THE ARCHEOZOIC ERA, 



147 



Fig. 38. — The white areas north of Mexico represent exposures of Archean with some 

 undifferentiated Proterozoic; 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 1 roterozoic, beneath later formations. The light 

 shading about the borders of the land represents the area of the continental platform 

 covered by sea-water, or what is the same thing, the area of the epicontinental sea 

 for this continent. 



