372 DONALD C. BARTON 
apparently must remain a matter of conjecture. Evidence of an 
organic origin is wanting. In slides of the St. Louis chert spicules 
were present only in one case. In his study of slides of the Mis- 
souri cherts Hovey reports that he found only one carrying sponge 
spicules. Likewise Van Tuy] in his study of the cherts of the Osage 
series found sponge spicules present in only one sample. Of the 
presence in numbers in the Mississippian seas of other silica- 
secreting organisms nothing is known. 
In the case of the cherts of the St. Louis area the theory that 
the chert is formed from the collection of colloidal silica on the sea 
floor is not applicable, since the chert is plainly secondary.  Hinde’s 
application of this theory to explain the presence of unsilicified 
shells in the chert is not necéssary, as the differential replacement 
is readily accounted for by the lower solubility of.the shell mate- 
rial. In this connection a brief series of tests was run on the 
relative solubility of recent and fossil pelecypod shells, fossil bra- 
chiopod shells, on crinoid stems, and on chert-bearing and chert-free 
limestone. The material was powdered to pass through a two- 
hundred-mesh sieve and was digested in 25 cubic centimeters one- 
half normal HCl, plus 350 cubic centimeters distilled water, and 
the time required for neutralization, as shown by methol orange, 
was noted. A marked tendency was shown toward less solubility 
on the part of the shells, crinoidal limestone, and chert-free lime- 
stones, such as the Salem, and greater solubility on the part of the 
limestones associated with chert. The experiments were not 
extended enough to be conclusive. 
That the chert was formed before the consolidation of the lime- 
stone from silica derived from the solution of siliceous spicules or 
tests is possible in the case of some of the chert. But it is definitely 
not possible in the case of most of the chert, as the chert did not 
form until after the limestone had acquired its granular character. 
The formation of the chert in a similar manner during, or later 
than, the consolidation is more possible. The chert is secondary 
and is pre-Pennsylvanian, and therefore must have formed dur- 
ing, or not long after, the consolidation of the limestone. There is, 
however, no positive evidence of the organic origin of the silica. 
The suggestion that the silica of the chert is exotic and that it has 
