e, and
in a less effective manner at Civita Vecchia and Naples. At
Marseilles, however, when the breakwater reached great depths, the
quay was abandoned on account of the increased exposure, and the
extension made of a simple rubble mound, protected on the sea side,
from the top down to 20 ft. below sea-level, by large concrete blocks
deposited at random.

[Illustration: FIG. 5.--Marseilles Breakwater, central portion.]

The superstructures at Holyhead and Portland, being built on the old
weak system of a sea wall and a harbour wall, with rubble filling
between, are protected on the sea side by raising the rubble against
them from low water up to high water of spring tides; whereas the
superstructure of Cherbourg breakwater, being built solid and less
exposed, is only protected on the sea side by large rubble and some
concrete blocks, forming an apron raised slightly above low water.
These three breakwaters are provided with a quay sheltered by a raised
wall or promenade on the sea side; but as the mound on the harbour
side is raised up to, or a little above low water, the quay is only
accessible for vessels near high water. This, however, is of
comparatively little importance, since these quays, though very useful
for access to the end of the breakwater in fairly calm weather, are
inaccessible in exposed situations with a rough sea; and quays for the
accommodation of vessels are better provided well within the sheltered
harbour.

The outer portions of the main breakwaters at Genoa and at Naples
(fig. 6), extending into depths of about 75 ft. and 110 ft.
respectively, have been provided with superstructures, similar in
type, but more solid than the superstructure at Marseilles; and the
sorted rubble mounds upon which the superstructures rest are protected
on the sea slope by stepped courses of concrete blocks from a depth of
26 ft. below sea-level, covered over at the top by a masonry apron
forming a prolongation of the superstructure. The outer extension of
the main breakwater at Civita Vecchia furnishes an interesting example
of a composite form of breakwater, in which the rubble mound has been
protected, and greatly reduced in volume and extent in deep water, by
stepped courses of concrete blocks carried up from near the bottom of
the mound (fig. 7).

[Illustration: FIG. 6.--San Vincenzo Breakwater, Naples.]

The breakwaters in front of