Brongniart was a French mineralogist, geologist, and naturalist, who first arranged the geologic formations of the Tertiary Period
(from 66.4 to 1.6 million years ago) in chronological order and described them. He made the first systematic study of trilobites that became important in determining the chronology of Paleozoic strata (from 540 to 245 million years ago).
He helped introduce the principle of geologic dating by the identification of distinctive fossils found in each stratum and noted that the Paris formations had been created under alternate freshwater and saltwater conditions. image. From Today In Science History
