WHITE.] FOTTSVILLE FORMATIOlSr IN THE TYPE REGION. 763 



COMPOSITION OF THE FORMATION. 



The character and composition of the Pottsville formation and its rela- 

 tion to the Umbral shale (Mauch Chunk formation) below and the 

 Productive Coal Measures above are well shown in the magniiicent 

 exposure in the' cutting along the Pennsvlvania Railroad through 

 Sharp Mountain on the east side of the gap below Pottsville. A some- 

 what detailed section of this exposure, extending from Tumbling Run 

 Valley to the Pottsville Valley is given on Pis. CLXXXl, CLXXXIL 

 This section includes the upper portion of the Mauch Chunk red shales 

 and extends to the Dirt bed, the third workable coal of the Coal Measures 

 at this point. The Pottsville formation itself may. for the present, be 

 considered as comprising that portion, nearly 1,200 feet in thickness, 

 of the section between the topmost bed of red shale and the "Twin" 

 coal, which in both the first and the second geological surveys of this 

 State has been agreed upon as the boundary between the Pottsville 

 formation and the succeeding Lower Coal Measures. 



On referring to the section it will be observed that the lower por- 

 tion represents a transition from the typical red, purplish-red, and 

 olive-green shales of the Mauch Chunk to the almost exclusively are- 

 naceous, ponderous quartz-conglomeratic terranes of the Pottsville 

 formation. The conglomerates intercalated in increasing proportions 

 in the upper beds of the Mauch Chunk consist of irregularly bedded, 

 poorly assorted, or sometimes apparentl}' unassorted pebble or bowl- 

 der accumulations in a matrix of coarse arkose sands colored by 

 reddish or greenish shale washes. The pebbles are mostly of quartz, 

 though sandstone, syenite, chloritic schist, limestone, and even red and 

 green shales and conglomerate fragments are also present. Occasion- 

 ally the pebbles, which are sometimes subangular, attain a diameter of 

 3 or 4 inches or more; but in most of the beds the coarsest materials 

 do not exceed a g'oose egg in size. For a long distance from the base 

 of the formation the conglomerate matrix consists of a micaceous, 

 chiefly arenaceous medium, poorly cemented and often colored with 

 a red or green argillaceous material. 



In passing upward the beds of red shale are less conspicuous, and at 

 about l,:iOO feet below the Twin bed the last distinct stratum of t3q3ical 

 Mauch Chunk red shale is seen. Above this the conglomeratic matter 

 prevails almost exclusively through a long interval. Nevertheless, the 

 olive-green shales occur here and there throughout an interval extend- 

 ing 200 or 300 feet higher, while most of the conglomerates in the 

 lower portion of the section derive their color from the greenish or 

 reddish mingled sediments. The irregular l^edding and the variety of 

 the rock materials in the pebbles, which are often imperfectly rounded, 

 are interesting features of the lower portion oi the Pottsville forma- 

 tion itself. This portion of the section is notably characterized by 



