A two-week cruise in the Persian Gulf during August, 1948, by several ships resulted in the collection of a large amount of information about the sediments and water characteristics. Because the area is a geosyncline (long narrow trough of thick sediments), the new data serve to test some concepts of sedimentation in geosynclines. The water is of high temperature but also of such high salinity that Persian Gulf water sinks and flows out of the Gulf beneath incoming surface water of the Indian Ocean. The high temperature and salinity aid removal of calcium carbonate by mollusks, leading to a dominance of shell fragments in the sediments. Oolites are common in the entrance area, but more soluble minerals such as gypsum and halite are present only in shallow marginal lagoons Non-calcareous detrital sediment is contributed mostly by rivers at the head of the Gulf, though wind-derived sediment is locally important. The non-calcareous detrital fraction is mostly fine-grained with light minerals dominated by quartz, and heavy minerals dominated by tough resistant species such as leucoxene, zircon, magnetite, sphene, epidote, and garnet. Contours of grain size, calcium carbonate content, and water salinity are more or less parallel with the axis of the Gulf, rather than transverse as might be expected of a geosyncline having its chief source of detrital sediment at one end and its opening to the ocean at the other end. The high percentage of calcium carbonate, quartz, and resistant heavy minerals is characteristic of an area of slow deposition and does not fit t e common concept of geosynclinal sediment, non-calcareous graywacke. Mesozoic and Tertiary strata of the geosyncline also contain much limestone. Evaporites, common in the older rocks, would be more widely deposited at present if the entrance strait were to become shallower than now by diastrophic uplift. A higher organic content, probably characteristic of the Mesozoic and Tertiary oil source beds, would exist in present sediments if the strait were more open and if the climate were cooler and wetter than now.