Havre, constructed in 1896-1907, for
sheltering the altered entrance to the port, were formed of a sorted
rubble mound, protected on the sea slope by concrete blocks, and
raised a little above low water of spring tides, upon which large
blocks of masonry, built on land, were deposited with their upper
surfaces about 18 in. above low water of neap tides. As soon as
settlement of the mound under the action of the sea appeared to have
ceased, these masonry blocks were connected together by filling the
spaces between them with masonry; and a solid masonry superstructure
was built during low tide on this foundation layer, as shown in fig.
8.

[Illustration: FIG. 7.--Civita Vecchia Outer Breakwater.]

The breakwaters constructed for forming harbours on the sea coast of
the United States are almost all rubble-mound breakwaters. The two old
detached breakwaters sheltering Delaware Harbour near the
south-eastern extremity of Delaware Bay, were formed of simple rubble
mounds raised about 13 ft. above low water; but in closing the gap
between them towards the close of the 19th century, the rubble mound
was stopped at low water, and a sort of superstructure, consisting of
stepped courses of large rectangular blocks of stone on the sea and
harbour sides, with tightly packed rubble between them and capped
across the top for a width of 20 ft. with a course of large blocks,
was raised to 14 ft. above low water, resembling, on a small scale,
the upper part of the Civita Vecchia mound (fig. 7). A similar
construction was adopted for the new breakwater formed in 1897-1901
for providing a harbour of refuge at the mouth of Delaware Bay; but in
this instance the mound was made considerably wider at the top, and
had to be protected along the toe of the superstructure on the sea
side by large stones. The same form of superstructure, also, on a
narrower base, was resorted to for a breakwater in deeper water at San
Pedro in California with satisfactory results. When, however, a
breakwater of the Delaware type was in progress for forming a harbour
of refuge in Sandy Bay, Massachusetts, in front of Rockport to the
north of Boston, the upper 13 ft. of the 600 ft. of completed
superstructure were carried away during a severe storm in 1898 leaving
only a portion about 5 ft. in height above low water, the average rise
of tide there being 8-3/5 ft. The design was, accordingly, mo