Fragments of muscovite-chlorite-graphite schist in a volcanic pipe agglomerate of the Pico Etereo area resemble schist exposed along the faulted western flank of Sierra del Carmen near Boquillas, Coahuila. The agglomerate consists mostly of rounded- to sub-rounded fragments of limestone and rhyolite or trachyte in a rhyolitic matrix. Minor amounts of baked shale, orthoquartzite, and schist are scattered throughout the matrix. Many of the limestone fragments were derived from the Glen Rose Limestone. The schist fragments were derived from the interior zone of the Ouachita structural belt which underlies much of the northern part of the Coahuila platform. Depth to the basement is at least 2800 ft in the Pico Etereo area.