LESSONS IN GEOLOGY. 351 



Laurentian tableland extending from Labrador southwestward 

 to the region of Lake Superior, thence north westerly to the Arctic 

 Ocean. They occur in the Adirondack and White mountain re- 

 gions, at many localities in the Western mountain regions 

 in the Alleghanies, in Arkansas and Missouri and in other 

 places, but only in the region of Lake Superior and some western 

 localities have the rocks of the two eras been clearly separated. 



These rocks as exposed in the Laurentian tableland are from 

 40,000 to 50,000 feet in thickness. Their formation and their ele- 

 vation into mountains is explained as follows : The area now oc- 

 cupied by these rocks was a shallow sea, adjacent to an extensive 

 land surface, probably on the north in the region of Hudson's 

 Bay. The sediments derived from the land were spread out over 

 this sea bottom. For some reason this area of the crust was 

 weak, so that it settled gradually, as the sediments were depos- 

 ited, till they became more than 50,000 feet deep. At length the 

 settlement was so great that the rocky masses underneath be- 

 came softened by the internal heat, and the whole mass became 

 so weak that it yielded to the lateral pressure of the sinking 

 crust and the sediments were mashed together, the crumpled, 

 folded strata slowly swelling up into mountains. 



Under this great pressure, through the agency of heat and 

 water, the organic matter of the sediments was destroyed and 

 they were changed into met amor phic rocks. 



Originally the substances which make up the earth's crust 

 were widely disseminated, and the great bulk of the Archaean rocks 

 were formed from a mixture of a wide variety of substances ; but 

 the assorting tendency of nature's operations is manifest in the 

 beds of quartzite, slate, limestone, iron ore, and graphite. 



The Laurentian tableland is the oldest portion of North Amer- 

 ica; it seems never to have been submerged. It has been subject 

 to eroding agencies for untold ages; it has furnished materials for 

 the rocks of all subsequent ages; yet rising by intervals or con- 

 tinuously, it has always kept pace with the degrading effects of 

 the eroding forces. 



