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Location and accessibility______________________ Surface features________________________________ Settlements and culture.________________________ Climate and vegetation_____,____________________ Prebatholithic geology___----_-.___---_____________ Upper Precambrian(?) and Lower Cambrian sedimentary rocks of the White Mountains.-________ Wyman formation__________________________ Reed dolomite______________________________ Deep Spring formation. _____________________ Campito formation________________________ Andrews Mountain member._____________ Montenegro member.

Transcontinental seismic, aeromagnetic, and gravity measurements, together with geologic observations, suggest that the conterminous United States is divided by the Rocky Mountain system into two crustal and upper-mantle superprovinces. In the eastern superprovince, the velocity of compressional waves in the upper-mantle rocks is everywhere greater than 8 km/sec, the mean crustal velocity is generally greater than 6.4 km/sec, and the crust is generally thicker than 40 km. In the western superprovince, the velocity of compressional waves in the upper-mantle rocks is everywhere less than 8 km/sec (except along the margin of the Pacific Ocean basin), the mean crustal velocity is generally less than 6.4 km/sec, and the crust is generally thinner than 40 km. Aeromagnetic data are characterized by anomalies of large amplitude in the eastern superprovince, indicating an abundance of magnetic minerals, whereas the magnetic field in the western superprovince is relatively featureless. The weakly magnetic crust of the western superprovince is relatively devoid of magnetic minerals at shallow depths and may be above the Curie temperature for magnetite (578øC) and therefore nonmagnetic at depth. Gravity measurements and considerations of isostasy indicate that crustal and upper-mantle densities vary with velocity. These observations, together with the Cenozoic geologic record of diastrophism and volcanism in the western superprovince and relative Cenozoic inactivity in the eastern superprovince, suggest a mobile upper mantle in the west and a predominantly silicic crust that is now receiving mafic and probably also silicic material from th.e mantle, whereas the upper mantle in the east is relatively stable and the now predominantly mafic crust has been extensively intruded with mafic material from the mantle; additional mafic material has been added by extrusion of lava. The primitive continental crust that evolved from the mantle was probably silicic, and it has been made slowly more mafic by addition of mafic material from the mantle and removal of silicic material from the continental surface by erosion and stream transport.

Seismic waves generated by underground nuclear and chemical explosions have been recorded in a network of nearly 2000 stations in the western United States as a part of the Vela Uniform program. The network extends from eastern Colorado to the California coastline and from central Idaho to the border of the United States and Mexico. The velocity of P waves in the upper mantle ranges from 7.7 km/sec in the southern part of the Basin and Range province to 8.2 km/sec in the Great Plains province, and, in general, the velocity tends to be nearly the same over large areas within individual geologic provinces. Measured crustal thickness ranges from less than 20 km in the Central Valley of California to 50 km in the Great Plains province. Changes in crustal thickness across provincial boundaries are not controlled by regional altitude above sea level unless the properties of the upper mantle are the same across those boundaries. The thickness of the crust tends to be large or small as the P wave velocity (and presumably the density) in the upper mantle is large or small. Within the Basin and Range province, the thickness of the crust seems to vary directly with regional altitude above sea level. Evidence that rocks of intermediate P wave velocity exist in the lower part of the crust has been accumulated from seismic waves that have traveled least‐time paths, as well as from secondary arrivals (particularly reflections). On a scale that includes many geologic provinces, isostatic compensation is related largely to variations in the density of the upper‐mantle rocks. Within geologic provinces or adjacent provinces, isostatic compensation may be related to variations in the thickness of crustal layers. Regions of thick crust and dense upper mantle have been relatively stable in Cenozoic time. Regions of thinner crust and low‐density upper mantle have had a Cenozoic history of intense diastrophism and silicic volcanism.

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