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sketch map (Fig. i). The second part of the paper begins with a summary of the known areal extent of the unconformity, and this is followed by a discussion of the magnitude of the stratigraphic break. Since the formation names are largely local and probably unfamiliar to most readers, the following partial columnar section is given. Jurassic, La Plata sandstone Dolores (Chinle) formation Triassic, Shinarump conglomerate DeChelly sandstone (local) Perian (), Moenkopi red beds ' W. T. Lee, "General Stratigraphic Break between Pennsylvanian and Permian in Western America," Bull. Geol. Soc. Am., Vol. XXVIII (i917), pp. 169-70. 6i C. L. DAKE Kaibab limestone Coconino sandstone Upper Aubrey group, Pennsylvanian Carboniferous Supai formation Redwall limestone, Pennslvanian and Mississippian DETAILED OCCURRENCES OF THE UNCONFORMITY Mule Twist Canyon.-The first locality at which the unconformity was noted is about four miles northwest of Mule Twist FIG. I.-The Colorado Plateau Canyon, a well-known pass through the La Plata "reef" or "ledge," a prominent escarpment on the east flank of the Water Pocket Flexure, west of Henry Mountains, Utah (Fig. i). At this point the unconformity is distinctly indicated by basal conglomerates, by the uneven surface on which the Moenkopi rests, and by the ' G. K. Gilbert, The Geology of the Henry Mountains, i880.

FIG. I.-Contact of thin-bedded marine Jurassic limestone on white cross-bedded sandstone of La Plata group. One-half mile southwest of Teasdale, Utah. (Contact at black line.

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