IN THE DEGRADATION OF THE LITHOSPHERE. 373 



agent of grinding. In the case of the Pottsville one of the marked 

 features of it is the occurrence at variable intervals of massive 

 siliceous conglomerates which alternate with fine sands and coal beds. 

 David White-*' has summarized the literature in regard to the Potts- 

 ville and the following quotations are from his paper :-^ 



" the Pottsville formation is in the type region composed chiefly of massive 

 siliceous conglomerates . . . which comprises a series of ponderous conglom- 

 erates which are more variable in color, composition and assortment in the 

 lower part and more quartzose, dense and light colored near the top . . . and 

 are interspersed with a number of carbonaceous beds workable over consid- 

 erable areas. . . . The conglomerates intercalated in increasing proportion in 

 the upper beds of the Mauch Chunk consist of irregularly bedded poorly 

 assorted or sometimes apparently unassorted pebble or bowlder accumula- 

 tions 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. Occasionally the pebbles which are sometimes subangular 

 attain a diameter of 3 or 4 inches or more; . . ." 



Further in the same report (pp. 861-863) White discusses the occur- 

 rence of these conglomerates and their possible origin and says : 



" The remarkable strength and varying activity and direction of the move- 

 ments of the early Pottsville sediments over the Mauch Chunk delta in the 

 Schuylkill-Swatara region during a period of oscillating tide level are proved 

 by the alternation and high degree of irregularity in the Pottsville beds, by 

 the transportation of the conglomerate building material to a long distance 

 from the present margin — i. e., by the long radius of the fan — and by the 

 size of the bowlders which are sometimes encountered far from the margin 

 of the field. In illustration of the latter circumstance, the occurrence of 

 bowlders, 7 or 8 inches in diameter in Head Mountain, described by Rogers 

 ('Geol. Pennsylvania,' Vol. II., pt. i, p. 22) may be cited. . . . The inter- 

 ruption of the general subsidence by short periods of elevation and stability 

 . . . accounts also for the readiness with which the conglomerated sediments, 

 which usually almost directly, when not immediately, overlie every Lykens 

 coal, were swept across the carbonaceous deposits on the recurrence of the 

 general downward movement." 



White, in discussing the origin of these Pottsville sediments of 

 so pronounced a character, seems to fall back upon the idea of a 

 rapidly depositing series of coarse sediments washed down from 

 high lands back. (See quotations as above.) There are indicated 



20 David White, " The Stratigraphic Succession of the Fossil Floras of 

 the Pottsville Formation, etc.," 20th Annual Report of the Geol. Survey, U. S., 

 pp. 749-953 (Part 2). 



-1 Idem, pp. 762-763, 861-863. 



