the Middle Atlantic Slope. 131 



mapped by C. E. Hall, who designated the deposit " Ferrugin- 

 ous Conglomerate," and followed Lewis in referring it to the 

 Tertiary.* The low-lying clays in the valley of the Delaware 

 are not well exposed, have yielded no fossils, are not known to 

 be connected with the plastic clays of Delaware, and' cannot be 

 certainly correlated with the upper member of the formation ; 

 but the gravelly outliers belong to a series traced from the 

 Rappahannock river northward, everywhere sustaining identi- 

 cal relations to the underlying rocks and to the topography, 

 and everywhere petrographically similar to a characteristic 

 phase of the formation as typically developed in Maryland and 

 Yirginia, and may be confidently correlated with the lower 

 member. Indeed, in the exposure in Media the most abund- 

 ant materials are characteristic kaolinic arkose and more hetero- 

 geneous brown ferruginous sands such as occur about the head 

 of Chesapeake bay, coarse gravel being but a subordinate ele- 

 nient.f In the vicinity of the Susquehanna the gravel of the 

 outliers is generally coarse and abundant ; over the plateau be- 

 tween that river and the Schuylkill it is finer and less abund- 

 ant: and toward the latter stream it again grows coarse and 

 the outliers become more conspicuous. 



North of the Schuylkill the lower member is typically de- 

 veloped at several localities in Rose valley, in the northwestern 

 part of the village of the same name. The deposit is an ob- 

 scurely bedded arkose, generally friable but sometimes lithifiecl, 

 containing well worn quartz and quartzite pebbles both dissem- 

 inated irregularly and in lines, together with pellets and flakes 

 of clay, the whole similar to the deposits found in Yirginia ; 

 and silicified wood occurs in small quantities, but neither leaf- 

 impressions nor the bands of clay in which they are commonly 

 preserved were found. The mass is irregularly stratified and 

 evidently undisturbed, and rests directly upon tilted Triassic 

 sandstone. Again, three or four miles north of Conshohocken, 

 extensive deposits of white, pink, and mottled plastic clay, 

 petrographically indistinguishable from the upper member of 

 the formation where typically developed, are found overlying 

 gravelly arkose. The clay beds are largely worked for pottery, 

 while the subjacent arkose is excavated and screened for build- 

 ing sand and road metal — indeed there is a considerable area 

 north of Conshohocken and Norristown in which the proxim- 

 ity of the Potomac is proved by the presence of its materials 

 in roadways and foot-paths, and in the mortar of houses and 

 barns. Still farther northward outliers of gravel generally 

 finer than that found in the vicinity of the Schuylkill occur, 



*2nd Geol. Surv. Pa., Eeport 05, 1885, 10-13, and map. 



f 0. E. Hall notes pebbles 6 inches in diameter at this point ; but so large peb- 

 bles are probably rare, and were not found by the writer. 



