686 THE JOURNAL OF GEOLOGY. 



should expect, for according to the supposition, the conditions 

 at that distance from the old shore line should vary little any- 

 where. 



So one finds, 



First. A decided increase in thickness of coal eastward, or 

 better, northeastward toward the anthracite region, and a less 

 marked increase northward in the Bituminous basins. 



Second. A decided decrease in volatile in the direction of 

 increased thickness of coal, the decrease being comparatively 

 gradual until near the anthracite fields. 



Third. That this decrease is gradual even in the Anthracite 

 Strip from the Cumberland Field to the semi-bituminous coals of 

 the Southern Anthracite field,' where the rapid increase in thick- 

 ness is accompanied by a rapid decrease in the volatile. 



When, in 1877, the writer called the attention of his col- 

 leagues on the Pennsylvania Survey to the fact that the decrease 

 in volatile is wholly without relation to increase or decrease of 

 disturbance in the strata, he suggested that the variation was due 

 to difference in conditions under which the coal had been formed 

 in the several localities discussed — a sufficiently comprehensive 

 hypothesis, but yielding in this respect to some others of later 

 date. Now, however, there seems to be no good reason for any 

 such suggestion ; all that was needed was longer exposure to the 

 process whereby ordinary bituminous coal was formed. In 

 origin, the anthracite coal of Pennsylvania differs in no wise from 

 the bituminous coal of other parts of the Appalachian basin ; 

 but because the great marsh, from which sprang the many beds, 

 originated in the northeastern corner of the basin and extended 

 thence again and again on the advancing deltas formed by streams 

 descending from the Appalachian highlands, the time during 

 which the successive portions of the marsh would be exposed 

 would be less and less as the distance from the northeastern and 

 northern border of the basin increased, so that the extent of chemi- 

 cal change would decrease as the distance increased. It is, there- 

 fore, to be expected that in the northeastern corner, where the 

 deltas were formed quickly after subsidence was checked, and 



