WHITE.] GENERAL GEOLOGIC STRUCTURE. 761 



ant tloor of a high plateau. Along the southern border of the coal 

 field we accordingly find that the upturned and nearly vertical hard 

 beds of the Pottsville formation compose the crest of a narrow mountain, 

 Sharp Mountain, between which and the upturned wall of the Pocono 

 (Vespertine, X), the basal member of the Eocarboniferous in the 

 Second IMountain, extends a narrow parallel valley cut in the likewise 

 upturned soft red shales of the Mauch Chunk formation (Umbral 

 shale, XI), which, in the Schuylkill region, reaches its maximum 

 thickness of over 3.000 feet. The steep inclination of the Pottsville 

 floor is similarly accountable for the rigid and cristate c-haracter of the 

 rim of the coal field in Sharp Mountain, in Stony or Fourth Mountain 

 on the north side of the Dauphin Basin, in Bear and Wiconisco moun- 

 tains in the northern prong of the "fish-tail,'' and in Locust Mountain. 

 Its more enduring composition, as compared with the terranes of the 

 Coal iSleasures, has resulted in the erosion of irregular valleys, gener- 

 ally corresponding to the axes of the basins. Throughout the South- 

 ern Anthracite field, wherever the elevation of the Pottsville has been 

 sufficient to cause its complete erosion, the consequent erosion of the 

 underlying Mauch Chunk shales has been so rapid as to form sharply 

 defined valleys, varying in width according to the area uncovered or 

 the inclination of the beds. It thus happens that Broad Mountain, on 

 which is spread the northern dilation of the coal field, comprises essen- 

 tially an elevated undulating plateau sustained by the rigid, flexuous 

 Pottsville floor of the Coal Measures, which here, as in the Coal 

 Measures of the other anthracite fields, prevailingly forms anti- 

 clinal ridges and synclinal valleys. The breaking through of the 

 Pottsville on the anticlinal axes along the western portion of Broad 

 Mountain is responsilile for the deep t!Ove-like valleys between the 

 lobes of the field, as noted above, the borders being often formed by 

 massive cliffs of the gently inclined Pottsville conglomerates. In the 

 region north of Pottsville the elevated Pottsville formation is not 

 entirely cut across at any point, the result being that there is a conti- 

 nuity of the conglomerates, which bridge the axis from the New Bos- 

 ton Basin into the ^Nlahanoy Basin of the Western Middle Anthracite 

 field. The line of division ])etween these fields, which, as may be 

 inferred, is somewhat conventional, is drawn along the axis south of 

 the northward plunge of the conglomerates into the Mahanoy Basin. 

 The irregularity of the margin of the Southern coal field along 

 Broad Mountain is quite in contrast with the relatively straight bord- 

 ers of the prongs of the '*fish tail," or of the southern margin of 

 the field along Sharp Mountain from the Susquehanna River to a point 

 east of Middleport. This condition is largely due to the closer fold- 

 ing and increased depth of the Coal Measures toward the south, so 

 that the soft shales beneath the Pottsville are not brought to light by 

 the minor flexures. The effect of the latter is, however, evident 



