572 



GEOLOGY. 



2-3' 



t-2' 



A3' 



/'-S' 



the growth and accumulation of vegetation was often brought to an 

 end by subsidence which let the water (sea or lake or aggrading 

 stream) in over the marshes, drowning the plants, and burying the organic 

 matter which had already accumulated, under deposits of mud, sand, 

 or shells which the submergence brought in its 

 train. A second coal-bed in the same region 

 points to the recurrence of swamp conditions, 

 and means either (a) that after submergence and 

 burial of the organic matter, slight emergence en- 

 sued, caused by land or water movement, thus 

 reproducing the conditions for bogs; or (b) that 

 by sedimentation the sea or lake bottom where 

 the first bog had been, was built up to the water- 

 level, restoring swamp conditions. On the former 

 hypothesis (a), two successive coal-beds would 

 represent the following sequence of events: (1) A 

 more or less static condition with the surface ill- 

 drained, during which the vegetable matter ac- 

 cumulated; (2) a subsidence of the land or a rise 

 of the water during which the vegetable matter 

 was buried by sediment; (3) a reverse change, 

 bringing the surface again to the condition which 

 it occupied in the first place; (4) a later sub- 

 mergence, effecting the burial of the later growth 

 of vegetation. On the second hypothesis men- 

 tioned above (b), but one movement is demanded. 

 Stages (1) and (2) would have been as above, but 

 the sedimentation of (2) continued until the 

 swampy condition of the region was reproduced 

 Fig. 254. — Generalized as a resu i t f aggradation. The burial of the 



section of the Coal . °° 



Measures in Kentucky, vegetation calls for further submergence. In other 

 80o1ee\ hick Th1hea ^ W0 ^ the first hypothesis ca n s f or oscillation of 



black lines = c o a 1 . level : the second, for successive movements of 

 (Norwood, U. S. Geol. ' , . 



Surv.) one phase only. 



The number of coal-beds is often great. In Pennsylvania it fre- 

 quently (but not everywhere) exceeds 20; in Alabama, 35 (not all 

 workable) have been enumerated; in Nova Scotia, the number, includ- 

 ing some dirt-beds, is said to be about 80; but in the Mississippi basin 



