PETER SANDSTONE 
289 
the sandstones of the Roubidoux formation are almost identical 
with the Peter terranes. The grains are commonly well-rounded, 
although this feature is often greatly obscured by secondary 
quartz-enlargements. The formation is notably cross-bedded and 
ripple-marked, the ripples being of both the current and oscilla¬ 
tory types. Sun-cracks are also abundant, despite the fact that 
the formation carries marine fossils throughout. The fossils 
thus far found are limited to the cherty beds; none being found 
either in the sandstones or the limestones. According to the fossils 
submitted to E. O. Ulrich for identification the Roubidoux sand¬ 
stone seems to correspond to the horizon of the lowest Beekman- 
town beds. 
Above the Roubidoux sandstone, in slight local unconformity, 
rests the thin-bedded, argillaceous Jefferson dolomite, 200 feet or 
more in thickness, containing thin, discontinuous sandstone lenses. 
On the latter are ripple-marks, and sun-cracks occur sparingly; 
marine fossils are found at several horizons. 
Following the Jefferson dolomite, with apparent conformity, 
is the cherty, argillaceous Cotter dolomite, with a dominant sand¬ 
stone horizon near the middle. The maximum thickness, to the 
south in Arkansas, is 500 feet. A characteristic Beekmantown 
fauna is reported from these beds in Arkansas. The Jefferson 
and Cotter dolomites may be the equivalent of the Shakopee dolo¬ 
mite of the north. 
The next formation, called Powell, which rests on the deeply 
eroded and weathered surface of the Cotter dolomite, consists of 
200 feet of magnesian limestones, with some sandstones. It oc¬ 
curs as far north, in Missouri, as Ste. Genevieve County, and is 
the highest formation of Beekmantown affinities known in the 
state. The Powell beds of Arkansas are correlated with the upper¬ 
most Beekmantown section. 
On the deeply eroded surfaces of the Beekmantown, or Canad¬ 
ian beds, and lapping northward directly upon the Late Cambric 
strata in Wisconsin, rests the Peter succession. At its base in 
Missouri and Arkansas are a thick basal sandstone and a dolomite, 
together described as the Everton formation. The sandstone, 
which contains chert and limestone conglomerate and red residual 
clays at its base, is very variable in thickness, carries numerous 
thin, limestone lenses yielding marine fossils in Arkansas, is 
