Abstract
Within the past twenty years, students of stratigraphy have drawn the base of the Permian system in the Grand Canyon district at progressively lower horizons. On the basis of fossil plants, first the Hermit shale and, more recently, the red beds of the Supai formation have been included in the Permian section with the Kaibab limestone. The Bird Spring formation, more than 5,000 feet thick, which lies beneath the Supai in southern Nevada, has heretofore been placed in the Pennsylvanian, except for a few hundred feet in the lower part, which Girty considers Upper Mississippian. Fusulinids collected in the Las Vegas Quadrangle indicate that the upper 2,900 feet of the Bird Spring formation is correlative with the Wolfcamp formation and part of the Leonard formation in the Glass Mountains of Texas.
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