MISSISSIPPIAN CHERT OF ST. LOUIS AREA 265 
more abundant in the chert than in the surrounding limestone. 
Pyrite was present in only a few cases and was in small cubes in the 
center of quartz-filled cavities. Im much of the chert there is a 
slight amount of iron staining present. 
The structure of the chert as revealed under the microscope 
varies considerably. Part of the chert is massively composed of 
very fine grains, whose boundaries cannot be made out. Much of 
the chert is similarly composed in large part, with ramifying, 
banded microscopic masses of calcedonic material or interlocking 
quartz grains, which seemed to have filled pre-existing cavities. 
The interior cavities of shells are in most cases filled with inter- 
locking quartz grains. The shells, even very minute ones, for the 
most part are composed of calcite in medium-sized grains, but in 
some cases show partial or complete replacement. Where replace- 
ment of a shell has taken place it is mostly by fine allotriomorphic 
quartz granules. Concentric banding is shown by much of the 
chert and is caused in some cases by very slight differences in the 
amount of staining, probably ferruginous, and in other cases by 
a variation in the amount of admixed calcite grains, the calcite 
rich areas tending to a chalky white color. In a chert much 
resembling in marking the mottled Ordovician limestones the 
mottling is likewise due to a rapid variation of the proportions of 
calcite to quartz grains. Banding due to stratification is present 
in some of the chert and is shown by the orientation of minute 
shells, by variation in the amounts of a cloud of fine black specks, 
and by variation in the staining. 
RELATION OF CHERT TO SURROUNDING LIMESTONE 
The character of the contact of the chert and the limestone 
apparently varies with the different chert beds. In many cases 
there is a visible transition extending over a zone of one to two 
centimeters. This is particularly the case with the spherical 
nodules of the St. Louis (Fig. 2). But the contact more commonly 
is sharp, at least under megascopic examination. In the Osage 
chert of Iowa such contacts are reported by Van Tuyl microscopi- 
cally to show gradual transition. In the thin sections of St. Louis 
