288 
PETER SAiNDSTONE 
TAXONOMIC SIGNIFICANCE OF PETER SANDSTONE 
By Prof. C. L. Dake 
Missouri School of Mines, Rolla 
In the course of field work during several seasons past on the 
Early and Mid Ordovicic formations of the Ozark region much 
light appeared to be thrown upon the origin and stratigraphic 
relations of that great arenaceous terrane widely known through¬ 
out the Upper Mississippi Valley as the St. Peter, or Peter, sand¬ 
stone. Starting out with the imposed assumption that this forma¬ 
tion was eolian in its nature evidences soon became overwhelming 
that the beds really were ordinary marine sediments. 
Inasmuch as a history of every geological formation is usually • 
closely tied up with the histories of overlying and underlying 
terranes, it serves greatly to elucidate events to outline briefly 
the main features of Early Ordovicic stratigraphy, and to place 
particular stress upon that of the Ozark uplift, making occasional 
comparisons with the salient characteristics of adjacent tracts. 
The Ordovicic section of Missouri, as delimited in the present 
connection, begins with the Roubidoux sandstone, which is be¬ 
lieved to be the southern equivalent of the New Richmond sand¬ 
stone of the north. Locally, at least, this formation embraces a 
thin basal conglomerate, composed chiefly of chert-pebbles pre¬ 
sumably from the underlying Gasconade limestone. Other evi¬ 
dences of an erosion surface at this horizon are obscure. The 
Roubidoux formation varies from 75 to 200 feet in thickness. 
It consists of a succession of beds in which lateral gradations 
from sandstone into chert or limestone are the rule, and, while 
one section may be almost wholly sandstone, another may be over 
three-fourths limestone. Limestones become more abundant to 
the east, west, and probably to the south. In the size of grain, 
