Three hundred core samples of argillaceous sedimentary rocks of several Tertiary formations were analyzed for mineralogy, grain size, and porosity. The samples were taken at depths ranging from 3,500 to 50 m in a restricted area in the Rheintalgraben. Clay mineral composition (chiefly illite, kaolinite, and chlorite) is almost constant throughout the sequence, whereas grain size is increasingly coarser in older sediments. Initial porosities were obtained by extrapolating the compaction curves drawn from porosity values. Grain size influence was eliminated by multiplying the initial porosity by the logarithm of the median. The porosity values thus obtained are called "modified initial porosity." These modified initial porosities show an excellent correlation with paleosalinity: marine rocks have high initial porosities while fresh-water sediments have lower porosities. Thus, porosity values of argillaceous sediments deposited in different depositional environments can be used as indicators of paleosalinity if the void ratios are recalculated to a common grain size.