ARCILEAN TIME. 



149 



3. Those once covered, like the last, but which, in the course of the 

 upturnings of mountain -making, have been pushed upward among the 

 displaced strata, and in this way have been brought out to the light. 



In cases like those of figures 203, 204, in which the Silurian rocks are 

 spread in nearly horizontal layers over the borders of an area made up 

 of tilted Archaean rocks, the Archaean area either has been always 

 uncovered, or has become so from denudation ; but, in mountain- 

 regions, where the Silurian rocks have been folded up in the mountain- 

 making, the Archaean below may have been brought to view in the 

 upturnings. Moreover, the Archaean, if it had not undergone flex- 

 ures before the Silurian beds were laid down, would partake of the 

 Silurian flexures, or, in other words, be 'conformable to the Silurian 

 strata. But. if it had been flexed or tilted in some previous period of 

 disturbance, then the Archaean would be uncomformable to the Silurian, 

 although both were finally upthrown together, in the making of the 

 mountains. 



In the study of Archaean regions, these points require special inves- 

 tigation. 



Fi S . 206. 



Archaean Map of North America. 



In the map, Fig. 206, the chief Archaean regions are the white 

 areas, while the dark-lined portion represents the rest of the continent 

 submerged beneath the continental sea. 



