The source of the geological disturbance responsible for the midcontinent gravity high is here reexamined on the basis of gravimetric, seismological, and geological data in consort, and both the size of the structure and the lithologies responsible are at variance to the results of earlier studies that did not employ long-range, seismological data. Its extension under Lake Superior, implied recently on gravity and seismic grounds and a new seismic inference that flanking areas that extend laterally are typified by gabbroic rather than granitic velocities, infers a disturbance laterally more extensive than that of the high and hence the name used, the central North American rift system. A refined method of analysis of existing long-range crust-upper mantle profile data has made possible the inclusion of this seismic data for the first time. The analysis method rests on the recognition of the existence of a common finite and discrete regional suite of velocities determined from shorter reversed profiles traversing key areas, the assumption of local planarity of interfaces, which then are approximated piecewise, the existence of a functional relationship between compressional velocity and bulk density for crustal materials of the region, and, finally, on the admission that long ‘reversed’ profiles are in the main unreversed and must be treated so that the resulting models simultaneously satisfy the seismic, gravimetric, and geologic data. A high velocity, 6.9 km/sec, forms the ‘core’ of the gravity high imbedded in material of 6.4-km/sec compressional velocity. The inferred density contrast, 0.14 gm/cm3, is substantially smaller than any used previously and results in a 40% increase of volume and an increase in the thickness of the anomalous body. The latter is in accord with minimum depths (25–30 km) estimated from Pn time term. The 6.9-km/sec material is associable with rocks of Mellen gabbro type, and the 6.4-km/sec material with rocks of Duluth gabbro type on the basis of geologic field relations and extrapolated seismic ‘contact outcrops’ supplemented with high pressure and temperature velocity data. The nature of the structural model suggests the 6.9-km/sec material as intruded into the 6.4-km/sec material, which in turn (at least in Wisconsin) is replaced in the upper tens of kilometers with 6.1-km/sec material at a lateral distance of several hundred kilometers from the high. Models across the axial zone of the gravity high in Lake Superior, Wisconsin-Minnesota, and Iowa all show the anomalous high-velocity mass increases in width upward, producing a velocity reversal in the vicinity of the gravity high. A preliminary search for analogous structure among the more modern rift or ridge systems shows that, on the basis of average width, length, compressional velocity, velocity contrast, gravimetric and magnetic expression, structure, etc., the most analogous tectonic feature (at least on the basis of present information) is the Red Sea rift rather than the continental rift valleys of Africa.