n deep water
sheltering Algiers harbour, and at the French parts of Cette and Bona
on the Mediterranean; whilst they furnish the protection of the top
and upper part of the sea slope of the rubble-mound extension of
Marseilles breakwater down to 20 ft. below sea-level. At Alexandria,
concrete blocks compose the outer half of the mound, sheltering the
inner half consisting of small rubble (fig. 2); at Biarritz the mound
breakwater is formed mainly of concrete blocks, with rubble stone
filling the interstices and on the top; whereas at the outer end of
the western breakwater at Port Said, protecting the entrance to the
Suez Canal, a bottom layer of rubble is surmounted by concrete blocks.
These blocks are generally deposited at random; but at Cette (fig. 3),
and at the breakwater in deep water at Civita Vecchia, the concrete
blocks covering the rubble have been laid in stepped, horizontal
courses. This arrangement necessitates more care and better appliances
in construction; but, in compensation, the blocks so placed are less
exposed to disturbance and injury by the waves.

[Illustration: FIG. 2.--Alexandria Breakwater.]

[Illustration: FIG. 3.--Cette Breakwater.]

Concrete blocks possess the great advantages for breakwaters that they
can be made wherever sand and shingle can be procured, and of a size
only limited by the appliances which are available for handling them.
In fact, in places where stone of any kind is difficult to procure at
a reasonable cost, as for instance at Port Said, concrete blocks are
indispensable for the construction of breakwaters. Large concrete
blocks, moreover, by enabling a comparatively steep slope to be formed
with them on the sea side of a mound breakwater, reduce considerably
the amount of materials required, especially at exposed sites, and
also for breakwaters extended into deep water, such as those of
Algiers and Marseilles.

[Illustration: FIG.4.--Port Said Western Breakwater.]

Concrete block mound.

Occasionally, in the absence of suitable rubble stone, a mound
breakwater has been formed entirely with concrete blocks; and of this
the main portion of the western breakwater at Port Said furnishes a
notable example (fig. 4). Sometimes, in exposed situations, the mounds
of the composite type of breakwaters have been constructed exclusively
with concrete blocks, such, for instance, as in the curved breakw