and also the liability to damage by sudden floods during the progress of
the works, dams of Portland cement concrete, on account of their quick
consolidation, possess advantages over those of hydraulic masonry apart
from the necessity in the latter instance of constant supervision to
prevent "scamping" by leaving chinks and spaces vacant, especially where
large masses of stone or Cyclopean rubble are used.

Again, should the dam be drowned by flood during its erection, no harm
would accrue were it composed of Portland cement concrete, whereas should
it be of hydraulic mortar masonry, the wall would probably be destroyed
or, at all events, considerably injured by the mortar being washed out of
the joints. Portland cement, however, is only suitable for situations
where the foundation is absolutely firm, as, should there be the
slightest settlement, fissures would certainly be produced.

As regards foundations, the dam of the Puentes reservoir in Spain is
somewhat remarkable--see Fig. 12. Its height is 164 ft., and the profile
or cross section is of precisely the same character as that of the
Alicante dam, the latter being 135 ft. in height, 65 ft. wide at the
crest, and 65 ft. at the base, and erected about 300 years ago. At the
Puentes dam the flanks of the valley were reliable, but, as must be
frequently the case in such situations, the bed of the valley was
composed to a great depth of gravel, _debris_, and shaky strata. The
difficulty was overcome by throwing an arch, or arches, across the
valley, the abutments being formed by the solid rock on each side, and
building the dam upon this arching and filling in below the latter down
to a sufficient depth with walling.

Bearing in mind the sudden and great floods to which dams constructed in
such situations must be subjected, and, if the valley be very narrow, the