In 1978 the third test (Hoe Creek III) in a series of underground coal gasification (UCG) experiments was completed at a site south of Gillette, Wyoming. The post-burn study of the geology of the overburden and interlayered rock of the two coal seams affected by the experiment is based on the study of fifteen cores. The primary purpose of the study was to characterize the geology of the overburden and interlayered rock and to determine and evaluate the mineralogical and textural changes that were imposed by the experiment. Within the burn cavity the various sedimentary units have been brecciated and thermally altered to form several pyrometamorphic rock types of paralava rock, paralava breccia, buchite, buchite breccia and hornfels. High temperature minerals of mullite, cordierite, oligo-clase-andesine, tridymite, cristobalite, clinopyroxenes, and magnetite are common in the pyrometamorphic rocks. The habit of these minerals indicates that they crystallized from a melt. These minerals and textures suggest that the rocks were formed at temperatures between 1200/sup 0/ and 1400/sup 0/C. A comparison of geologic and geological-technological factors between the Hoe Creek III site, which experienced substantial roof collapse, and the Hanna II site, which had only moderate roof collapse, indicates that overburden thickness relative to coal seam thickness, degree of induration of overburden rock, injection-production well spacing, and ultimate cavity size are important controls of roof collapse in the structural setting of the two sites.