70 J. J. STEVENSON — PENNSYLVANIA ANTHRACITE. 



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 basin to the semi-bituminous coals of the Southern 

 anthracite field, where the rapid increase in thickness of coal is accom- 

 panied by a rapid decrease in the volatile. 



When, in 1877, the writer called the attention of his colleagues on the 

 Second Geological Survey of Pennsylvania to the fact that the decrease 

 in volatile is wholly without relation to increase or decrease of disturb- 

 ance 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 yield- 

 ing in that 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 of Pennsylvania diff'ers in no- 

 wise from the bituminous coal of the Appalachian basin ; but because 

 the great coal marsh, from which sprang the many beds, originated in 

 the northeastern corner of the basin and extended thence on the ad- 

 vancing 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 north- 

 east and northern border of the basin increased, so that the extent of 

 chemical change would decrease as the distance increased. It is therefore 

 to be expected that in the northeastern corner, where the deltas were 

 formed quickly after a subsidence and beyond which they advanced 

 slowly, as shown by the changing type of rocks, the chemical change 

 should have been almost complete, especially in the eastern Middle 

 and eastern extremity of the Southern field, which occupy that part of 

 the area in which the coal marsh, in almost every instance, appears to 

 have thrust itself first upon the advancing delta. 



It is quite possible that when detailed study of the anthracite areas in 

 Arkansas and Russia have been made, the same explanation may be 

 found applicable there also, and that the anthracite will be found near 

 the old shoreline, whence the marsh advanced as new land was formed. 



