MISSISSIPPIAN CHERT OF ST. LOUIS AREA 373 
been introduced from other siliceous formations by the underground 
circulation, possibly that of the geologic present, does not seem 
valid. The formation of the chert cannot have taken place through 
the agency of the present-day circulation, since the presence of the 
chert at the base of the Pennsylvanian shows the period of forma- 
tion to have been late Mississippian or early Pennsylvanian. The 
jasperoid of the Joplin district, furthermore, is a chertlike siliceous 
deposit that is said to have been deposited by the same under- 
ground circulation that is responsible for the mineralization of the 
region. But the jasperoid is later than the chert and distinctly 
different. The more serious objection, however, is the conformity 
of the chert with the stratification. Vertical zones following 
the joints are not found. The chert is found widespread, but 
is not found in an adjacent formation and is more or less 
similar over wide areas, but different in aspect in the different 
formations. 
The derivation of the silica of the chert through precipitation 
from sea-water is a possibility. Silica is precipitated from solution 
by calcite and replaces it when H,CO, is present, and, as the 
accumulating sediments of the ocean bottom usually contain 
decaying organic matter, H,CO, should be present. A tenth of the 
yearly increment of saline material in the ocean is silica, but the 
silica content of sea-water is practically nil, 1 part in 220 to 460 
thousand. The very considerable annual increment of silica must 
therefore be removed quickly, either by direct chemical precipita- 
tion or through the action of organic agents. In the case of the 
Mississippian beds of the St. Louis area it is not known that sili- 
ceous organisms were present to any important extent at the place 
and time of the deposition of the beds. The degree of concentra- ° 
tion of the silica in the ocean-water, in connection with the slow 
rate of diffusion and proximity or distance from the mouth of a 
river, may be an important factor, contributing to the lack of 
chert in some limestones, as, for instance, the Salem and Kemms- 
wick limestones. Such chert-free limestones may have formed at 
a distance from the mouths of rivers, and the silica may have been 
completely precipitated before currents brought these waters to 
the place of deposition of these beds. 
