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Across 4. an index fossil 5. object created when a mold is filled with sediments that become rock 8. priniciple that states that the geological processes that are acting on the Earth today are the same as those that acted upon the Earth in the past 11. type of trace fossil 13. insects are often preserved in this 14. atoms of the same element with different masses 17. occurs when an organism's tissue is completely replaced by minerals 18. four major parts of the geologic time scale 19. era where reptiles dominated 22. ideal sequence of rock layers that contain all known fossils and rock formations 24. "old life" 27. trace fossil of shelters made by animals

Down 1. the time it takes for one half of a radioactive sample to decay 2. process where organisms do not decay because the bacteria that feeds on the dead organism can't live without water 3. a process in which mineral fill in the pore spaces of an organism's tissue 6. type of fossil where the animal's acitivity is preserved 7. missing part or gap in the geologic column 9. most of the fossils of saber-toothed cats are found here 10. fossils of organisms that lived during a relatively short, well defined time period 12. principle that states that the younger rock layers lie above older rock layers in undisturbed sequences 13. dating technique that establishes the age of an object 15. preserved feces 16. era where mammals dominated 20. the process where an unstable isotope changes into a stable isotope 21. dating technique that uses the principle of superposition to determine if an object or event is older or younger than another object or event 23. a cavity in ground where a plant or animal was buried 25. unconfomity that exists between horizontal rock layers and rock layers that are tilted or folded 26. naturally preserved evidence of life