Extensive lava flows, like those of the Columbia plateau of Washington state, require thicknesses only of the order of meters and temperatures only slightly above the melting point to spread over distances of the order of hundreds of kilometers. Other things being equal, the spreading distance is proportional to the cube of the thickness of the flow.

Detailed surface geology and gravity maps of the Puget Sound area, combined with limited magnetic observations, earthquake epicenters, and well records, have been studied in order to determine the geologic structure of the region. The Puget Sound lowlands are a deep crustal depression separated from the Olympic Peninsula by an active fault of about 4000 m throw. The basin is divided into three main parts by an igneous horst with an older gabbroic core and younger basaltic layers. The northeastern part of the basin, centered around Seattle, has sedimentary thicknesses of about 10,000 m. The southern part, centered near Tacoma, and the western one, around Skokomish, are about 4000 m deep. Faults along the horst are active, the northern one more so. Surface geological work is hampered by a thick layer of glacial till.

The development of salt domes is formulated as a case of Taylor instability in the salt‐sediment system. It is assumed that the salt layer rests on a rigid substratum, while the upper surface of the sediment is free. Both salt and sediment are assumed to behave as highly viscous media. Of the entire spectrum of infinitesimal perturbations the component of the fastest growth is assumed to become dominant and to determine the spacing of domes. Exact solutions are derived for the linear theory in equations (3.7) and (4.24). Corrections due to the nonlinear terms are analyzed qualitatively in (5.8) and (5.9).

A new method for quantitative interpretation of gravity anomalies is presented. The disturbing body is represented by a finite number of vertical prisms arranged on a pre‐determined, regular grid. The horizontal dimensions of the individual prisms are small enough that they can be approximated by vertical‐line mass elements at the axis of the prisms. Formulas for gravity due to one prism are derived and, for the case of Gulf Coast salt densities, plotted on Figure 1. Gravity due to the whole body is an algebraic sum of the contributions of all prisms at the appropriate depths and distances. This method makes possible the direct interpretation by successive approximations by introducing proper geologic limitations. All the numerical work can be conveniently done on a high‐speed digital computer. The method is especially suitable for features with predominant vertical dimension such as salt domes and igneous plugs. It gives at least the same, and possibly higher, degree of accuracy as the graphical dot chart methods and, carried out on a digital computer, should be about two thousand times faster.

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