x LIST OF ILLUSTRATIONS 



FIGURE PAGE 



28. Upper Cambrian (Potsdam) sandstone in the Ausable Chasm of 



northeastern New York 62 



29. Paleogeographic map of North America during Lower (early) Cam- 



brian time 64 



30. Paleogeographic map of North America during late Middle and 



Upper (late) Cambrian time 66 



31. Structure section in Saratoga County, New York, showing how 



Upper Cambrian strata overlap upon a hillock of pre-Cambrian 

 rock (Grenville) 67 



32. Calcareous Algae, Cryptozoon proliferum, forming a reef in Upper 



Cambrian limestone near Saratoga Springs, New York 72 



33. A Cambrian Sponge, Leptomitus zitteli 73 



34. A Cambrian Jelly-fish, Brooksella altera ata 73 



35. A Cambrian Sponge or Coral, Archeocyathus rensselaericus .... 73 



36. Cambrian Brachiopods 74 



37. A Cambrian Pelecypod (Fordilla troyensis) 74 



38. Cambrian Gastropods 74 



39. Cambrian Trilobites, restored forms 75 



40. A Middle Cambrian Trilobite, Neolemus serratus, with well-pre- 



served appendages 76 



41. Map showing the surface distribution (areas of outcrops) of chiefly 



Middle and Upper Ordovician strata in North America .... 79 



42. Generalized structure sections through various parts of New York 



State, showing the attitude and relations of the various great 

 rock systems 80 



43. The Trenton (mid-Ordovician) limestone at its type locality, Tren- 



ton Falls, New York 81 



44. Geologic (columnar) section in eastern Tennessee, showing the pre- 



dominance of limestone in the Lower and Middle Ordovician, and 



of shale and sandstone in the Upper Ordovician 83 



45. Paleogeographic map of North America during Middle Ordovician 



time 85 



46. Structure section through a portion of the Highlands of the Hudson 



in southeastern New York 87 



47. Sketch map showing the relations of land and water in Europe 



during Ordovician time 89 



48. Ordovician Seaweeds, Callithamnopsis fructicosa 91 



49. Ordovician Graptolites 92 



50. Ordovician Echinoderms 93 



51. Various Ordovician Bryozoans on a slab of limestone 94 



52. Ordovician Brachiopods 95 



53. Ordovician Pelecypods 96 



54. Ordovician Gastropods 96 



55. Ordovician Cephalopods 97 



56. An Orthoceras restored 98 



57- Bits of Ordovician sea-bottom 100 



58. Ordovician Trilobites 101 



59. Map showing the surface distribution (areas of outcrops) of Silurian 



strata in North America 105 



