The Rockies concerned are those of Montana, Wyoming, Colorado, New Mexico, and the Colorado Plateau of Utah and Arizona, east of the Paleozoic miogeosyncline.

Evidence is presented that suggests that all the Rocky

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Mountain features of this region are the result primarily of Laramide vertical uplifts of oval or irregularly broad shape. They generally lack linear, narrow, or sinuous aspect. Some are conspicuously asymmetrical; others are fairly symmetrical; others approach gentle quaquaversal form. The structural relief ranges from 500 feet (Bowdoin dome) to 40,000 feet (Wind River uplift). Later Tertiary faulting, subsidence, sedimentation, and igneous activity have modified these Laramide uplifts considerably in places.

When the thrust faults are charted, they are found to be for the most part marginal to the uplifts. The uplifts of low and intermediate amplitude generally do not have associated border thrusts, but those in which Precambrian rock is exposed in the core commonly are bordered on one side or both by outwardly displaced thrusts. A firmer tie of uplift to border thrust is found in those where a structural relief of 25,000 feet or more exists.

These relations suggest that vertical uplift was the primary deformation and that thrusting was a secondary lateral deformation caused by gravity sliding and flowing. Since the basins were filled with sediments as the uplifts rose, it appears that thrusting is apt not to be related directly to the structural relief of uplift over adjacent basins, but to absolute relief at any one time as uplift exceeded sedimentation.

Anticlines suitable for oil and gas accumulation seem to be related to the marginal gravity creep from the uplifts. The locale is one of interplay of thrusting and folding of the surficial strata, and of sedimentation.

The Rocky Mountain region of uplifts is essentially the igneous province of alkalic and calc-alkalic rocks. Consideration of the origin of these rocks, of the nature of the uplifts, and of geophysical data lead the writer to postulate that the uplifts are due to megasills or lopoliths deep in the silicic (granitic) layer, perhaps near the boundary of the silicic and basaltic layers. It is expected that model experiments will indicate size, shape, and depth of intrusion to produce the various surface structures, and the nature of the border faults.

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