MANSFIELD: ROXBURY CONGLOMERATE. 109 
——:— Pottsville Conglomerate. The Pottsville formation of 
Pennsylvania and the region southward has been ascribed to marine 
origin. Accounts of it have been given in the state reports and in 
various separate papers. According to David White the rocks consist 
of ponderous conglomerates, which are more variable in color, com- 
position, and assortment in the lower part and more quartzose, dense 
and light colored near the top. The conglomerates alternate near 
the base with washes of purple and olive mud or soft greenish sand- 
stone, and in the higher portion with thin arenaceous shale and coal 
seams (D. White, a, p. 762). The section in Sharp Mountain below 
Pottsville shows a transition from the Mauch Chunk to quartz con- 
glomerates. The conglomerates intercalated in the upper beds of 
the Mauch Chunk are irregularly bedded and poorly assorted or 
sometimes apparently unassorted pebble or boulder accumulations 
in a matrix of coarse arkose sands, colored by reddish or greenish 
shale washes. The pebbles are mostly quartz though other erystal- 
line and clastic rocks are also present. Occasionally the pebbles, 
which are sometimes subangular, attain a diameter of three or four 
inches but usually they do not exceed the size of a goose egg. For 
along distance from the base the conglomerate matrix consists of 
micaceous, chiefly arenaceous material, poorly cemented and often 
colored with red or green argillaceous matter. Irregularities of bed- 
ding and variety of rock constituents in the pebbles, which often 
are imperfectly rounded, are interesting features of the lower por- 
tion of the Pottsville formation. Subangular pebbles in imperfectly 
bedded arkose conglomerates are not rare throughout the lowest third 
(ibid., p. 763-764). 
The study of fossil plants shows that the Sharon conglomerate, which 
constitutes the basal member of the Upper Carboniferous over the 
greater portion of western Pennsylvania and part of Ohio, and which 
has been regarded as the basal member of the Pottsville in general, 
belongs in the upper part of the typical Pottsville; and that several 
thousands of feet of Pottsville sediments in the southern Appalachians 
and a great thickness of beds in the southern anthracite region were 
laid down before an encroaching sea began the assortment of Sharon 
materials in western Pennsylvania or Ohio (D. White, b, p. 268). 
The thick sections along the eastern border of the Appalachian coal 
region contain floras distinctly older than those present in the lowest 
beds of the thin northwestern sections and the characteristic floras 
of the thin sections occur in their natural order in the upper part only 
of the thick sections (ibid., p. 271). 
