e
necessarily unfavourable for the insertion of stakes in the ground, if
such were, in fact, their practice in building dams. The theory upon
which beaver-dams are constructed is perfectly simple, and involves no
such necessity. Soft earth, intermixed with vegetable fibre, is used
to form an embankment, with sticks, brush, and poles embedded within
these materials to bind them together, and to impart to them the
requisite solidity to resist the effects both of pressure and of
saturation. Small sticks and brush are used, in the first instance,
with mud and earth and stones for down-weight. Consequently these dams
are extremely rude at their commencement, and they do not attain their
remarkably artistic appearance until after they have been raised to a
considerable height, and have been maintained, by a system of annual
repairs, for a number of years."[108]

[108] L. H. Morgan, _The American Beaver and his Works_,
Philadelphia, 1868, pp. 82-86.

There are two different kinds of beaver-dams, although they are both
constructed on the same principle. One, the stick-dam, consists of
interlaced stick and pole work below, with an embankment of earth
raised with the same material upon the upper or water face. This is
usually found in brooks or large streams with ill-defined banks. The
other, the solid-bank dam, is not so common nor so interesting, and is
usually found on those parts of the same stream where the banks are
well defined, the channel deep, and the current uniform. In this kind
the earth and mud entirely buries the sticks and poles, giving the
whole a solid appearance. In the first kind the surplus water
percolates through the dam along its entire length, while in the
second it is discharged through a single opening in the crest formed
for that purpose.

The materials being prepared in the manner I have previously
described, the animals make ready to establish their dyke. They
intermix their materials--driftwood, green willows, birch, poplars,
etc.--in the bed of the river, with mud and stones, so making a solid
bank, capable of resisting a great force of water; sometimes the trees
will shoot up forming a hedge. The dam has a thickness of from three
to four metres at the base, and about sixty centimetres at the upper
part. The wall facing up-stream is sloping, that directed down-stream
is vertical; this is the best arrangement for supporting the pressure
of the mass of water which is thus expended on an