The writers have obtained radiogenic dates on three different types of igneous rock in the southern Laramie Range to test the hypothesis that Paleozoic intrusion of mafic rocks was connected with structural activities involved in the preservation of near-by early Paleozoic sedimentary rocks. A small olivine diorite body was dated at 1320 ± 70 million years, a basalt dike at 1420 ± 70 million years, and the Sherman Granite, principal basement rock in the area, at 1380 ± 50 million years. Structural and petrologic evidence suggest that the diorite and the basalt are younger than the granite, but the dates preclude Paleozoic origin.

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