4i8 ROCKS AND BUILDING STONES OF THE EARTH 



and hard, granular in structure, and usually of a reddish or 

 grayish color. Obsidian (Fig. 283), another igneous rock, 'is 

 solid, but glassy in character and black in color. Pumice, still 

 another igneous rock, is a very light-weight, porous rock, full of 

 minute air spaces. 



For building purposes granite is the most important of the 

 igneous or lava-formed rocks. It is so strong and compact that 

 it can endure enormous strain, and it is not easily weathered. 

 Granite structures are among the most durable known. But 

 the hardness and compactness which make granite valuable as 

 a building stone also make it expensive, because it is hard to 



i i split and dress. Granite 



is quarried abundantly 

 throughout the United 

 States, particularly in 

 the East. 



Sandstone. This im- 

 portant building stone 

 is simply a mass of tiny 

 grains of sand cemented 

 together into a firm solid 

 rock. It is formed from 

 the sediment deposited 

 by running water, and 

 for this reason is called 

 sedimentary rock. Sedi- 

 mentary rock is some- 

 times called stratified 

 rock, because the de- 

 posits of which it is composed are frequently in the form of 

 layers (Fig. 284). Limestone is a sedimentary rock, because it is 

 formed from the vast deposits of decaying shells which accu- 

 mulate at the bottom of rivers and oceans, or from the deposits 



FIG. 284. Stratified rock. 



