220 J. J. STEVENSON CARBONIFEROUS OF APPALACHIAN BASIN 



locality is almost 50 per cent, so that the bed is at its best only in the 

 western part of these fields. It is not reached in the eastern Middle. 

 The Diamond is a variable bed, occasionally workable in the Southern 

 field, but disappearing in the western part. It has been recognized near 

 Mahanoy City in the western Middle, but, unlike the lower beds, it seems 

 to be wanting in the Shamokin area. The Tracy and Little Tracy show 

 similar variations. These are the highest beds recognized in the western 

 Middle. The Peach Orchard remains in an area so small that its varia- 

 tions are unimportant, but in the Pottsville and Llewellyn districts it is 

 from 4 to 10 feet thick and is one of the best beds in the whole column, 

 the coal being of exceptional purit}\ It was apparently the last im- 

 portant deposit, for, although the section extends 700 feet above it, none 

 of the higher beds, excepting perhaps the Tunnel, appears to be worth 

 working. 



The important period of coal accumulation within the Southern and 

 Middle fields ended with the Mammoth; for while the total amount of 

 coal formed during the remaining time was probably as great, yet accumu- 

 lation was continuous nowhere for long enough time to form a great bed 

 over any considerable area. The irregular local movements causing the 

 splitting of the Mammoth and Buck Mountain were no greater than those 

 occurring in later intervals of similar length, as is evidenced by the 

 varying intervals between higher beds. The variation is more striking 

 in the Mammoth and Buck Mountain only because confined in each case 

 to what becomes at times a single bed. It must be remembered that the 

 time required for accumulation of coal in those beds was probably longer 

 than that required for accumulation of half the mass above the Mam- 

 moth; so that the only cause for wonder is that, in any locality, the sub- 

 sidence could be so slow and so regular long enough to permit accumu- 

 lation of 105 feet of anthracite coal. Great variations in conditions 

 existed during the formation of all beds except the Mammoth, for even 

 the Buck Mountain shows a broad area in which the carbonaceous matter 

 is distributed through a mass of coal slate, while some other beds, usually 

 alternating thin layers of coal and slate, occasionally become sufficiently 

 good to repay mining. 



In this connection it is well to consider the variations in some in- 

 tervals. The sections suggest that in the Southern field the intervals 

 between Buck Mountain and Mammoth and between Mammoth and 

 Holmes increase toward the west ; but one may not offer a generalization, 

 as complete presentation of details might prove this only an apparent 

 condition. But variations in intervals below the Mammoth are im- 

 portant, as they are wholly clear in the multitude of sections gathered by 



