CHAPTER XI 

 THE ARCHEOZOIC ERA 



The oldest rocks. The most ancient part of the earth's 

 history of which we have any tangible record in the rocks is 

 quite as shadowy and obscure as the dim legendary period of 

 human annals. The record has been in large measure de- 

 stroyed, and that which remains is confused and difficult to 

 interpret. The rocks which were made during the Archae- 

 ozoic era are the oldest of which we have any knowledge. 

 They compose what is called the Archcean system. This un- 

 derlies all later systems of rocks, and over most of the globe 

 it is still buried beneath thick layers of younger rocks (Fig. 

 317). In most places where Archaean rocks can now be 



FIG. 317. Ideal cross section of North America, showing how the Archaean 

 rocks underlie all others. 



studied they have been uncovered by erosion. Such regions 

 may have been more deeply eroded than others because they 

 have been repeatedly uplifted. In the eastern half of Canada 

 the Archaean rocks reach the surface over a large area, inter- 

 rupted only by bands or patches of the younger rocks. They 

 are exposed again in a long strip between the Appalachian 

 Mountains and the Coastal Plain, stretching from Alabama 

 to New England, in the cores of certain anticlines among the 

 mountains of western United States, and in other situations. 

 In other countries similar exposures of Archaean rock have been 

 found, particularly in Brazil, central Africa, and Scandinavia. 

 B. & B. GEOL. 18 317 



