40 PROCEEDINGS OF THE ANN ARBOR MEETING 



these represent the climax of his interpretative researches on sedimentary 

 formations. Nearly all of his antecedent work on sediments led up to 

 this masterpiece, and out of it grew another masterpiece, his papers bear- 

 ing the general title "Strength of the earth's crust." ^^ He says regard- 

 ing the Upper Devonian delta : 



"The Portage and Chemung are seen to be the shallow sea equivalents of 

 the Oneonta and Catskill. a subaqueous top set plain. The Skunnemunk eon- 

 glomerate is a down-folded remnant of a piedmont alluvial gravel plain which 

 lay between the flat delta surface and the mountains. The Pocono sandstone, 

 into which the Catskill passes by transition, is seen to be divided into two 

 phases — a marine phase in western Pennsylvania and Ohio, a fluviatile phase 

 in eastern Pennsylvania. In the Pocono the sharp delimit-ation of the two 

 phases is obscure, but between the Catskill and Chemung the color contrast 

 draws the dividing line separating the subaerial and subaqueous topset beds. 

 The margin of the delta no doubt held lagoons varying from brackish to fresh 

 water, so that marine fossils should be somewhat more restricted in their 

 range than the gray and olive shales." ''- 



"The red shale formations, the Catskill and the Mauch Chunk, show transi- 

 tions on the east into the overlying formations. The Pocono. on the contrary, 

 passes abruptly at its top into the Mauch Chunk shale. Both the Pocono and 

 Pottsville conglomerates are made up 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 climatic pendulum which carried the world from Upper Devonian 

 warmth and semi-aridity to Upper Carboniferous humidity and possible cool- 

 ness." ^" 



As regards the former extent of the Upper Devonian delta deposits 

 and the source of the material composing them, Barrell says : 



"The margins of the gravel plain may therefore be estimated at pcn-haps 

 from 20 to 35 miles to the southeast of the Green Pond syncline. making the 

 original limits of the Upper Devonian deposits from 45 to 60 miles southeast 

 of the present outcrops in Pennsylvania. . . . The distance to the north- 

 western edge of the crystalline floor concealed beneath the Coastal Plain is 

 now about 65 miles, and from beyond this line it would appear that the greater 

 portion of the (juartzite must have been derived." 



The site of the source of most of the sediments forming the Upper 

 Devonian delta is now the Atlantic Continental Shelf, or even farther 

 seaward, for a source must have existed for 63,000 cubic miles of material. 



The Triassic deposits of New Jersey and Connecticut are mentioned 

 in many of Barrell's papers, and their origin is particularly considered 



31 Am. Jour. Sci. (4), vol. 37, 1914, p. 250. See footnote. 



3= Am. Jour. Sci. (4), vol. 36, 1913. p. 46.5. 



33 Am. Jour. Sci. (4), vol. 37, 1914. pp. 241. 242. 



