On the basis of potassium-argon mineral ages, plutonic rocks in an area of approximately 22,000 square miles in the southern Alaska Range and the Aleutian Range can be assigned to age groups that show differences in chemical characteristics and geographic distribution. The plutonic groups are Early and Middle Jurassic, Late Cretaceous and early Tertiary, and middle Tertiary in age. Most of the plutonic rocks in the Aleutian Range south of Iliamna Lake appear to be Jurassic, but north of Iliamna Lake, Jurassic plutonic rocks seem to be restricted to a belt on the southeast side of the Chigmit Mountains—Alaska Range. In the western or northwestern part of the Alaska Range north of Iliamna Lake, only Cretaceous and Tertiary plutonic rocks have been found. Rocks rich in K-feldspar are predominant in the Cretaceous and Tertiary plutons, but subordinate in the Jurassic plutons. Most of the mineralization in the region is associated with the Cretaceous and Tertiary plutons.