860 FLORAL ZONES OF THE POTTSVILLE FORMATION. 



From the above tuIHe it appears that over 3i><.» feet of transition 

 series has been inchided within the Pottsville formation in some of the 

 measurements pul)lished 1)y the State survey. Among the deductions 

 to be drawn from the table, perhaps the most important are: (1) 

 "Whatever the arbitrary ])ase line employed in the measurements, the 

 formation is found to ]»e thickest in the central portion of the field, 

 i. e., the region including Pottsville and Lincoln. (2) The forma- 

 tion appears to be as thick at 7 or 8 miles from the present southern 

 ])order of the field as in Sharp Mountain. Thus on the Broad Moun- 

 tain, near Altamont colliery No. 2, the diamond drill bore hole can 

 hardly have begun nuich higher than the Twin coal, while the section 

 at Kohlers Gap in Bear Mountain, which was carefulh' described and 

 measured by Rogers, appears to be as thick as all those measured by 

 myself in Sharp Mountain. It seems not improba])le that the great 

 thickness of the formation in the Williamstown tunnel, as platted in 

 columnar-section sheet vii, may be due to error in the identification of 

 the liuck ^lountain bed, or in the computation of the the thickness of 

 the beds. (3) The diminution of the thickness of the Pottsville Ix^tween 

 the type section at Pottsville Gap and the Lansf ord railroad tunnel 

 in Locust ]Mountain is well marked, as appears to be also the rapid 

 increase which is noted in the region of Nesquehoning Gap. I am 

 disposed to believe that in the Panther Creek Basin the B bed is per- 

 haps nearer the level of the Twin coal, or supposed Buck jNlountain 

 bed, than is coal A, which, although distinctly referable at Tamaqua 

 to the Lower Coal Measures, seems to carry a rather less recent flora 

 than that of the Twin coal. Neither is it certain that the A bed at 

 the Nesquehoning Gap is identical with that similarly designated at 

 Tamaqua. (4) Another diminution in the thickness of the section 

 seems to occur along Sharp ^Mountain from Pottsville to Swatara 

 Gap, where the interval from the supposed Twin bed to the top of the 

 red shale is perhaps less than i»50 feet. (5) One of the most interesting 

 facts brought to light in this comparison is the apparently but slight 

 decrease of the formation in Sharp ^Mountain in passing westward 

 along the Dauphin Basin, where at Rattling Run, near the Avestern 

 end of the field, it still retains a thickness of 1,100 feet. This observa- 

 tion is of greater weight because it is based on careful measurements 

 apparently extending only from the iqipermost bed of r(Ml shales. 



The more marked variations in the thickness of the Pottsville are 

 perhaps due to diff'erences in the horizons taken as (he upper or the 

 lower limits, or to changes in the, thickness of tiic sc\crul t(M-ranes 

 from i)oiiit to point, rather than to the existence of an uiiconforniily 

 at the base of the formation. Even at Tamaqua, where the discrep- 

 ancy between the thickness of the Pottsville, as measured in tht> two 

 gaps, points, perhaps, toward discordance, the difference may )»(• due 

 eitlici- to \iiriatioii. without uiicoiiformitv at the base, or to the a))solute 



