This paper is a synopsis of the late Paleozoic orogeny in the south-central states of North America. Recent papers by American geologists and personal work of the writer and his collaborators are coordinated into as complete a treatise as present knowledge permits.

Two independent mountain systems, the Wichita and the Ouachita, are distinguished. The Wichita system comprises the Wichita Mountains, the Criner Hills, the Arbuckle Mountains, the buried folds along Red River including the Muenster arch, and the buried Amarillo Mountains. These folds, though belonging to the Permo-Carboniferous phase, originated within an intra-continental pre-Middle Devonian geosyncline, in which the major accumulation of predominantly marine limestone sediments was pre-Devonian. This system strikes generally west and west-northwest. It meets nearly at right angles the front of the second complex, the considerably more important Ouachita system.

The Ouachita system (in a wider sense) is a large arcuate feature of which only a small part of the widely overthrust outer rim of the northernmost loop is exposed in the Ouachita Mountains of southeastern Oklahoma and Arkansas. Most of this system is buried, though folds exposed in the Marathon uplift belong to the southwestern extension of the same system, which, as the writer believes, in Oklahoma and Arkansas is itself an extension of the Appalachians, which disappear in Alabama under the Cretaceous blanket of the Gulf Coastal Plain.

Unlike the Wichita complex, the Ouachita ranges originate from an inter-continental Carboniferous geosyncline, with major post-Devonian deposition, principally in the latest Mississippian or earliest Pennsylvanian. The facies of the rocks is also intrinsically different. The exposed part consists of great overthrust masses, of a structural type that can best be compared with the northern front of the European Variscan system in France and Belgium.

The principal phases of folding in both the Wichita and the Ouachita systems are Pennsylvanian; the final phase of the overthrusting of the Ouachitas is possibly Permian. Precursory Mississippian and older movements are indicated.

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Recent geological subsurface work makes possible the tracing of the rim of the buried Ouachita front range through east-central Texas, first by the characteristic, depressed early Pennsylvanian foreland basin, which exactly reproduces the coal basin foredeep of southeastern Oklahoma and southern Arkansas, and then by wells which actually have penetrated to the mountain front.

The present Llano-Burnet uplift marks the existence of a resistant buttress in the foreland, causing a salient around which the chains turn, to reassume a southwest course in southwestern Texas and northern Mexico; here the folds reappear at Marathon, beyond which they disappear beneath the Cordillera in Mexico.

The Hunton arch is another important buttress, causing the uplift of the folded foreland fault-slices of the Arbuckle Mountains.

The character of the nappe (overthrust) structure of the Ouachita chains is discussed and compared with the very similar build of European mountain chains. These mountains are not merely the edge of an ancient "Llanoria," but a great range of world-wide importance.