THE CRUST OF THE EARTH 185 



The broad valley between the two great mountain systems 

 of the United States furnishes sedimentary rocks but no 

 metamorphic rock. In this region there are beds of lime- 

 stone, hundreds of feet thick. In many places the cover- 

 ing of mantle rock is thin and quarries are easily opened. 

 These furnish an inexhaustible supply of material for build- 

 ings and for making lime. Lime is necessary for all masonry 

 and concrete work, and is prepared by heating limestone. 



i i 



FIG. 94. FOLDED COAL-BEARING ROCKS 



Dotted line = surface when strata were folded. a, b = fragmental 

 rock, c = seam of coal, d = present surface. / = faulted fracture. v = 

 an eroded valley. 1. In which vertical section would the discovery of coal 

 be most likely to occur? Why? 2. In which least likely? Why? 3. How 

 do you account for the thicker covering at c'f 



The oldest mountains are in the eastern states and Cali- 

 fornia. These furnish the finest metamorphic rocks, granite 

 and gneiss, both of which are called granite commercially. 

 The granite of the Rocky Mountains is a coarse granite. 

 The lavas of the Rocky Mountains are used for trimming 

 stones, but are too porous for other use in regions where 

 the atmosphere is humid. 



A source of great industrial prosperity in this country is 

 the coal-supply. Beds of coal lie between beds of shale and 

 sandstone in large areas from Nova Scotia to Iowa and south 

 to Texas, and farther west on the slopes of the Rocky Moun- 

 tains through Mexico and Central America. Where the 

 strata have been lifted up in mountain making, the coal is 

 fairly accessible. If the strata have been very much com- 



