PHYSICAL AND FAUNAL EVOLUTION 239 
of cross-bedding; and just below the upper white quartzite. In east- 
central New York it is found at the base of the Oneida conglomerate, 
which is the approximate equivalent of the upper white sandstone of 
Niagara. In the Appalachians, it is found mostly in the upper part of 
the Tuscarora and Clinch sardstones, the stratigraphic equivalent of 
the Medina. Sarle' has recently interpreted this structure as due 
to worm borings. So far as I have observed in the field, the raised 
ridges of this fossil always occur on the under side of the sandstone 
layers, representing, therefore, the relief molds of grooves generally 
formed in the clays beneath. These grooves had a median ridge and 
a regular succession of transverse ridges separated by broad concave 
grooves. A similar structure, known as Climatichnites trails, but of 
a much broader type, occurs in the Potsdam sandstone of New York. 
Woodworth? has suggested that it represents the trail of an animal 
comparable to some extent to modern Chiton. 
There are no known remains of organisms in the Medina or 
Clinton capable of making such an impression, and the organism 
which made it either had no parts capable of preservation or else 
it was a terrestrial type frequenting the shores and sandy wastes, 
where it left its trail in the mud, but not its remains, just as the 
Triassic Dinosaurs left their footprints but seldom their skeletons. 
The Tuscarora has a thickness of 820 feet in Logan’s Gap, Jack’s 
Mountain, Mifflin County, Pennsylvania, but thins perceptibly west- 
ward and southward, being 400 to 500 feet thick in Bald Eagle 
Mountain and 287 feet in Wells Mountain and the Pennsylvania- 
Maryland line. This thinning appears to be due to failure of the 
lower beds, showing a true case of non-marine progressive overlap. 
In New York, the upper part is represented by the true Medina, 
which has a thickness of 125 feet, and begins and ends with a pure 
white quartz sandstone. More strictly speaking, the upper white 
sandstone alone represents the true Tuscarora, but the lower beds, 
still partly red, and the shales, probably are the equivalent of the lower 
reddish sandstones and greenish shales underlying the true white 
Tuscarora, and sometimes referred to the Upper Juniata. The 
Oneida conglomerate of central New York, 40 feet thick, is likewise 
t Rochester Acad. of Sci. Proc., 1906, No. 4, p. 203. 
2 New York State Pal. Rep., 1907, Bull. New York State Museum, No. 69, p. 959. 
