THE STORY OF OUR ROCK FOUNDATION 41 



and eroded that we may now often examine their upturned 

 edges as they become exposed. The accessible rocks of these 

 early eras are exceedingly thick, possibly one hundred thousand 

 feet thick, thicker than all the formations that came after them. 

 The amount of erosion that they have undergone is tremendous. 

 Rock layers miles in thickness have been removed from these 



FIG. 27. Folded and crumpled rock on the face of a vertical cliff of jasper 

 and hematite at Ishpeming, Michigan. 



early land masses that were of continental proportions. Pre- 

 sumably then, the Archaeozic and Proterozoic eras represent 

 longer periods of geological time than any of the later eras, and 

 are to be counted in tens of millions of years, possibly hundreds 

 of millions. 



The early lands, made up of these primitive rocks that served 

 as the nuclei for the formation of the North American continent, 

 were a group of great islands. In general the oceanic depressions 

 and the continental elevations have existed from Archaeozoic 

 time, possibly from the very first, much as they now are. These 



