ch is
closed by brush, the current can no longer sweep away the earth and
stones used for the embankment.

The third kind is the strongest and easiest to build. It is for places
where big trees lean out over the stream. Three or four beavers gather
about a tree and begin to cut, sitting up on their broad tails. One
stands above them on the bank, apparently directing the work. In a
short time the tree is nearly cut through from the under side. Then
the beaver above begins to cut down carefully. With the first warning
crack he jumps aside, and the tree falls straight across where it is
wanted. All the beavers then disappear and begin cutting the branches
that rest on the bottom. Slowly the tree settles till its trunk is at
the right height to make the top of the dam. The upper branches are
then trimmed close to the trunk, and are woven with alders among the
long stubs sticking down from the trunk into the river bed. Stones,
mud, and brush are used liberally to fill the chinks, and in a
remarkably short time the dam is complete.

When you meet such a dam on the stream you are canoeing don't attempt
to break through. You will find it shorter by several hours to unload
and make a carry.

All the beaver's cutting is done by chisel-edged front teeth. There
are two of these in each jaw, extending a good inch and a half outside
the gums, and meeting at a sharp bevel. The inner sides of the teeth
are softer and wear away faster than the outer, so that the bevel
remains the same; and the action of the upper and lower teeth over
each other keeps them always sharp. They grow so rapidly that a beaver
must be constantly wood cutting to keep them worn down to comfortable
size.

Often on wild streams you find a stick floating down to meet you
showing a fresh cut. You grab it, of course, and say: "Somebody is
camped above here. That stick has just been cut with a sharp knife."
But look closer; see that faint ridge the whole length of the cut, as
if the knife had a tiny gap in its edge. That is where the beaver's
two upper teeth meet, and the edge is not quite perfect. He cut that
stick, thicker than a man's thumb, at a single bite. To cut an alder
having the diameter of a teacup is the work of a minute for the same
tools; and a towering birch tree falls in a remarkably short time when
attacked by three or four beavers. Around the stump of such a tree you
find a pile of two-inch chips, thick, white, clean cut, and arched to
the curv