, solid obstacle, tend in their recoil down the face to
scour out the materials of the mound at the outer toe of the
superstructure, and thereby undermine it, especially where the
superstructure is founded on the mound near low-water level, and there
is, therefore, no adequate cushion of water above the mound to diminish
the effect of the recoil on the foundation.

The mound constituting the lower portion of the composite type of
breakwater has been formed in the same varied way as simple mound
breakwaters, namely, of rubble, sorted rubble, rubble protected by
concrete blocks, and wholly of concrete blocks. The only differences
introduced in the mound in this case are, that it is not carried up so
high, that the top portion covered by the superstructure needs no
further protection, and that special protection has to be provided on
the slope of the mound adjacent to the outer toe of the
superstructure.

Superstructures.

The forms of the superstructures exhibit considerable variations,
ranging from a few concrete blocks laid in courses on the top of the
mound, or a paving furnishing a quay protected by a narrow parapet
wall on the sea side, up to a large, solid st