February 5, 1770 (a Monday)

Alexandre Brongniart

On this date, the French mineralogist, geologist, and naturalist Alexandre Brongniart was born. He was the first person to arrange the geologic formations of the Tertiary Period (from 66.4 to 1.6 million years ago) in chronological order and describe them. He made the first systematic study of trilobites, an extinct group of arthropods that became important in determining the chronology of Paleozoic strata (from 540 to 245 million years ago). He also helped introduce the principle of geologic dating by the identification of distinctive fossils, called index fossils, found in each stratum and noted that the Paris formations had been produced under alternate freshwater and saltwater conditions. [Notice that the use of index fossils for the relative dating of rocks and fossils was established long before the use of radioisotopes for their absolute dating, contrary to what some “creationists” would have you believe.]

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