7H4 FLORAL ZONES OF THE POTTSVILLE FORMATION. 



the occurrence of olive-green or slightl}' reddish mud beds, apparently 

 red('p()sit«>d from the older formation. These muds often conclude 

 rapid transitions from greenish conglomeratic sandstones into fine 

 argillaceous silts of no great thickness. On the conspicuousl}^ uneven 

 surfaces of the latter, coarse conglomeratic strata or typical bowlder 

 beds are directly imposed, in knife-edge contacts, at a number of 

 horizons in the lower half of the section. These irregular, interca- 

 lated nuids. which are similar to others in the upper Y)art of the ]Mauch 

 Chunk formation, sometimes appear as thin lenses interspersed among 

 the irregular layers of the conglomerates. Without further detailed 

 description of the type section, for which the reader is referred to 

 Pis. CLXXXI. CLXXXII, it will appear that we have a series of beds 

 of passage — i. e.. a transition series — consisting of coarse, heteroge- 

 neous, semiassorted. conglomeratic materials, intercalated in the upper- 

 most l>eds of red shale, above which, for a distance of several hundred 

 feet, many of the conglomerates preserve essentialh^ the same charac- 

 ters, although typical deposits of the red and green shales are want- 

 ing. Subangular pebbles in imperfectly l)edded arkose conglomerates 

 are not rare throughout the lowest third of the formation in this 

 vicinity. Although the quartz material preponderates, pebbles of 

 sandstone and shale are not infrequent. Occasionally some of the 

 pebbles attain the proportions of goose eggs, and farther east, in a 

 section near the Hacklebarney tunnel, soiue of them measure 5 to 6 

 inches in diameter. 



As already indicated, the conglomerates in the lower portion of the 

 PottsviHe formation are prevailingly greenish, arkose, and poorly 

 cemented, l^sually. in the more freshly cut sections, they otier little 

 resistance, and frequenth-^ they are but slightl}'^ displayed. When, 

 however, the erosion has been very slow, as along the summit of Sharp 

 Mountain, the ferruginous material so cements the pebbles that the 

 lower ledges of the formation often predominate and form, for consid- 

 erable distances, the crest of the mountain. This featui'e is more 

 noticeable to the eastward of Swatara (iap. In the upper half of the 

 formation the conglomerates become more rigid, more distinctly are- 

 naceous, and more persistent, the pe))bles l)eing better rounded, more 

 compactly disposed, and regularly assorted. Sandstone without peb- 

 bles is rare and is alwa3's thin in the section. Cross bedding, indica- 

 tive of current movem«Mit from the northeast, is conspicuous. In the 

 more shaly conglomeratic sandstones in the middle of the section con- 

 cretionar}' weathering is especially noticeable. 



Generally speaking, the relatively small amount of shales and of coaly 

 matter in the type section is, for the most part, contained in the mid- 

 dle third. Toward the top the conglomeratic material becomes lighter 

 colored, as well as more exclusive, and at a distance of 200 or 800 feet 

 below the Twin coal, in that portion of the exposure opposite the 



