THE ARCHEOZOIC ERA 



445 



Fig. 341. The white areas north of Mexico represent exposures of Archean; 

 the white areas south of the United States represent lack of knowledge; 

 the black areas represent exposures of Proterozoic, while the lined areas 

 represent Archean or Proterozoic, beneath later formations. The light 

 shading about the borders of the land represents the area of the con- 

 tinental platform covered by sea-water, or what is the same thing, the 

 area of the epicontinental sea for this continent. 



