66 



OUR PHYSICAL WORLD 



rocks are divided into certain groups according to the dominance 

 of certain minerals. In the granite group at the left, the domi- 

 nant minerals are quartz and orthoclase feldspar. As you read to 

 the right in this table through the succeeding groups, the quartz 

 becomes a less and less conspicuous element in the rocks. The 

 feldspars decrease in amount and those present are of the plagio- 



TABLE OF IGNEOUS ROCKS 



clase varieties; finally the feldspars disappear entirely. While in 

 the granite group we may have hornblende or pyroxene present 

 in small quantities, in the right-hand groups these minerals come 

 to be the dominant ones. 



Reading down in any one of the groups, the texture of the 

 rocks varies from a spongy texture, through a glassy texture to 

 the crystalline texture, and the latter is first fine, then coarse. 

 In some of the groups these spongy and glassy rocks are missing. 



