MISSESSTPPIAN: CHERT OF ST. LOUIS AREA 263 
was impossible to correlate with certainty throughout the area the 
higher cherty horizons. 
MORPHOLOGY OF THE CHERT BEDS 
The general form of the chert varies considerably from bed to 
bed, although it seems to be more or less constant for any individual 
horizon. It is possible to distinguish several distinct types of 
occurrence of the chert. Most of the chert in the Burlington- 
Keokuk limestone and some of the chert in the St. Louis limestone 
are in nodular bands or bands of flattened nodules. The nodules 
are irregularly elliptical with horizontal diameters subequal and one 
and a half to twice the vertical diameter, and are seldom less than 
three centimeters or more than ten centimeters in diameter ver- 
tically. The nodules are rounded and usually sharply delimited, 
at least to megascopic examination, from the inclosing limestone. 
In any given chert bed these nodules show distinct distribution 
parallel to the stratification and by coalescence form the nodular 
bands intercalated between the thin limestone beds. In a few 
chert beds in the Burlington-Keokuk the chert is irregularly rami- 
fying, with angular outlines, and in general pattern resembles some 
of the mottled Ordovician limestones. Although the chert of 
this type usually crosses many distinct layers of the limestone, 
the greatest development shows distinct distribution parallel to 
the stratification. A form of the chert very characteristic of the 
St. Louis limestone and found but very rarely in the Burlington- 
Keokuk are the bands of nodules, spherical or ovoid in shape and 
six to sixty centimeters in diameter. The contact of the nodule 
and the limestone is apparently sharp, but there is usually present 
a thin chalky-looking transition zone. The nodules in a given band 
characteristically are well aligned to some horizon, in many cases 
the middle of a thick limestone bed. A form of the chert that is 
found both in the St. Louis limestone and in the Burlington- 
Keokuk limestone is a thin band, pancake-like in the St. Louis and 
platelike in the Burlington-Keokuk. These bands most commonly 
are six to ten centimeters thick, but in some cases much more, and 
fifteen to fifty meters long. The contact of the chert of these bands 
with the limestone seems to be sharp. 
