242 ./. Barrell — Uj)j)er Devonian Delta of the 



erates are made np dominantly of much water-worn white 

 quartz pebbles, and their whole areas are characterized by a 

 great dominance of siliceous over argillaceous contents. All 

 of these features correspond to the theoretic results upon a 

 broad piedmont slope of increasingly wide swings of the cli- 

 matic pendulum which carried the world from Upper Devo- 

 nian warmth and serai-aridity to Upper Carboniferous humidity 

 and possible coolness. Such are the major changes through 

 the later Paleozoic ; changes which find parallels in the geo- 

 synclines of the Maritime Provinces of Canada and. of north- 

 western Europe. They form a crescendo leading from the 

 Middle Devonian and terminating in the Permian, a period of 

 mixed and unparalleled aridity and glaciation. 



On these great climatic movements were superimposed the 

 minor climatic rhythms and from them must be separated the 

 effects of the crustal movements. In the continental interior, 

 the region of shallow seas, the effects of diastrophism are more 

 in evidence; an unconformity separating the Mississippian and 

 Pennsylvanian and the latter exhibiting progressive overlap. 

 The great thickness of the Pennsylvanian deposits of the south, 

 measured by many thousands of feet, following the limestones 

 of the Mississippian and the black shales of the Devonian, also 

 testifies to great crust movements during the Pennsylvanian in 

 those regions. The Pennsylvanian deposits of the Acadian 

 and Boston basins seem also to imply orogenic movements of 

 considerable magnitude in the north. 



The facts which have been previously cited regarding the 

 formations in eastern Pennsylvania seem, however, to corre- 

 spond most largely to a climatic cause. The far-carried con- 

 glomerates and sandstones of the Pottsville associated with 

 some black shales and coal ; to a lesser degree the same rela- 

 tions in the Pocono, contrast with the absence of conglom- 

 erates in the Mauch Chunk, and the presence there of thick 

 red shales and the marks of aridity. In the Catskill these 

 features resemble the Mauch Chunk but conglomerates are not 

 wholly absent on the east and the indications in the red shales 

 are those of semi-aridity. The climatic control which explains 

 these relations of conglomerates and shales also points strongly 

 in consequence toward the former existence of a piedmont 

 plain, now eroded, which during the Catskill and Mauch 

 Chunk epochs became a storehouse of siliceous gravelly waste, 

 which by the stronger rivers of the Pocono and Pottsville 

 epochs was spread over the delta plain and given to the waves 

 of the shallow sea. The amount eroded from the Piedmont 

 slope was but a fraction of the volume of the whole deposit. 

 It was that amount which was contained between the stable 

 river grades before and after the climatic change. The 



