19. Sketch of the Grand Canyon of the Colorado River 134 20. Map of the Great Basin 137 21. Map of Crater Lake, Oregon 153 22. Mount Rainier, Washington 154 23. Map of Puget Sound 161 24. Map showing isobars for January and July 175 25. Ice-palace, Montreal 199 26. Map showing tracks of West Indian hurricanes 209 27. Map showing depth of evaporation in the United States 212 28. Map showing distribution of forests 215 29. Douglas firs, Vancouver 241 30. View in redwood forest of California, from photograph by U. S. Bureau of Forestry 242 31. Bison at Silver Heights, Winnipeg 275 32. Map showing range of bison 276 33. Maps showing the growth of the North American continent 303 34. Map showing the distribution of coal-fields 336 35. Ideal section through an oil and gas pool beneath an anticlinal 339 36. Lodge or tepee of Blackfoot Indians, Manitoba 387 37. Panorama of Uxmal, Yucatan 390 38. Pointed arches in Central American ruins 393 39. Carved stonework, Uxmal, Yucatan 394 NORTH AMERICA CHAPTER I THE MARGIN OF THE CONTINENT--THE CONTINENTAL SHELF In beginning the study of the physical geography of North America, one of the first facts to claim attention is that the true continental border is in general many miles seaward from the present margin of the land. The boundary of our field of study is defined with considerable accuracy by a line drawn on the bottom of the sea adjacent to the present coast-line of the continent so as to pass through all points where the soundings show a depth of 100 fathoms of water. This 100-fathom contour in the topography of the sea-floor chances to coincide in a general way with the outline of the submerged border of the continent; landward from it the bottom rises with a gentle slope, while seaward the descent is usually steep down to a depth of 2,000 or more fathoms.