I9I3-] STEVENSON— FORMATION OF COAL BEDS. 63 



may represent, in time-value, the intervening deposits where the 

 spHts are most widely separated — in which case the total thickness 

 of coal may be approximately the same throughout. When a split 

 loses thickness away from the place of union, it may be that sub- 

 sidence began at some distance from that place and was, so to say, 

 rapid ; but where the split thickens, the subsidence was at first ex- 

 tremely slow, permitting accumulation to continue after it had 

 ceased beyond the place of union. Some of the splits increase, 

 others lose in thickness. A study of the benches in each split proves 

 independent history. 



One may not regard a coal bed as a single deposit, the result of 

 consecutive deposition, broken only by pretty irruptions of clay or 

 sand. It is the record of accumulation in a given area interrupted 

 by longer or shorter intervals of no accumulation, which are marked 

 by the partings. These intervals in one locality may be synchronous 

 with continued accumulation in another. It is very evident that 

 this accumulation did not begin simultaneously in all portions of 

 the area now marked by a coal horizon and it is equally certain that 

 its termination was not simultaneous throughout. Unquestionably 

 the opening and closing of the work at any given horizon were 

 embraced within a definite period, but one must recognize that only 

 a very small part of the bed may be actually of synchronous origin 

 throughout. Study of the benches of the Pittsburgh coal bed has 

 led the writer to conclude that very little coal accumulated in 

 northern Ohio and much of Pennsylvania until after a notable thick- 

 ness had accumulated in southern Ohio and in West Virginia. The 

 diminishing importance of the portion below the Bearing-in coal 

 seems to indicate a northward advance of coal-forming conditions. 

 It is equally clear that coal accumulation ceased after the Bearing- 

 in within most of the southern portion, for the Breast is unimportant 

 or absent, whereas it continued long time at the north, as appears 

 from the increasing importance of the Breast in that direction. 

 Changes of similar kind are shown by the Middle Kittanning or 

 Hocking Valley coal of Ohio, which has been studied in detail 

 throughout an area of more than i,ooo square miles, where it has 

 great economic importance. Enough is known to make clear that, 



