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Isla Desecheo is in Mona Passage between Puerto Rico and Hispaniola. The bedrock, 700-1,100 meters of middle Eocene volcaniclastic rocks, consists chiefly of dacitic volcanic sandstone, fine-grained volcanic breccia, and mudstone. Deposition in at least moderately deep water is indicated by the abundance of graded beds and by the fact that interbedded mudstone contains exclusively planktonic faunas. Sparse rounded grains and shallow-water fossil debris in the graded beds indicate that the volcaniclastic debris came from volcanoes in shallow seas. Steep submarine slopes are suggested by submarine-slide deposits and penecontemporaneous deformation structures. The rocks are extensively zeolitized. The southern two-thirds of the island, south of an eastwest vertical fault, has west-trending nearly isoclinal folds which are overturned to the north. North of the fault, the main structural feature is a gentle anticline which plunges south. The contrasting structural features are believed to represent different tectonic levels that are separated by a low-angle thrust fault. The strongly deformed beds may be the remnant of a gravity-glide sheet that was eroded from the northern part of the island. Isla Desecheo lies along the seaward projection of the great southern Puerto Rico fault zone, which is, perhaps, the major fracture zone of Puerto Rico. Large-scale transcurrent and vertical faults and associated gravity-glide structures seem closely related in age and geometry to similar features in Hispaniola and Cuba. Surficial deposits include marine terrace deposits, colluvium (in part phosphate cemented), and beach deposits. There is evidence of two stillstands of the sea, one at 12-13 meters and the other at about 2 meters higher than present sea level. The higher level is probably of Pleistocene age and the lower, of Holocene age, perhaps about 3,300 years before the present. Tectonic uplift is believed responsible for the 2-meter level and, therefore, at least in part for the 12-13-meter level.

More than 250 pebble counts were done on conglomerates from the Salinian block and nearby terrane in California, Baja California, and Arizona. The regional pattern of conglomerate compositions suggests that the Salinian block originated adjacent to the Mojave block and is not an exotic terrane as interpreted from paleomagnetic results. Upper Cretaceous, Paleocene, and Eocene conglomerates in the Salinian block consist of two remotely derived detrital suites. The principal suite is rich in clasts of felsic volcanic rocks with subordinate amounts of granitic rocks and quartz sandstone. Granitic rocks are more abundant in lower Tertiary conglomerates of this suite. The second suite, only found locally near the base of the sequence, is rich in quartz sandstone. Both suites are very poor in chert. Conglomerates of the Salinian block contrast with those in regions which were located to the north and south before San Andreas fault displacement. Cretaceous and lower Tertiary conglomerates in the Diablo and Temblor Ranges to the north and Cretaceous conglomerates in the Peninsular Ranges to the south are richer in chert than are Salinian block conglomerates. Lower Tertiary conglomerates both to the north and south are poorer in granitic rocks than are Salinian conglomerates. Salinian conglomerates are compositionally similar to conglomerates in native North America in areas bordering directly on the western part of the Mojave block. Although there are no remotely derived Upper Cretaceous conglomerates in the western Mojave region instead only locally derived basal conglomerates conglomerates in the overlying and onlapping Paleocene sequence are similar to the principal Cretaceous conglomerate suite of the Salinian block. Paleocene conglomerates in the Tehachapi and El Paso Mountains and in the San Gabriel Mountains are very similar to Paleocene conglomerates of the Salinian block. An Eocene conglomerate in the San Emigdio Mountains, located at the south end of the San Joaquin Valley and near the western Mojave, is similar to Eocene conglomerates in the Salinian block. Jurassic or Cretaceous conglomerates composed of volcanic rocks and chert in the lower half of the McCoy Mountains Formation of southeastern California and adjacent Arizona are very similar to conglomerates in the Winterhaven Formation in extreme southeastern California. This supports other evidence indi-Manuscript approved for publication February 19, 1992. PLACE OF ORIGIN OF THE SALINIAN BLOCK AS BASED ON CLAST COMPOSITIONS OF CONGLOMERATES are found to be compositional mismatches and on this basis should no longer be used as reference points in estimating fault displacements. 122°M ETHODS AND DEFINITIONS 118° 114°1 10°C OLORADO.