Non-opaque heavy minerals have been examined from contemporaneous sandstone-shale pairs of Tertiary formations on the Texas Gulf Coastal Plain. Total weight of heavy minerals in the shale is less than in the sandstone, but the same grain sizes are present in both. Sediment in the coarse and medium silt sizes contains the same variety of minerals as the very fine sand size. In shales the variety of minor minerals is only slightly larger than in contemporaneously deposited sandstones. However, the number of species present in only trace amounts is significantly larger in shales than in sandstones; the sandstones contain much larger percentages of altered grains than do the shales. Thus, shales rather than sandstones should be examined in heavy-mineral studies. Unstable species, post-depositionally dissolved in sandy beds, may be preserved in contemporaneously deposited shales.