out exposing themselves to dangers on land, and 

 also offer an easy means of intercommunication 

 between ponds when these are ice-covered. Pond 

 subways also afford a place of refuge or a means 

 of escape in case the house is destroyed, the dam 

 broken, or the pond drained, or in case the pond 

 should freeze to the bottom. Commonly these 

 are full of water, but some are empty. On the 

 Missouri and other rivers, where there are several 

 feet of cut banks above the water, beaver com- 

 monly dug a steeply inclined tunnel from the 

 river's edge to the top of a bank a few feet back. 

 Most of this tunnel work is hidden and remains 

 unknown. A striking example was in the Spruce 

 Tree Colony, elsewhere described. These colon- 

 ists, apparently disgusted by having their ponds 

 completely filled with sediment which came down 

 as the result of a cloudburst, abandoned the old 

 colony-site. A new site was selected on a mo- 

 raine, only a short distance from the old one. 

 Here in the sod a basin was scooped out, and 

 a dam made with the excavated material. The 

 waters from a spring which burst forth in the 

 moraine, about two hundred yards up the slope 

 113 



