FORMATION .OF COAL. 105 



brought down annually by this stream into the 

 Slave Lake. The trees which have been torn 

 down by the impetuous current, generally 

 retain a considerable mass of earth and stones 

 entangled in their roots ; they, therefore, readily 

 sink, and by so doing form considerable shoals, 

 which time converts into wooden islands. 

 " Then," says Dr. Eichardson, " a thicket of 

 small willows covers the newly-formed island, as 

 soon as it appears above water, and their fibrous 

 roots serve to bind the whole together firmly. 

 The trunks of the trees gradually decay, until 

 they are converted into a blackish brown sub- 

 stance, resembling peat, but which still retains 

 more or less of the fibrous structure of wood." 

 If we now suppose this island sunk to the bot- 

 tom of the river, covered over with many beds 

 of mud, and then left for a long period to per- 

 fect the chemical changes already begun, we 

 have a complete idea of this interesting process. 



The chemistry of it is as follows. In contact 

 with but little air, and a large mass of water, 

 the changes which take place are necessarily 

 somewhat different from those occurring in 

 woody fibre exposed to the air. Under these 

 circumstances, a decomposition somewhat ana- 

 logous to putrefaction or fermentation takes 

 place in the wood. Its elements undergo re- 

 arrangement, the ultimate effect of which, com- 



