A geomagnetic survey, using an Askania magnetometer, of portions of northern Westchester, Putnam and Dutchess counties in New York state totaling approximately one hundred and thirty square miles is described and more than five hundred observations of the vertical component of the earth's magnetic field are analyzed. These data are plotted on a scale of 1:30,000 and the resulting isogam chart is used to correlate the observed major changes in the vertical component with variations in the geology of an area in which basement complex rocks outcrop.This survey demonstrates that a rather well defined relationship exists between changes in the vertical component values and the major geologic characteristics, using an average (regional) station spacing of four stations per square mile. In addition, the probable presence, on the basis of the magnetic data, of additional unexposed amphibolite lenses and associated igneous intrusives is indicated in those areas of the Eastern Schist Zone where the glacial overburden and/or soil obscure the underlying formations. The use of statistical methods is demonstrated for evaluating magnetic data for various purposes including the determination of interformational boundaries and the correlation of pronounced changes in geologic and magnetic characteristics.

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