A geologic study was made of an area on the southwest corner of Isla Tiburon, located in the northern half of the Gulf of California. The emphasis of this report is placed on the description and interpretation of a 988 meter thick section of fossiliferous marine conglomerate (Fig. 4). Other geologic, as well as archaeologic, observations were made and are discussed in this report. The area studied is divided into two distinct physiographic provinces, one of older and one of younger volcanic terrane, which are separated by a major fault. Outcrops of a marine conglomerate are found in the younger volcanic rocks. The conglomerate has been divided into three members, an upper fossiliferous member (mcg-1), a middle nonfossiliferous member (mcg-2), and a lower fossiliferous member (mcg-3). Foraminifera collected from member mcg-1, of the conglomerate, indicate deposition took place during Neogene time; possibly upper Miocene. The conglomerate was deposited in the marine tropical waters of a shallow, nearshore trough, located near the mouth of a fluvial system which supplied the sediment to form the conglomerate. Subsidence of the trough approximately equaled the rate of sedimentation. Genesis of the trough was probably the result of the dilation that took place during the formation of the proto Gulf of California. This type of rift produced trough (or basin) is analogous to the rift valleys of Africa or the Triassic basins of the east coast of North America which formed durinf the rifting of Laurasia. Isla Tiburon is also of archaeological interest. Abundant shell mounds, pottery fragments, and stone tools, left by the past inhabitants of the island, are found undisturbed along the coast. These artifacts show the inhabitants (The Seri Indians) were of a Stone Age culture; although they did have the technology to produce fired clay pottery.