596 THE ICE AGE IN FORTH AMERICA. 



mains against aerial decay. The floating carpet of moss be- 

 comes still more solid, and is then overspread by many species 

 of larger swamp-plants and small arborescent shrubs, espe- 

 cially those of the heath family ; and so, in the lapse of years 

 by the continual vegetation of the mosses, which is never inter- 

 rupted, and by the yearly deposits of plant remains, the carpet 

 at last becomes strong enough to support trees, and is changed 

 into & floating forest, until, becoming too heavy, it either breaks 

 and sinks suddenly to the bottom of the basin, or is slowly 

 and gradually lowered into it and covered with water. . . . 



The absorbing power of the peat-mosses enables them to 

 grow higher and higher above their original water-level, from 

 which they thus gradually emerge. The name emerged bogs 

 has been therefore given them. 



The peat of emerged bogs is less compact ; the annual 

 layers are more distinct, generally well defined in their succes- 

 sion. At the top of the bog the layers measure about one inch 

 in thickness, at the bottom less than one eighth inch, and 

 in old bogs still less. The growth, therefore, though not very 

 rapid, is easily observed and registered in several ways. 



It may be measured by compass and level from the border 

 of the swamp, the central portion of which becomes gradually 

 higher and higher, screening from the view of a spectator on 

 one side of it objects which had been before observable on the 

 other side of it. 



It may be estimated also by a time-scale, in cases where 

 ancient bridges, pavements, etc., whose epoch of construction 

 is certified by documents, are discovered buried under beds of 

 peat of known thickness. 



Again, in places where peat-bogs have been worked for a 

 number of years, old pits are encountered, now entirely re- 

 filled ; and when this happens with peat, during the life of 

 the proprietor, who has himself dug the old pits and can recall 

 the exact date, very precise data are thus furnished for learning 

 the amount of time necessary for the reproduction of a given 

 thickness of peat. 



The rate of growth depends, of course, on atmospheric or 

 other local circumstances, but, putting together many such 

 pieces of documentary testimony obtained in different conn- 



