Ancient land masses of northern South America and the Caribbean Sea region have had an important influence on the position and shape of the Maracaibo Basin in northwestern Venezuela. Gondwana and Antillia, the two major land masses involved, lie southeast and northwest, respectively, of the basin. Minor land masses of the Venezuelan Andes, Sierra de Perija, and Island of Toas are located on the south, west, and north edges of the basin, and have a close structural relation to secondary folding. Maracaibo Basin is a flask-shaped structural geosyncline 150 miles long, is flanked on the east, south, and west by high mountains, and is connected by a narrow neck with the Caribbean Sea on the north. Igneous, metamorphic, and sedimentary rocks within the basin range in age from t least Ordovician to Recent. Acidic and basic dikes and batholiths cut igneous and metamorphic rock. In places, sediments are intruded. Structural influence of igneous intrusions on surrounding rocks is believed to be negligible. En echelon arrangement of structures in the Maracaibo Basin is suggested. The more highly inclined flank of the folds is generally toward the axis of the basin. Marginal faults locally are present, but are considered to be incidental to folding. Rotational movement of orogenic forces in the basin is suggested by a northward plunge of structures west of the lake, and a southward plunge of structures on the east side of the basin.