The Earth and Space

3. A rock formed from consolidated clay sediments
5. Organic material is converted into stone with the replacement of minerals
15. Physical force used against an object
16. A layer of rock from a certain time period
19. An era from 230 million to 190 million years ago (disappearance of dinosaurs)
20. The extinction of a large number of species within a relatively short period of geological time
21. The star around which the earth orbits

1. An era from 500 million to 425 million years ago
2. A long and distinct period of history with a particular feature or characteristic
4. A discharge of electricity
6. A change where a species becomes better suited with its environment
7. A group of living organisms who look similar and are able to reproduce
8. The eruption of a volcano
9. Hard, whitish tissue making up the skeleton in humans
10. A specialist in paleontology
11. An era from 135 million to 63 million years ago
12. An era from 405 million to 345 million years ago
13. A solid inorganic substance of natural occurrence
14. The process of being eroded by natural agents
17. An era from 280 million to 230 million years ago
18. The remains of an organism preserved in petrified form