MAMMALS OF MINNESOTA. 173 



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 high ten 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. Sach 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 



