The porosity of sedimentary rocks varies inversely with depth of burial; shales change regularly, and sandstones less predictably. Experiments with quartz are summarized, pointing up the functions of temperature, solubility, and time, and the possible prediction for natural sandstones. Data assembled for Ordovician, Pennsylvanian, and Tertiary oil and gas reservoir sandstones, show reductions in porosity with increasing depth. In areas with high temperatures the porosities tend to be lower than for similar sandstones in areas with low temperatures, but in younger sands this relation may be reversed. The observed curves provide a basis for estimating porosity of untested sandstone where geologic age and thermal gradient can be approximated.