861 



WHITE.] THICKNESS OF POTTSVILLE IN SOUTHERN FIELD. 



failure of the Twin coal to appear in the Locust Gap section/ Allow- 

 ance for reduction by pressure and crushing should also ]>e nmde m 



some sections. -o^ff ,,r;iu in 



The rcktivdv slight diminution in tho thickness ot the PottsMlle m 



passing along 'the 'Oauphin Basin to Rattling Run as conclttsively 



hown°i„ th! table given above. '■™<l"\'l'i'-''l';V*'"H;"n '1 ™ 

 thickness of the fomiation before reac-hing the Broad Top Basm. abou 

 75 miles distant, where it is said' to be only l.iO feet tb.ck. somewhat 

 remarkable In view of the geographic position of the Broad Top 

 tield on the east side of the Appalachian trough, between the ^e.,^ 

 thick Virginia and the Schuylkill sections, the alternatives-uncon- 

 formitv or diagonaling of the PottsviUe base in tune-d,s<ussed m 

 o Action with tho subject of the lower li.nit of the format.on u, 

 another part of this report are a^ain called to mind. The surpr.smg 

 difference in the measurements of the sections seems not wholl^ sat s- 

 factorilv explained bv the theoretically farther offshore position of the 

 Broad Top Jlountain, although that may account lor a large part ot 

 the difference. The more probable explanation, as t appears to me, 

 is that Broad Top was not directly in the influence of the strong, fluctu- 

 atin.. detritus-kden currents, which may have budt a large portion of 

 the great, broad, shoal-watcr terrace in the SchuylkiH-bwaara region 

 while red argillaceous shale was still being deposited m the Huntingdon 

 Coiuttv regfon. Unfortunately, no plants have lieen co lected trom 

 the latter region to show the relative age of the lower beds. 



The remailable strength and the varying activity ^«^^ An-^f""" °* 

 the movements of the early PottsviUe sediments over the Mauch Chunk 

 delta in the SchuvlkiU-Swatara region during a period of osciUa ing tide 

 level arc proved by the alternation and high degree of irregularity m 

 the PottsviUe beds, by the ti.nsportation of the conglomerate-building 

 material to a long distance from the present margin-.. e.,b> the long 

 radius of the fan-and by the size of the bowlders which are sometimes 

 elun^ered far from the margin of the Keld. In illustration o the 

 latter circumstance, the occurrence of bowlders , or b inches m diam- 

 eter in Head Mountain, described by Rogers,' may be cited. 



As illustrating the thinning of the beds to the northwest, as well as 

 indicating the radius of the thickened formation o the Southern 

 Anthracite tield, it may be of interest to quote a number of measure- 

 ments of the PottsviUe in other regions, in both the anthracite anc the 

 iUuuinous basins. From a thi.kncss of about 1,200 feet in the type 

 lection, ornearly^h^^ame^^^Ul^^ 



Anthracite Survey for 1883, p. 80 [202 feet on p. IOC], 

 -I.e. White, Bull. U. S. Geol. Survey No. 05, p. 18o. 

 3Cieol. Pennsylvania, Vol. II, Pt. I, p. 22. 



