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Observations made on the 1940 E. W. Scripps cruise to the Gulf of California add information concerning the later geologic history of the Gulf and of Lower California. Granitic rocks are found in Lower California, on some of the islands, and in Sonora; they may all be part of the late Cretaceous batholith of Lower California. The islands along the western margin of the Gulf consist largely of tilted volcanic rocks (Comondú formation) of middle to late Miocene age, overlain unconformably by sediments of Pliocene age (Salada group). In Lower California the Comondú dips gently west. The Pliocene sediments, conglomerates, sandstones, and limestones were deposited in small embayments with the older rocks rising topographically above, identical to the environment of recent similar sediments. Evidence shows that the Gulf of California and peninsula of Lower California were blocked out essentially in their present form by early Pliocene time. Locally the lower and middle Pliocene rocks have been folded and faulted, whereas the upper Pliocene sediments have only been upwarped and faulted. Numerous Pleistocene terraces occur on most of the islands, and some have been faulted. Pliocene and Pleistocene volcanic rocks have been recognized on some of the islands, but only one island possibly represents a volcanic cone built on the Gulf floor.

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