The environment of deposition and diagenetic changes of some of the Late Pennsylvanian and Early Permian limestones of the Glass Mountains are closely reflected in their field relations and in their texture and particle composition as determined by thin section investigations. From analysis of stable C isotopes, the fine-grained, light-colored, algal limestones indicative of an oxidizing environment have distinctly positive delta C 13 values, whereas fine-grained, dark-colored fetid limestones indicative of reducing environments have markedly negative delta C 13 values. The coarse grained limestones having abundant sparry calcite cement have delta C 13 values near zero, that is, a C 13 /C 12 ratio near the standard. Chicago PDB, a belemnite. These C 13 values suggest that the source of the sparry calcite cement is at least in part from CO 2 produced by methane fermentation of organic material in the substrata.