511 



chute, the main channel of the stream, the sediment was fine, heavy, 

 black, and but little offensive, and contained few tubificid worms, 

 the lighter organic matter evidently being carried down by the cur- 

 rent; but in the more sluggish water of the south chute it was light, 

 soft, and grayish, and full of Tubificid cc. For the first half mile 

 above the dam the river bottom is mostly rock, covered by a thin 

 layer of very soft ooze; but below the dam the swift flow of the 

 stream keeps the bottom bare. 



The chemical condition of the bottom deposits at this time is 

 shown by the following table of the results of analyses of gases col- 

 lected from the bottom sediments of the Illinois, at Morris, in 

 August, 19 II. 



This table is made more significant if it is brought into com- 

 parison with the following data of analyses obtained from the gases 

 of the septic tanks of two sewer systems of Illinois towns. 



It will be seen that all of these samples are alike characterized by 

 the virtual absence of oxygen, by the large percentages of carbon 

 dioxide, and by the predominance of marsh gas or methane (CH4), 

 and that the Illinois River gases are indistinguishable from those of 

 the tank sludges of the town sewers. The abundance of carbon diox- 

 ide and the absence of oxygen are, of course, to be understood as 

 due to organic decomposition in the presence of oxygen ; and the 

 methane is evidence of the continuance of decomposition after the 

 available oxygen was exhausted. 



