THE SILURIAN SECTIONS 551 



Silurian. 



Medina or Tuscarora quartzites. Total thickness of "White Medina*' 820 

 feet. A series of very hard, fine-grained, white, and cleanly washed 

 quartzites in thick strata below and thinner bedded above, decidedly 

 cross-bedded and without conglomerate. Toward the top more and 

 more of thin sandy shales is interbedded, and between 200 and 300 

 feet above the base there is a zone, 50 feet thick, of a pinkish color. 

 The white quartzites are much spotted with ferruginous specks, the 

 result of oxidized siderite grains. At about 300 feet above the base 

 of the formation Arthrophycus alleghaniense is very common, and 

 apparently the same burrows were seen at 20 feet above the base. 

 At many levels may also be seen the vertical burrows of Scolithus 

 verticalis, and this fossil hereabouts is a more usual guide to the 

 Medina than is Arthrophycus. 



The contact with the Ordovician is a disconformable one and may be 

 seen on the north side of the stream just where the trolley line begins 

 to cross it. On the upper side of the break are white quartzites, 

 somewhat darker than those higher in the formation, and these rest 

 in marked contrast on the red quartzites of the Ordovician which 

 have interbedded shale zones of a maroon color. 



In the "Narrows" of the Juniata River, 3 to 5 miles southeast of Lewis- 

 town, the Upper Tuscarora sandstone is well displayed, but mainly 

 as talus. Here it is a thin-bedded, gray to milky white and locally 

 pinkish, cleanly washed, cross-bedded, fine-grained quartzite, inter- 

 bedded with thin layers of greenish shale and an occasional thick one 

 of black shale. Arthrophycus alleghaniense is common here, espe- 

 cially at the very top ; the original locality for this guide fossil to the 

 Medina is 10 miles east of Lewistown, on the Juniata River. 



Probable break in sedimentation. 



Ordovician. 



Juniata formation, 1,590 feet thick. The upper member, "Medina Red 

 Sandstone and Shale" of Dewees, is 1,280 feet thick. This soft for- 

 mation, probably of continental origin, erodes into valley form and 

 consists of brick-red, more or less cross-bedded, sandy shales, inter- 

 bedded with muddy sandstones that become finer grained upward. 

 The sandstones are replete with intraformational flat pebbles of red 

 shale. There is also much rippling and sun-cracking. The upper- 

 most 30 feet are much harder and cleaner red quartzites with thin 

 separation zones of maroon colored shales. No fossils of any kind 

 are present, and because of its stratigraphic position the formation is 

 correlated with the similar Queenston of western New York, the con- 

 tinental phase of the marine Richmondian. 



"Oneida Red conglomerate" of Dewees. Thickness, 310 feet. This is a 

 mountain-making formation and forms the western ridge of Jacks 

 Mountain. It consists of a series of heavy-bedded, coarse and con- 

 glomeratic, reddish quartzites, the material of a quick-running river 

 laid down on the floodplain of a delta, since the cross-bedding and 

 channeling are most marked. The formation is replete with zones of 



