370 DONALD C. BARTON 
the pancake-like chert bands appear, for example, is more or less 
contorted, although in the closely associated, chert-free beds it is 
even and regular. The slight vertical displacement in the slicken- 
sides that are found at the lateral contacts of some of the larger 
nodules has already been mentioned. 
Yet while the chert seems definitely to be secondary, there is 
an aspect on its part of contemporaneity with the limestone. This 
is evidenced by the widespread development of the chert in the 
St. Louis limestone and in the Burlington-Keokuk limestone, 
although the intervening Salem limestone—Bedford odlite lime- 
stone—is practically chert-free. The constancy of habit of the 
chert in the St. Louis limestone and in the Burlington-Keokuk, 
- respectively, over a wide area has already been noted. The devel- 
opment of the chert also is parallel to the stratification. In a band 
of isolated nodules there is characteristically a striking alignment 
of the nodules at some level in a bed, often a massive one. There 
are in some cases several bands, and in a few cases there is no align- 
ment of the nodules, but where the bands are present they are 
parallel to the stratification. The nodule-containing bed is in some 
cases three or four feet thick, with no shale partings, is uniform in 
grain and character throughout, and shows no apparent cause for 
percolating waters, whether silica-bearing or not, to flow at certain 
definite and localized levels. The pancake-like chert masses and 
the chert lenses likewise show conformity to stratification, although 
in this latter case there is in many places coalescence of several 
lenses by lateral thickening. As far as could be seen there was 
no possibility of localization of the chert at definite levels by 
control of percolating waters by shale partings or such. Ii 
the chert were purely epigenetic, it would seem probable that 
the chert bands would show some tendency to cut across the 
stratification. 
The various theories that have been proposed to explain the 
origin of chert may be said in essence to be six. 
I. The silica is of organic origin, derived chiefly from the spicules 
of siliceous sponges. The silica may be derived from other of the 
siliceous organisms, as, for instance, in the case of the radiolarian 
cherts of California. 
