Abstract: The boron content of sea water increases linearly with salinity within the range tested. The boron content of the illite minerals in the clay mineral size fraction of many types of sedimentary rocks has been shown to increase with the “ geologically known ” salinity prevailing during deposition of the rocks. It is believed that boron is abstracted by the illitic clay minerals from sea water in proportion to the salinity of the sea water. This boron content is not substantially changed by other geological processes normally operative in sedimentary rocks. It is concluded that the boron content of illite minerals in sedimentary rocks can be used to evaluate the salinity of the sea prevailing at the time the sediment was deposited.