18C.9 1 103 [Agassis 



and commenced to build their dams that they began to clear the 

 forest just in the immediate vicinity of the dams extending in every 

 direction, first up the stream as far as the nature of the creek would 

 allow, and then laterally by means of their canals, as far as the level 

 of the ground would allow, thus little by little clearing a larger area, 

 according to the time they have occupied any particular place. 

 Soon after the commencement of their dams, usually selected on such 

 a spot of the creek as is sluggish, and where a slight elevation will 

 give a large area for flow age, the ground must have become saturated 

 with moisture, rank grass must have started up. Sphagnum soon made 

 its appearance, and' little by little, as the dam was raised, the area 

 extended, and the marshy meadow prepared the ground for the ulti- 

 mate formation of the peat beds observed, which extended little by 

 little as far as the possibility of the ground would allow, covering by 

 degrees the base of the stumps of the trees cut by the beavers, as 

 well as of the bushes, covering the sticks left about with a coating 

 of grass and peat, and either decomposing or preserving, as was the 

 case in the peat bog observed, the stumps which are to tell us now 

 how long they have been occupied in raising their dams. 



The rapidity with which peat is deposited varies greatly in some 

 districts of Switzerland, according to Lesquereux, to whose kindness 

 I am indebted for the needed figures, taken from the exploration he- 

 conducted so successfully in various parts of Europe to ascertain the 

 nature and growth of peat bogs. We find localities where at a fixed 

 date no bogs existed, and after a lapse of fifty years had grown one 

 and one half feet, so that we can take as a general thing a growth of 

 about one foot in a century as the average, though the rate of growth 

 varies according to localities, three feet in a hundred years having 

 been observed, the lacustrine deposits growing at a much slower rate 

 than the peat bog deposited in mountainous districts. So that in the 

 case of bogs of the depth of nine feet, we can safely assume that the 

 probable age of the dam is about nine hundred years, which would 

 give a rational explanation of the possibility of building such huge 

 structures by such small number of animals as are evidently the 

 dwellers on the shores of the beaver pond of any one dam. 



New dams are started, as stated by Morgan, by a pair of young 

 beavers, and I find that considerable exploration of localities suited 

 for new structures is made by beavers during the winter, when the 

 crust of the snow h suitable, and their trails have been found at a 

 distance of two am one half miles from the nearest dams, prospecting 



