On Punta Concepcion in Baja California, a group of volcanic rocks of Miocene age is intruded by igneous rocks of the same age, and both are overlain unconformably by lower Pliocene sedimentary rocks of non-marine and marine origin. Veins of pyrolusite and other minerals are abundant in some basalts of the volcanic series and also cut the intrusives and the lowermost lower Pliocene rocks but form detrital grains in other members of the lower Pliocene series. The veins of almost pure pyrolusite are very narrow but very rich and locally abundant. They provided a small supply of chemical grade ore during World War II. The veins are believed to be of primary hydrothermal origin.

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