Variations in the grain size, amount, and mineralogy of magnetic phases in layered volcanic rocks define a magnetic stratigraphy that can be identified by means of magnetic susceptibility measurements. In ash-flow tuffs, grain size and other variables that control susceptibility are to a large extent a function of the cooling and alteration history. The phases responsible for the susceptibility of ash-flow tuffs consist of post-emplacement high-temperature precipitates of Fe-oxides in volcanic glass, and the phenocrystic Fe-Ti oxides. At Yucca Mountain, Nevada, laterally-continuous high-susceptibility (~10-2 SI) horizons exist in the Paintbrush Tuff due to the presence of either precipitates or phenocrystic Fe-Ti oxides. The frequency dependence of magnetic susceptibility, &khgr;(&ohgr;), can be used to discriminate between horizons with superparamagnetic predcipitates and horizons with multi-domain phenocrystic material. The &khgr;(&ohgr;) exhibits a 26% decrease per decade of increasing frequency for precipitate grain sizes where the superparamagnetic single-domain state gives way to stable single-domain behavior, and thus &khgr;(&ohgr;) offers an indirect method for rapid estimation of magnetic grain sizes. The interpretation of variations established by field and laboratory susceptibility data has been constrained by petrography and transmission electron microscopy (TEM). ¿ American Geophysical Union 1990