The El Rosario Quadrangle, situated 100 kilometers northwest of the capital, Tegucigalpa, on the Carretera del Norte, straddles the boundary between the Volcanic Ranges and Plateaus and the Central American Cordillera morphotectonic units, and includes pre-Mesozoic metamorphic rocks, Mesozoic and Tertiary sedimentary rocks, and Tertiary intrusive and volcanic rocks. The oldest rocks exposed are two facies of the pre-Mesozoic Cacaguapa Schist: the Humuya Member of sheared conglomerate, containing evidence of two metamorphic periods, and schist with interlayered, boudinaged meta-andesite; and the Las Marias Member of sericite-quartz schist with inter-layered marble and quartzite. Nonconformably overlying the metamorphic rock are interbedded conglomerate, sandstone, and shale, with intercalated volcanic rocks of the early Mesozoic Todos Santos Formation. The early Cretaceous Yojoa Group of carbonate rock overlies the Todos Santos Formation. Two formations comprise the Yojoa Group: Cantarranas Formation of thin-bedded, marly limestone and the overlying Atima Formation of massive limestone. The Late Cretaceous to early Tertiary (?) Valle de Angeles Group includes red sandstones and shale with minor conglomerate. Padre Miguel Group (Miocene?) siliceous ignimbrite and reworked pyroclastic rock are represented by the La Sabana and Cerro Le Cañada ignimbrite members. The La Sabana ignimbrite has an olivine basalt flow within its lower part. Intrusive igneous rocks include rhyolite with tuffaceous, devitrified-tuff, or granophyric textures, basalt and gabbro dikes, and intruded gabbro. These rocks record three major episodes of deformation: pre-Mesozoic folding and development of schistosity; the Montaña de Comayagua structural belt, a N. 60° W. structural high with high-angle reverse faults formed during the Laramide orogeny; and Tertiary faulting that formed north-trending grabens. The Opoteca silver mine may be the richest mineral deposit within the El Rosario Quadrangle. / text