The Coban-Purulha area, comprising approximately 400 sq. mi. in central Guatemala, lies in the folded and faulted front ranges of the Antillean Mountain system which extends eastward from Chiapas, Mexico, across Guatemala, and into the Caribbean Sea. The outcropping rocks are of Paleozoic and Mesozoic age. The Tactic and Chochal formations consisting lithically of shale and limestone, respectively, are of Permian age and comprise the oldest stratigraphic sequence. Unconformably overlying the Chochal formation is a sequence of continental deposits, the Todos Santos formation, of Jurassic-Cretaceous age. These are conformably overlain by a thick limestone sequence of Cretaceous age which is subdivided into the lower Ixcoy formation and the upper Coban formation. Nomenclatorial problems encountered were dealt with by proposing the new name Tactic for lower Permian shales which have been erroneously called Santa Rosa, discarding the name Santa Rosa in favor of Todos Santos for the continental beds, and the redefinition of the Ixcoy and Coban formations. Folding has produced, in addition to a large E.-W.-trending anticlinorium, a number of complexly fractured anticlines and synclines. The area is cut by a zone of major E.-W. faulting which appears to align with the N. boundary fault of the Cayman trench (Bartlett trough) in the Caribbean Sea. The thick sequence of limestone, which has been extensively fractured where folded, together with the heavy rainfall, has given rise to an exceptionally well developed karst topography.