364 DONALD C. BARTON 
LITHOLOGICAL CHARACTER OF THE CHERT 
To the naked eye the chert characteristically appears stony, but 
in some cases granular or chalky, although in the latter cases the 
chert is not appreciably less tough or hard. In color the chert 
varies from dirty white to dark gray and, except for one thin band 
of chalcedonic chert, is mottled and banded. The mottling is simi- 
lar in effect to that given by the grain to the limestone and is appar- 
ently a pseudomorphic character of the chert reflecting the granular 
character of the limestone. The banding is concentric with the 
form of the nodule, or horizontal. In the latter case it is a retention 
of the stratification markings. The chert with rare exceptions is 
fossiliferous wherever the limestone of that horizon is fossiliferous. 
Crinoid stems are by far the most common fossil. Bryozoa, Spiri- 
feri, Producti, and other brachiopods, Lithostrotion in the St. Louis 
limestone and Fusulina are also common. A notable feature about 
the fossils is that in a very great number of cases they are still cal- 
careous and do not show the effects of the siliceous replacements of 
the rest of the rock. 
In thin section under the microscope the chert is seen to be com- 
posed chiefly of quartz with more or less calcite, and in some cases 
with chalcedony, opal, dolomite, pyrite, and iron staining. The 
quartz making up the mass of the chert is in excessively fine grains, 
less than o.o1 mm. in diameter, which are not clearly distinguished 
even under the high power, and it is to the compensation due to the 
superposition of these small grains that many of the areas dark 
under crossed nicols are to be attributed. Locally, in many cases 
within a shell, there are patches of allotromorphic grains of larger 
size (0.1 mm.). Ina few thin sections there were seen larger, sub- 
angular, clastic grains. The calcite, abundant through much of the 
chert, is in small rounded grains that cloud certain areas or form 
patchy aggregates through the chert, or is in large grains forming 
the unreplaced shells. The calcedony, when present, is in thin, 
fibrous, wavy bands that permeate part of the chert, lining shells, 
and microscopic cavities. ‘The presence of the opal is inferred from 
the presence in part of the chert of much isotropic material with a 
moderately low index of refraction. ‘The dolomite, where observed, 
was in small rhombs scattered through the chert and was distinctly 
