(our AL OF GEOLOGY 
Wales Vibe h—~ OCTOBRE TK. 1907 
THE PRE-RICHMOND UNCONFORMITY IN THE 
MISSISSIPPI VALLEY 
STUART WELLER 
Ulrich and Schuchert’ have called attention to the widespread 
emergence of the land in the interior of North America at the begin- 
ning of the Richmond stage of later Ordovician time, and the resub- 
mergence of the same during the later portion of the same stage. 
This epoch in the geologic history of the North American continent 
is one which has usually been overlooked, but which, nevertheless, 
is of vast importance. 
During mid-Ordovician time a great interior sea reached westward 
from Appalachia to the Rocky Mountain region. In this sea the 
Trenton limestone of New York, New Jersey, and Pennsylvania was 
being deposited, as well as the equivalents of this formation farther 
west, the Galena limestone of Wisconsin, Minnesota, northern Illinois, 
and Jowa, and the Kimmswick limestone of southern Illinois and 
southern Missouri. In the Big Horn Mountains of Wyoming the 
greater portion of the Big Horn limestone is also contemporaneous in 
age with the Trenton and Kimmswick limestones of the east. In the 
regions between these outcropping areas, now occupied by younger 
formations, limestones of similar age are doubtless buried beneath the 
younger strata. 
Following mid-Ordovician or Mohawkian time, this great interior 
sea became much contracted, so that the beds of Utica and Lorraine 
1. O. Ulrich and Charles Schuchert, “Paleozoic Seas and Barriers in Eastern 
North America,” Report of the New York Paleontologist for 901, pp. 645; 646. 
Vol. XV, No. 6 519 
