THE STORY OF OUR ROCK FOUNDATION 



39 



Concerning the earliest rocks that made up the outer cool 

 portion or crust of our earth we know nothing except by inference, 

 for they are nowhere subject to observation. Probably they 

 were lavas that had outflowed from the deeper layers on to the 

 surface, forming not only 'what lands there were, but also the 

 bottoms of the seas and oceans. These rocks were attacked by 



Fig. 25. — Basaltic dyke intrusive in granite, the latter lichen-covered, south 

 shore of Partridge Island (Marquette), Lake Superior. 



the various agents of erosion, and disintegrated. The material 

 so eroded was used to build up other rocks as indicated in the 

 preceding chapter. These newer rocks, sandstones, shales, etc., 

 such as are forming today under similar conditions, accumulated 

 through long ages, were upheaved by the crumpling and folding 

 of the crust, occasionally into great mountain ranges, and thereby 

 metamorphosed into quartzites, slates, and schists. The rock 

 layers were by the same convulsions fractured, and into the 



