The two patch reefs investigated are within Leonardian (Middle Permian) strata in the northwestern part of the Finlay Mountains, about 16 mi (25.8 km) northwest of Sierra Blanca, Texas. The Leonard Series in these mountains was deposited on the southwestern margin of the Diablo platform bordering the Marfa basin and consists largely of marlstones, with smaller amounts of calcirudites and bioclastic limestones. The environment of deposition was one of relatively shallow, quiet waters that periodically sustained many benthonic calcareous organisms. Fragments of the skeletons of this fauna were deposited as bioclastic limestones on which patch reefs could become established. In the two patch reefs studied in detail, four facies are readily recognizable: subreef, reef flank, eef core, and suprareef. The principal reef-building organism in the reef-core facies is the algal genus Tubiphytes Maslov. Other organisms include calcareous sponges, fenestrate bryozoans, brachiopods, and rugose corals. Abundant crinoid stems and fusulinids are found in the associated bioclastic rocks. These reefs are similar to, but smaller than, the oil producing reefs of the same age on the platform edges around the Delaware and Midland basins on the east, but this is the first account of such reefs in the Finlay Mountains.