EXTENT OF FOLDING AND SUBSEQUENT EROSION 467 



The Tuscarora in most of the sections is characterized by Arthrophy- 

 cus harlani. In Pennsylvania its lower part may contain very coarse 

 quartz and other pebbles, forming in rare cases an "egg conglomerate." 

 Cross-bedding is commonly of the torrential type, though excellent ex- 

 amples of eolian bedding are also seen. The thickness varies greatly, but 

 on the whole it is thicker in the central part of the State and thins out 

 in all directions north, west, and south. Thus along Bald Eagle Moun- 

 tain it has a thickness of 1,068 feet at Tyrone Gap, though of this fully 

 one-half may belong to the Medina. It decreases to 400 feet in Wills 

 Mountain at Milligans Gap and to 287 feet at the Maryland line. Here 

 it is continuous, with the red Medina underlying, which also shows good 

 torrential cross-bedding. It is a snow white to light gray quartzite, fre- 

 quently cross-bedded, the quartz grains being of medium size. Yellowish 

 green, hard clay pebbles occur in it and Arthrophycus harlani is largely 

 restricted to the upper beds. In the lower beds, in the center of the Wells 

 Creek section, I have observed eolian cross-bedding and the grains are 

 fine, uniform, and well rounded, suggesting an eolian origin. Northward 

 along Bald Eagle Mountain the thickness is also variable. Thus at 

 Bellefonte Gap, in Center County, it is 400 or 500 feet thick; in Mill 

 Hall Gap, 695 feet. In the Lycoming County gaps it is only 100 feet. 

 Farther eastward the thickness also varies. Thus in Logans Gap through 

 Jacks Mountain it is 820 feet and is composed of hard, massive layers 

 from 2 to 4 feet in thickness. Some argillaceous material occurs, but the 

 sand is exceedingly pure. Secondary enlargement of grains has made 

 them angular and sharp where they separate out. Twenty-five miles west 

 of this point, at the Juniata Gap of Jacks Narrows, the Tuscarora is 450 

 feet thick, and 10 miles farther south, at Orbisonia, it is 400 feet and con- 

 tains some red layers. In the northeastern counties of Pennsylvania and 

 Maryland its thickness varies between 200 and 300 feet, rarely becoming 

 more. In the Mercersburg-Chambersburg quadrangle, Stose records an 

 average of 270 feet, which in the Maryland and Virginia regions (Paw- 

 paw-Hancock folio) is similar or decreases to 200 feet. A remarkable 

 feature of the sandstone in the southern region is its whiteness and 

 purity, though occasional shale bands occur. These should contain 

 Eurypterid fragments. Arthrophycus harlani is the only fossil so far 

 found. 



The Clinch sandstone. — The Clinch sandstone is in all respects the 

 equivalent of the Tuscarora of Pennsylvania. It resis on the red beds 

 (Bays), which, like their northern representatives, the Juniata, generally 

 represent a double formation, an older of late Ordovicic age, the equiva- 



