Significant lateral variations in upper mantle velocities across the western U. S. were observed in the gnome experiment. This makes necessary a reinterpretation of crustal thickness measurements made with the assumption that velocities in the various layers of the crust remain constant while their thickness changes. Four examples of the work of Ewing and Press have been reinterpreted. The crust is thinner (30 km) in the Basin and Range Province and thicker (55 km) under the Rocky Mountains than indicated by previous interpretations.

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