Agassiz.] 102 [June 16, 



Morgan, in the work just quoted, after showing that the dams are 

 the work of a comparatively small number of beavers, naturally also 

 comes to the conclusion that they must be of great age from the 

 amount of solid material they contain, from the destruction of the 

 primitive forests within the area of the ponds, and other causes from 

 which he concludes that these dams have existed in the same places 

 for hundreds and thousands of years, and have been maintained by a 

 system of continuous repairs. 



In building an artificial dam across a beaver meadow, I came acci- 

 dentally upon data showing that Morgan's view of the antiquity of 

 beaver dams is correct. For the purpose of obtaining a secure founda- 

 tion for a mill dam (erected at a short distance above a beaver dam) , 

 it became necessary to clear away the soil of the bottom of the 

 beaver pond, which had been exposed by cutting the beaver dam 

 below it. 



This soil was found to be a peat bog of variable depth, attaining a 

 thickness of six feet. A belt of this peat, twelve feet wide and some- 

 what over twelve hundred feet long, was removed, and imbedded in 

 the peat below the surface peat of the bottom of the pond were 

 found the traces of a number of stumps in various stages of decompo- 

 sition, and here and there what looked like beaver cuttings, and finally 

 the workmen came across several stumps where the marks of the 

 beavers' teeth were still plainly visible, showing that the stumps found 

 in the peat were probably all stumps of trees cut by beavers in former 

 times ; the bottom of this peat bog was two and one half feet above 

 the base of the beaver dam. We have here positive evidence that in 

 this case, at least, the peat bog was formed by the pond flowed by the 

 dam, and the same state of things exists in several other dams which I 

 examined. We find that they are always accompanied by larger or 

 smaller peat bogs, all of which, as in the case above alluded to, owe 

 their existence to the beaver dams. On sounding them, the depth of 

 one of them was found to be as great as nine feet. In these instances 

 careful levels were run from the dam towards the source of the creek 

 upon which they are built, and on reconstructing the appearance of the 

 country, as it must have been before the beaver dams existed, I found 

 that from the nature of the surrounding country, the open spaces now 

 joining the beaver ponds, the beaver meadows where the trees are 

 scanty or small, must at one time have all been covered by forests 

 similar to those which are found on the banks, and fully as luxuriant. 

 It was only when the beavers established themselves upon the creek 



