18 



PHYSICAL GEOLOGY 



Although solid rocks are exposed only occasionally in the 

 interior of the United States, as in quarries, mines, along the 



FIG. 5. Horizontal stratified rocks and 



ding planes. 



courses of certain streams, and in a few other situations, they 

 outcrop (come to the surface) over large areas in eastern 



FlG. 6. Metamorphic rock. Contorted gneiss 

 (Young, Can. Geol. Sure.) 



