Evaluation of isotopic ages of granitic intrusive rocks of large batholiths in circum-Pacific North America indicates that Mesozoic and Cenozoic plutonism was episodic but not periodic. Three intrusive epochs have been defined in the Alaska-Aleutian Range batholith of Alaska on the basis of concordant potassium-argon ages of coexisting biotite and hornblende or muscovite. Only two intrusive epochs based on concordant ages of mineral pairs and an older plutonic episode can be recognized in the Sierra Nevada and southern California batholiths. Detailed intrusive histories for other parts of Alaska, British Columbia, and Yukon Territory cannot be inferred from available data. The intrusive epochs in the western United States do not correlate with the epochs in the Alaska-Aleutian Range batholith. The spacing, duration, and synchroneity of intrusive epochs in the circum-Pacific batholiths of North America are not yet well established.

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