Clifton College Scientific Society. 27 



The favourite site for a Beaver colony is a bend in a stream 

 with high and clayey banks on either side. Here the cur- 

 rent will offer the least resistance to the dam, and clay in 

 abundance is forthcoming for its consolidation. The first 

 step is to fell down a large tree at the selected site, or two, 

 if the stream is a. wide one. The trees are cut down so as to 

 fall across the stream, and if the current is sluggish they 

 may extend in a straight line from shore to shore ; but if it 

 be a rapid one and two trees have been employed, a sort of 

 arch is at once formed, with the crown down the stream, an 

 effect of course produced by the greater rapidity of the 

 current in the middle.' The only jjerpeudicular portions 

 of this rude framework consist of the tree's branches — it is 

 erroneous to say that the Beaver fastens any posts vertically 

 into its dam. But the consolidation of the structure is 

 secured in this way : many sticks (four to six feet long) are 

 cut down some little way up the stream and are then freed of 

 their branches, dragged into the current, and guided towards 

 the dam. Here they are fastened together by means of mud, 

 small stones, leaves, roots, &c., carried from shore by the 

 builders, and very soon a great degree of resistance is 

 afforded to the current, and the water speedily rises above 

 the obstruction. The Beavers raise their dam still higher, 

 and when the required level is attained they cease their 

 work, leaving the rest to the action of the water itself. 

 Great quantities of sand, gravel, or mud are soon deposited 

 on its upper side by the stream, and any chance trees or 

 branches which may float down help to make the dam still 

 more complete. Bye and bye the sticks and logs of willow 

 (which are sure to form part of it) take root, and send forth 

 their shoots ; decaying leaves begin to form a soil, vegetation 

 appears, and the permanence of the whole dam is ensured. 

 It is true that the recurriiig floods of spring often carry 

 away large portions of the surface part, but the under struc- 

 ture generally remains proof against such attacks, and with 

 the return of the building season the Beavers repair any 

 inroads that may have been made. 



The dams are often as much as three hundred feet wide, 

 but their height above the water is inconsiderable, and their 

 thickness is of course greatest at the bottom of the water. 

 An opening in the centre of the dam permits the efiiux of 

 the current. 



' Yet at least two writers on ' popular ' Natural History state the very reverse, 

 affirming that the convexity, and not tlie concavity, is turned towards the stream ! 

 It is difficult to see how this could be. On this point Mr. Green's evidence is more 

 reliable {vide op. cit. pp. 363-4). 



