PAPERS ON GEOLOGY 263 
stone is known, and where it is only a few feet in thickness, 
it seems probable that a period of general peneplanation in this 
region intervened between the deposition of the latest of the 
Fernvale limestone and the earliest of the Thebes sandstone. 
The latter formation could thus not be older than about 
middle Richmond, and possibly is late Richmond in age. 
The unconformable relations of the Thebes sandstone with 
the underlying Fernvale limestone or older rocks in this region 
indicates that the remarkable succession of oscillatory move- 
ments of advance and retreat of the sea that occurred in the 
lower Mississippi embayment during the Alexandrian epoch of 
the Silurian period began here during the early part of Rich- 
mond time. 
THE ORCHARD CREEK SHALE 
The Orchard Creek shale occupies the distinguished position 
of being the oldest fossiliferous marine formation of Silurian 
age known in the United States. It is even more restricted in 
its distribution than the Thebes sandstone, being known in 
Illinois in only a few places in Alexander county in a distance 
of 4 or 5 miles north and south of Thebes. The formation was 
named from Orchard Creek which joins the river south of 
Thebes in Alexander county, near the mouth of which stream 
this shale is exposed in the bank of Mississippi River. 
The best known exposures of shale of this formation is along 
the Chicago and Eastern Illinois railroad about one-half mile 
south of Gale, and in the river bank near this place. The strata 
consist of bands of blue or bluish-gray calcareous shale, 4 to 
6 inches thick, alternating with layers of impure, argillaceous 
limestone which in the lower and middle parts are 2 to 4 inches 
thick. The calcareous layers become thicker and closer to- 
gether in the upper part where the bed grades upward without 
any distinguishable sedimentary break into the overlying 
Girardeau limestone. The total known thickness of the for- 
mation is about 22 feet, 
The calcareous layers of this formation furnished the fol- 
lowing species of fossils: 
Fossils from the Orchard Creek Shale : 
Strophostylus ornatus n. sp. 
