110 ILLINOIS ACADEMY OF SCIENCE 
not subsequently infiltrated with the liquid hydrocarbons Of 
the bog to the same extent as that of the bright laminae. 
The foregoing interpretation of the structural features of 
the coal beds leads to the following very definite conclusions : 
1. That the beginning of vegetable accumulation of the coal 
beds was in a very shallow swamp. 
2. That the swamp deepened so slowly, either by subsi- 
dence of the area, or from the gradual building up of the bor- 
der or outlet by sedimentation, or both, that the plants were 
able to adjust themselves to the changes, and the accumulation 
of vegetable matter in a general way kept pace with the in- 
creasing depth. 
3. That throughout the entire period of accumulation of 
the coal beds the water of the swamp was so shallow that, dur- 
ing the oft-recurring cycles of drought, successive levels of 
the vegetable mass were temporarily exposed and so modified 
by partial atmospheric decay as to result in the formation of 
the dull mineral charcoal laminae. 
4. That the time involved in the accumulation of the veget- 
able material of a coal bed was the time necessary for the 
growth of the plants, plus the time recorded in the partial in- 
terruptions of normal accumulation, indicated by the dull lam- 
inae and charcoal partings of the coal, which would very con- 
siderably increase the usual estimate of time required. 
