THE BEAVERS OF NORTH AMERICA 35 
done, and beavers vary in the quality of their work 
just as men do, the structure is finally practically 
watertight. Yet the word finally is scarcely the 
right word to apply to a dam, for so long as the 
‘beaver have any need of it they continue adding 
to both its height and length. What begins by 
being a perfectly complete dam, perhaps twenty 
feet long and a foot or two in height, ends with 
a length of many hundreds of feet and a height 
of six or seven feet or even more. Besides the, 
above-mentioned materials stones are frequently 
employed in the construction of dams. In fact it 
is rare to find any that have not at least a few 
stones worked in with the sod, particularly 
towards the ends. When so few are used it is 
hard to say what purpose they serve. But 
there are instances of dams being built in 
which stones form the larger part of the 
material. These are not common and I have 
never seen but one example. Unfortunately the 
photograph shows only those stones which are above 
water, where there is a fair proportion of other 
material, but below the surface there was little 
else than stones of small size, none weighing more 
than three or four pounds. Very much larger- 
ones are frequently used; in fact, I have been 
told by trappers of some that weighed about thirty 
or forty pounds and even more, and Mills speaks 
of stones weighing upwards of one hundred and 
twenty pounds being moved into position on the 
D2 
