Trilobite - extinct marine arthropods, used as an index fossil for the Paleozoic era, Absolute dating - The actual age, or date, in years when a geological event occurred, or a rock was formed, Cross-cutting relationships - principle of geology that states that the geologic feature which cuts another is the younger of the two features, Fossils - the remains, impressions, or other evidence of the former existence of life preserved in rock., Epoch - A smaller division of geologic periods (Eon  Era  Period  Epoch), smallest division of geologic time, Era - the second longest division of geologic time; combined into eons and divide into periods (Eon  Era  Period  Epoch), Period - a smaller division of an Era (Eon  Era  Period  Epoch), Eon - The longest division of geologic time (Eon  Era  Period  Epoch), Geologic Time Scale - a chronological model of the geologic history of Earth using the division of eons, eras, periods and epochs, Half-Life - the time required for half of the atoms in a given mass of a radioactive isotope to decay, or change, to a different isotope, Index Fossils - fossils that lived in a wide geographic area, lived a short time and is easily recognizable, Radioactive Decay - the natural spontaneous breakdown of the nucleus of unstable atoms into more stable atoms of the same or different elements Nuclear Decay, Relative Dating - places events in a sequence without assigning any numerical ages. (Compares ages), Superposition - states that in a sequence of undisturbed sedimentary rocks, the oldest rocks will be at the bottom of the sequence and the youngest will at the top, Uniformitarianism - the theory that changes in the earth's crust during geological history have resulted from the action of continuous and uniform processes,

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