xvi CONTENTS. 



PAGE 



The Proterozoic of the Lake Superior Region. 175 



The Huroman System 175 



Composition and thickness, 175. Disturbing move- 

 ments, 177. Sections of the Huronian, 179. Erosion 

 of the deformed Huronian system, 181. Contempora- 

 neous events elsewhere, 182. 



The Animikean System. 183 



General relations, 183. Composition, 183. Igneous 

 rocks, 184. Thickness of sediments and duration of 

 the period, 184. Deformation and erosion, 185. Alter- 

 ation of the rocks, 185. Distribution, 186. 



Iron Ore in the Lake Superior Region 190 



Production, 190. Geologic positions, 191. 



The Keweenawan System 192 



Constitution and thickness, 192. Copper, 198. 

 General Considerations Concerning the Lake Superior Pro- 

 terozoic 199 



Duration of time, 199. The sources of the sedi- 

 ments, 199. Succession of events, 200. Metamor- 

 phism, 202. Sequence of events elsewhere, 202. 



Proterozoic Rocks in Other Regions 203 



North of Lake Huron, 204. The great northern 

 area, 204. The eastern provinces of Canada, 204. 

 The original Laurent ian area, 204. The Adirondack 

 region, 205. Isolated areas in the Mississippi basin, 

 206. The Cordilleran region, 210. In the eastern part 

 of the United States, 212. 



Proterozoic Formations in Other Continents 216 



Life During the Proterozoic Era 217 



Climate. . . . , 217 



CHAPTER V. 



THE CAMBRIAN PERIOD. 



Formations and Physical History 218 



The Subdivisions of the Cambrian and their Distribution. ... 219 



The Lower Cambrian 219 



Distribution, 219. Geography of the Early Cam- 

 brian, 222. " Lost " interval, 222. Extent of uncon- 

 formity at base of Lower Cambrian, 224. 

 The Middle Cambrian 224 



