The Rex chert member of the Phosphoria formation of Permian age was studied petrographically from specimens collected across the stratigraphic sections at five localities in Idaho, Wyoming, and Utah. The Rex is a fine-grained quartzite with chert in its upper part in the Big Hole Mountains of Idaho. Southward toward the Utah-Idaho lines much of the clastic character is lost, calcareous chert predominates, and the member increases in thickness. It becomes thinner again farther south toward the type locality.
Microcrystalline quartz predominates in the chert, but some thin sections were studied in which three varieties of silica—clastic quartz grains, chalcedony, and microcrystalline quartz—all occurred in a single section. Petrographic evidence suggests that the chert of the Rex is primary with probably considerable diagenetic replacement of carbonate. It is inorganic in the main but contains some siliceous sponge spicules. Subsequent replacement is believed to be insignificant.