Across
- 2. three time periods
- 5. animals without a backbone
- 6. part of life in water and land
- 7. earths atoms becoming unstable
- 8. actual age in years
- 10. different
- 13. traces of living things
- 14. to guess
- 16. lava that hardens on the surface
- 17. egg that has a special fluid
- 18. helps scientists date rocks
- 21. radioactive elements to date fossils
- 22. magma rises up through rock layers
Down
- 1. animals with a backbone
- 3. 80% to 90% of animals die
- 4. determine the age of rock layers
- 9. timescale
- 11. animals that give live birth
- 12. age compared to other rocks
- 15. cold blooded animals
- 19. tiny sections
- 20. all the continents