SILURIC FORMATIONS IN MISSOURI 
135 
defined plane of unconformity. This fact may indicate that the 
terrane is the correlative of the Goweran series of the region 
to the north, best displayed in northeastern lowa.^^ 
The nether horizon of the Siluric succession is sharply defined. 
Everywhere throughout the area under consideration in which they 
are exposed the Siluric limestones are underlain by blue shales 
of Ordovicic age. In Pike county these shales attain a thickness 
of more than seventy feet. They doubtless belong to that part of 
the general geological section which, in Iowa, is known as the 
Maquoketan series,^^ although in northeast Missouri the special 
title Buffalo shales is sometimes applied to them.^^ In southern 
Missouri and southern Illinois these shales are known as the 
Thebes formation. In the east, in Indiana and Ohio, they pass 
under the title of Richmond, or Cincinnati, shales. 
South of the Missouri river there comes in between the Noix 
limestone or basal terrane of the Siluric of the northeastern part 
of the state, and the Maquoketan shales, other formations which 
are wholly unrepresented by formations in the north. Most 
important of these is the Girardeau limestone. 
The Siluric beds thus rest in marked unconformity upon the 
Ordovicic strata. This fact is a significant one in all stratigraphic 
considerations of the Siluric section of the region. It points to 
far-reaching diastrophic movements which ushered in Siluric 
deposition in the area and to profound changes in the local sedi¬ 
mentation in the region. 
Whether represented by the conspicuous white oolite, with its 
prolific fauna, or by the soft, yellow limestone, which is non- 
oolitic and highly fossiliferous, the Siluric succession of rocks in 
northeast Missouri is everywhere at the base sharply set off from 
all rocks beneath. 
The delimitation of Siluric terranes is also sharply marked. 
At Louisiana there immediately overlies the Bowling Green dolo¬ 
mite a black shale having a thickness of about three feet. Above 
the black layer are a few feet of green shale; then follows the 
thick Louisana limestone. West of the city five or six miles, on 
Grassy creek, the black shales attain a thickness of 40 feet; and 
12 Iowa Geol. Surv., Vol. XXII, p. 155, 1813. 
13 Iowa Geol. Surv., Vol. XXII, p. 155, 1913. 
14 Proc. Iowa Acad. Sci., Vol. V, p. 61, 1898. 
