THE VEGETABLE MATTER OF ILLINOIS COAL BEDS 765 



selves to the changes, and the accumulation of vegetable matter 

 in a general way kept pace with the increasing depth. 



3. That throughout the entire period of accumulation of the 

 coal beds the water of the swamp was so shallow that during the 

 oft-recurring cycles of drought successive levels of the vegetable 

 mass were temporarily exposed and so modified by partial atmos- 

 pheric decay as to result in the formation of the dull laminae with 

 their mineral charcoal. 



4. That the time involved in the accumulation of the vegetable 

 material of a coal bed was the time necessary for the growth of 

 the plants plus the time recorded in the interruptions of accumula- 

 tion indicated by the dull laminae and charcoal partings of the coal, 

 which would very greatly increase the usual estimate of time 

 required. 



