MAMMALS OF MINNESOTA. Aiea 
firm banks. The dam is begun by laying sticks in regular 
order parallel to the current and loading with rocks and mud. 
Nothing of the nature of stakes or piles is employed. The 
poles may be twelve feet long, and are arranged with great 
precision. The inner slope is more abrupt, and is faced with 
earth and turf, forming below an impervious layer. The dam 
requires constant repairing, and may be added to annually 
until it becomes a very considerable and solid structure. The — 
upper part of the dam being more permeable than the lower, 
the water is permitted to percolate through without gathering 
head at any place, thus preventing injury. The best authori- 
ties state that there is no co-operatiou in the work of repairing, 
but that each beaver examines and builds as he sees fit. Others 
claim that there is a systematic supervision by older members 
of the clan and appointed relays, reserves and the like. This 
may be due to the close concert of action observed. When 
busily engaged upon their structures, carrying earth and trow- 
eling it upon the inside of the dam, or patting it firmly on the 
lodges with their tails, the appearance is very like that of 
a disciplined force of builders, and at the approach of danger, 
the simultaneous disappearance of the party, each with a dis- 
tinct slap upon the water with the tail, tends to highten the 
effect of concerted action. 
The beaver not only forms dams and lodges, but excavates 
extensive canals through the swampy ground adjacent in order 
to transport the cuttings and boughs used in construction. 
Often the trees used by the beaver are not found in immediate 
proximity to the pond, and such canals are carried to the higher 
ground where suitable growths of poplar, birch or maple are 
situated. The boughs used upon the dam are often so large as 
to be carried with great difficulty on land, but can be trans- 
ported by water with ease. Such canals also connect the pond 
with the burrow where the margins are soft and swampy. 
Canals of this sort are often over 100 feet long. Where the 
banks are abrupt the beaver excavates inclined passages or 
slides, somewhat similar to otter slides, along which the brush- 
wood is carried to the stream. This reminds one of what may 
be seen along the white cliffs in the Tyrol, where well-worn 
slides several hundred feet long mark the ways along which 
peasants send the faggots used for fuel in the valleys. 
When abundant, the beaver is easily trapped, and its fur was 
once a staple commodity among the early settlers, often taking 
the place of currency. The trap, which is a medium-sized 
