The Laramide1 (late Cretaceous—early Eocene) igneous rocks or “porphyries” of the Front Range are largely confined to a belt about 15 miles wide that extends diagonally across the pre-Cambrian core of the range, from Breckenridge northeast to Lyons (Fig. 1). The intrusives show a wide variety both of composition and of form. Dikes are abundant throughout the porphyry belt, but stocks are concentrated along the northwest side, and sills are almost confined to the sediments bordering the pre-Cambrian near Breckenridge and between Lyons and Boulder. South of Boulder, late Cretaceous surface flows and a few dikes and sheets occur near Golden. Co-extensive with the porphyry belt are important ore deposits of lead-silver, gold, gold-telluride, tungsten, and fluorspar.
Many contributions have been made on local problems of petrology and petrography, and a few papers dealing with some of the broader regional aspects have been published, notably the . . .