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Across 4. an era than spans about 66 million years ago also known as the age of mammals 11. atoms of the same element that have the same number of protons but different numbers of neutrons 12. the rate of decay of unstable isotopes can be used to estimate the absolute ages of fossils and rocks 13. one of the four long units of geologic time between the Precambrian and the present 14. a core sample that is typically removed from an ice sheet 16. the era of time prior to 600 million years ago, covering 90% of Earth’s history 17. a group of extinct segmented hard-shelled marine arthropods that lived over 520 million years ago

Down 1. a record of the geologic events and life forms in Earth’s history 2. a continuous process by which rocks are created changed from one form to another, destroyed, and then formed again 3. an interval of time known as the age of reptiles from about 252 to 66 million years ago 5. one of the three main rock types and formed from the cooling and solidification of magma and lava 6. a hypothetical super-continent that included all current land masses 7. a fossil which minerals replace all or some of an organism 8. the gradual movement of the continents across the earth's surface through geological time. 9. rocks that are subjected to high heat and pressure which causes a profound change in form 10. known for the breakup of the super-continent 15. one of the units of geologic time into which geologists divide eras