98 Wisconsin Academy of Sciences, Arts, and Letters, 
which is of Trenton age. Accordingly the section here lacks the 
important Eden and Maysville groups of the Cincinnatian series. 
Farther south in this great valley, as in southeastern Missouri and 
northern Arkansas, the hiatus beneath the Eichmond is increased 
by elimination of the Galena so that the former lies on a limestone 
of either early Trenton or late Black Eiver age. In the western 
half of the continent it is still further increased, the first of the 
Eichmond deposits there being in contact with beds ranging in age 
from Middle or Lower Ordovician to Canadian, Ozarkian, or even 
Upper Cambrian. 
In the Appalachian Valley region the hiatus between the Ordo¬ 
vician and the Silurian is of lesser time value than in the Missis¬ 
sippi Valley. But here, locally, as in the vicinity of Lewistown, 
Pa., the red Juniata sandstone, which is of Eichmond or Lower 
Medina age and there rests on a thick mass of the latest Ordovician 
gray Oswego sandstone, is not only much thicker than usual but 
contains in its lower 400 feet many often thick beds filled with 
rounded quartz pebbles. In size the pebbles vary from very small 
to three inches in diameter. In central Pennsylvania, therefore, 
we find all the physical evidence that even the most critical ob¬ 
server could demand on which we may confidently base the con¬ 
viction that the boundary between the base of the Eichmond (or of 
formations corresponding in age to it) and the deposits beneath 
its base is of systematic value; (1) great thickness of red muddy 
sandstone indicating preceding, long enduring land-surface decay, 
(2) thick beds of quartz conglomerate, and (3) extensive trans¬ 
gressions of new seas over previously long emerged areas of much 
older rocks. It is important further to note that in Pennsylvania 
where the first and second of these conditions are more clearly in¬ 
dicated than usual the boundary between the Juniata and the over- 
lying Tuscarora sandstone (Upper Medina with which Schuchert, 
Grabau, and other paleontologists propose to begin the Silurian) is 
practically indeterminable. And yet the part of the section in 
which it must lie is perfectly exposed in the section between Lewis- 
town and Eeedsville. Aside from the Oswego-Juniata contact 
above discussed the only other reasonably possible plane at which 
the boundary between the two systems might be drawn is at the 
locally clearly defined contact between the base of the Oswego 
sandstone and the top of the Eeedsville shale (~Eden plus Lower 
