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Two maps (Pls. 11-1 and 11-2, in pocket) have been prepared to show the general distribution, lithology, and age of Cenozoic igneous rocks in the Western United States. Plate 11-1 shows igneous rocks that range in age from 65 m.y. (start of the Cenozoic Era) to 17 m.y., and Plate 11-2 shows rocks that are 17 m.y. or younger. The 17-m.y. age is a geologically significant division point. This age is approximately the low point of the mid-Miocene lull in igneous activity that has been recognized in the Great Basin (McKee and others, 1970), in the Basin and Range province...

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