Empirical determinations of the mean surface velocity of Lg indicate a range from 3.47 to 3.54 km/sec., with one set of velocities off the east coast of Mexico as low as 3.20 km/sec. Computations based on ray theory show that Lg with these velocities may be shear waves guided in the upper crust by an alternation of refractions and surface reflections. The slower velocities would result from shear waves traveling almost entirely in the sedimentary layer. Polarization of Lg is related to the angle at which the guided shear waves are incident at the surface.

Calculations show that Lg would travel in a continental crust covered by a considerable thickness of water, thus supporting the hypothesis that the absence of Lg indicates that an oceanic crust underlies such bodies of water as the Gulf of Mexico.

All computations were run on the Univac 1103 at Southern Methodist University.

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