WHITE.] VARIATION IN PUTTSVILLE TERRANES. 863 



of the Pottsville .stratii, l)ut uLso the astonishing laek of continuit}' 

 among even eonspicnous and important strata. In fact, I know of no 

 region in the Appalachian trovigh in which the local irregularities of 

 the coal-bearing formations are more marked than in the Southern 

 Anthracite iield. It is not diHicult to account for this irregularity on 

 the hjq^othesis I accept in explanation of the conditions attending the 

 deposition of the Schuylkill-Swatara and Virginia sections. The for- 

 mation of beds of coal under such conditions seems to necessitate the 

 assumption either that there existed, at various times on the surface of 

 the Pottsville terrace or fan, coastal lagoons or protected basins, the 

 sluggish water supply of which was laden for short periods with little 

 else than vegetable matter, or, as appears more probable, that, as the 

 result perhaps of occasional uplifts, large areas lying within bars or 

 shoals were converted during short intervals of quiescent stability 

 into Carboniferous swamps or lagoons in which considerable irregular 

 deposits of plant matter accumulated before the current erosion of the 

 l)arriers or the renewal of the general movement of submergence ter- 

 minated the conditions favorable for coal formation and permitted 

 the invasion of the coarsely deti"itus-laden waters. The interruption of 

 the general subsidence by short periods of elevation and stability, 

 while permitting at once the accumulation of vegetable matter in one 

 region and the seaward extension of the submarine terrace in another 

 during the periods of higher level, accounts also for the readiness with 

 which the conglomeratic sediments, which usually almost directly, 

 when not immediatel}', overlie every Lykens coal, were swept across 

 the carbonaceous deposits on the recurrence of the general downward 

 movement. 



The varialjilitv in the thickness of the coals, their irregular inter- 

 vals and distribution, as well as the fact that the areas containing the 

 lower Lykens coals are so restricted, compared with the area of the 

 anthracite tields, appear to sustain this hypothesis as explaining both 

 the deposition of the coals and the extent of the formation. 



As partially illustrating the variation of the several members of the 

 Pottsville formation in the mining district of the Southern Anthracite 

 field, while showing the prevailing intervals between the coals, the 

 following incomplete table is presented, although it is extremely 

 fragmentary and evidently insufficient to serve as the basis of any 

 important generalizations. 



