The Mayagüez area forms the southwestern corner of Puerto Rico, west of 67° W. and south of 18° 15' N. One-third of the 640 square kms is covered by thick alluvium. Unconformities separate a basal complex, two sequences of highly folded igneous and sedimentary rocks, and a younger sequence of gently dipping sedimentary rock. The basal Bermeja complex contains serpentinite, silicified porphyritic volcanic rock with some sedimentary rock, and minor spilite, amphibolitized spilite, and amphibolite. It is exposed chiefly in some anticlinal cores in southwestern Puerto Rico. Limestone, mudstone, andesite, and basalt form the older folded sequence. The Río Loco formation, bronzite andesite porphyry in part with pillow structures, was extruded perhaps in the Cenomanian. The Mayagüez group includes most of the rocks in southwestern Puerto Rico: the Yauco mudstone, Parguera limestone, Brujo limestone, Melones limestone, Maricao basalt, Sabana Grande andesite, and El Rayo volcanic rocks. The maximum possible age range is Turonian to Maestrichtian. The group ranges in thickness from about 800 m in the south to 3800 m in the north, and it varies in lithology from limestone in the south to mudstone and volcanic rock in the north, indicating a volcanic center to the north during that time. The second folded sequence contains andesitic volcanic rock, bedded tuff, and massive limestone. The San Germán formation (Maestrichtian) includes andesite, the Cabo Rojo agglomerate member, and the Cotui limestone member. The Jicara formation, massive limestone and bedded tuff, is Paleocene; there is one exposure of an unnamed ? Eocene marl. Post-Eocene limestone and conglomerate are also exposed in the area. The structure of the basement complex is obscured by its massiveness and by the cover of younger rocks. Two major deformations have affected the rocks of southwestern Puerto Rico since Cenomanian to Santonian time. In the Maestrichtian, the first of these formed folds with a N. 60° W. trend, asymmetric or overturned to the south. Near the south coast the folding of thin Mayagüez group rocks was probably influenced by trends in the Bermeja complex which caused deviations in the regional trends and also some overturning to the north. The San Germán formation, deposited unconformably on the eroded surface of the folded Mayagüez group, contains large allochthonous blocks of older and contemporaneous rocks. These blocks, up to 2 km by 1 km in exposure, were deposited by slumping or sliding due to gravity within and at the base and top of the San Germán formation near Lajas and San Germán. Most rocks in the blocks are extremely contorted and contain deformed Foraminifera. The San Germán and Jicara formations and perhaps the ?Eocene marls were deformed into gentle open folds trending east in the area covered by this report. Oligocene, Miocene, and younger sedimentary rocks have been tilted and uplifted. Large east-west left-lateral transcurrent faults cross the area, offsetting and offset by two sets of transverse faults (N. 45° E., N. 20° W.): most faults are probably Maestrichtian to Oligocene, although minor faulting has continued to the present. Dikes and sills of quartz diorite porphyry and mica-quartz dacite porphyry intrude the ?Maestrichtian San Germán formation and older units. A diorite plug cuts the Bermeja complex, and a granodiorite plug intrudes the Mayagüez group.