576 



GEOLOGY. 



bifurcation may be brought about. Vegetable matter for a coal seam 

 is accumulating along a-b (A). The outer part of the bog then sinks 

 (B) so that sediment is deposited over a part of the area (a to c) 

 where vegetation formerly grew. When the depressed area (a to c) 

 is aggraded or elevated (relatively) sufficiently (C), the bog is restored 

 at the left, and the two layers of organic matter at the left, unite at 

 the right, where the accumulation of vegetable matter has not been 

 interrupted. It will be readily seen that other sequences of events 

 might bring about the same result, and that, if this process is fre- 



a 



hnir^i.rrwrlirfi^ii. 



B 



C 



Fig. 232. — Diagram illustrating the sequence of events which might lead to the bifur- 

 cation of a coal bed. In the first diagram, vegetal matter is accumulating from 

 a to b; in the second, a part of the area, a to c, is represented as submerged; in 

 the third, the tract from a to c has been built up or lifted up (relatively) so as to 

 become a marsh again, and vegetal matter is accumulating from a to b. 



quently repeated, great differences might exist in the number of coal 

 seams in different parts of the same general region. 



Varieties of coal. — The exact manner in which the different varie- 

 ties of coal have arisen has never been satisfactorily determined. In 

 general it is true that the anthracite coal occurs in mountainous regions, 

 where the coal and other layers of rock with which it is associated 

 have been subject to more or less intense dynamic action. Thus, 

 in the mountains of eastern Pennsylvania (Fig. 263) the coal is mainly 



