Chert in the Cow Head Group is mainly a replacement of limestone
and shale and, to a lesser extent, an interparticle cement. Its
field occurrences are distinct as: (1) silicified margins on
coarse conglomerates and thinly bedded limestones; (2) nodules
within limestone and shale; (3) pervasively silicified beds of
limestone and shale; and (4) clasts or partial replacement of
clasts within conglomerate.  Radiolarians and sponge spicules
are composed of microquartz or calcite and are particularly
common in the Ordovician part of the succession where most chert
occurs. In limestone spatially associated with chert, the use of
cathodoluminescence demonstrates that calcite-replaced
radiolarians and spicules are volumetrically more important than
realized through transmitted-light petrography. Petrographic
relations between siliceous and rare pyritized radiolarians
further indicate that these particles may be dissolved prior to
compaction. No trace of their former existence remains, other
than indirectly through the