Rocks of Late Cretaceous age (Turonian to Maestrichtian) in Puerto Rico are of 3 types: 1) primary volcanic rocks, including tuffs, tuff breccias, and lavas; 2) intermixed pyroclastic and epiclastic rocks, including volcanic conglomerates, volcanic sandstones, and volcanic siltstones; and 3) limestones, most of which were formed as reefs around volcanic islands. These rocks, which have a maximum thickness of more than 20,000 ft. , crop out along the crest and flanks of a complexly faulted, northwestward-trending anticlinorium that forms the mountainous core of Puerto Rico. In the central and S.-central parts of the Commonwealth, 3 formations are recognized. The Robles formation, which probably ranges from late Coniacian to early Campanian in age, is characterized by laminated siltstones and sandstones but includes the Lapa and Las Tetas lava members and locally at the base the rudistid-bearing Rio Maton limestone member. The Cariblanco formation, which unconformably (?) overlies the Robles formation, is probably middle Campanian to middle Maestrichtian in age. It is made up predominantly of thick coarse conglomerates separated by sandstones and siltstones. The Coamo formation of late Maestrichtian age consists chiefly of massive tuff breccias and bedded tuffs including ashflow deposits. In northeastern Puerto Rico the Upper Cretaceous section consists of the Fajardo formation, chiefly siliceous, ammonite-bearing mudstones and siltstones of Turonian and Coniacian age; interbedded sandstones, siltstones, thin limestones, pebble conglomerates, and tuff breccias that are believed to be equivalent to the Robles and Cariblanco formations of S.-central Puerto Rico; and conglomerates, sandstones, and lenticular reef limestones that are believed to be equivalent to the Coamo formation. Rocks equivalent to the Fajardo formation in the central part of the island consist of massive volcanic breccias, lavas, thin tongues of sandstone, and the Aguas Buenas limestone member, a reef deposit that lies at the base of the Fajardo formation. From N.-central Puerto Rico eastward the upper part of the Robles formation is represented by a thick sequence of pillow lavas and by volcanic breccia and agglomerate that appear to have accumulated along a W.-northwestward-trending belt of volcanism. The strata of known Late Cretaceous age overlie massive volcanic rocks whose age has not been determined. They are conformably overlain in most places by strata probably Paleocene and Eocene in age, but are locally overlain unconformably by limestones of Oligocene age. Rocks of the Fajardo, Robles, and Cariblanco formations and their probable equivalents are chiefly subaqueous, stratified deposits that accumulated in a subsiding basin adjacent to volcanic source areas. Cross-bedding is rare in all of these rocks, but graded bedding, cyclic repetition of unsorted coarse clastics and fine-grained clastics, and slump structures indicate that turbidity currents were an important agent in the transportation and deposition of the debris. The Coamo formation, characterized by redbeds, consists chiefly of sub-aerial deposits including tuffs and tuff breccias. Some of the Coamo formation tuffs were transported and redeposited by mudflows or lahars, others are ashflow or hot avalanche deposits. The present anticlinal structure of the Late Cretaceous rocks is believed to be the result of doming during intrusion of a batholith during early Tertiary time. Complex faulting that accompanied the batholithic intrusion in places helped to accentuate the anticlinal structure but in other places has modified it. Faulting has been most intensive in a belt that trends W.-northwestward across the central part of the island. Normal, high-angle reverse, and transcurrent faults are recognized, but the principal fault of northern Puerto Rico appears to be a transcurrent fault of regional extent.