MISSISSIPPIAN CHERT OF ST. LOUIS AREA Biya: 
t. The chert is supposed to be derived from colloidal silica which 
formed from the decomposition of siliceous sponges and collected 
in the depressions of the sea floor. The chert bands are supposed 
to represent former sponge beds, where the sponge remained accu- 
mulated in place over a considerable area. 
2. The chert is supposed to form before consolidation of the 
limestone through the solution of scattered siliceous spicules and 
the almost immediate replacement of parts of the limestone. 
3. The chert is supposed to form after the consolidation of the 
limestone through the solution by percolating waters of the siliceous 
spicules and the replacement of part of the limestone by this dis- 
solved silica. 
II. The silica is supposed to be of inorganic origin. 
4. The chert is supposed to form by the precipitation of silica 
and the replacement of the limestone in the presence of circulating 
waters which have passed through sandstones, arenaceous rocks, or 
rocks containing silicates. 
5. The chert is supposed to result from the reaction of the dis- 
solved silica of sea-water with the limestones, with the consequent 
precipitation of the silica and with possibly a later concentration. 
6. The chert results from the diffusion of silica in solution 
through a limestone. The concentration will vary in the direction 
of the diffusion, and the deposition resulting when the concentration 
is sufficient will be in zones perpendicular to the direction of 
diffusion. As the conditions for diffusion are more favorable in the 
early days of the consolidation and as the most likely direction for 
the diffusion is upward toward the surface or downward from it, 
the deposition will be parallel to the stratification, although inde- 
pendent of it. _The development of the chert in successive zones is 
due to the lowering of the concentration immediately around the 
first started zone or zones of crystalizing material. The silica may 
be derived from organic or inorganic sources. 
ORIGIN OF THE CHERT OF THE ST. LOUIS AREA 
The source of the silica of the chert of the St. Louis area is not 
as clear as in the cases of the English cherts and flints, the radio- 
larian cherts of California, and the cretaceous cherts of Texas, and 
