LIST OF ILLUSTRATIONS 



FIGURE PAGE 



1. A Protozoan (Amoeba) without a shell. Greatly enlarged .... 14 



2. Shelled Protozoans (Foraminifers) 14 



3. Sponges on a shell 14 



4. Modern Hydrozoans. Part of a colony much enlarged 15 



5. A group of modern Corals showing the internal structure of one 



individual 16 



6. Stemmed Echinoderms (Pelmatozoans) 16 



7. A modern Asterozoan ("Starfish") . . . . 17 



8. Modern Echinoids ("Sea-urchins"), one with spines in position . . 18 



9. Bryozoans 18 



10. Brachiopod shells (fossil forms) 19 



11. A modern Pelecypod 19 



12. Gastropods 20 



13. A modern chamber-shelled Cephalopod (Nautilus) showing the in- 



ternal shell structure 21 



14. A modern Squid 21 



15. Diagram to illustrate correlation of rock formations by continuity 



of deposit 24 



16. Diagram to illustrate correlation of rock formations by similarity 



of sequence 24 



17. Diagram to illustrate correlation of rock formations by degree 



of change or structure 25 



18. Diagram to illustrate the significance of unconformities ....... 27 



19. A very symmetrical spiral nebula in Pisces (M. 74) 37 



20. Diagram to illustrate the formation of a spiral nebula 38 



21. Archean (Grenville) sedimentary gneiss in the central Adirondacks. 43 



22. Map showing the surface distribution of pre-Cambrian (Archeozoic 



and Proterozoic) rocks in North America 44 



23. Diagram showing the principal subdivisions of the Proterozoic and 



their relation to the Archeozoic in the Lake Superior district . . 49 



24. A view in the Grand Canyon of Arizona (Shinumo quadrangle) 



exhibiting the relations of Archean, Algonkian and Paleozoic rocks 



to each other 52 



25. Part of a pre-Cambrian (Huronian) Alga 54 



26. Map showing the surface distribution (areas of outcrops) of Cam- 



brian, and some very closely associated Lower Ordovician, strata 



in North America 58 



27. Geologic section through northeastern Iowa, showing how character, 



thickness, and distribution of deeply buried rock formations can 



be determined by a comparison of well records 60 



ix 



