28 THE STORY OF THE EARTH. 



pressure becomes gabbro, after flowing on the 

 earth's surface becomes basalt. 



Rkyotite* — The name rhyolite indicates the 



fiuidal structure of the cooled lava, which results 

 from the movement Of minute crystals about 

 larger crystals in the flow of the molten stream. 



Some of the crystals are visible to the eye. The 



material between them is named the ground mass. 



Under the microscope, this ground mass is seen to 

 be formed of microscopic crystals, with an un- 



Stalline material between them, distinguished 

 as the base. The visible crystals are principally 

 quartz, with th< y variety of Olthoclase fel- 



spar named sanidine. These minerals may form 

 the entire mass of a granite rhyolite. But rhyolite 

 may be free t'r<»m crystals, formic. ISS, such 



as obsidian ; or be expanded into a froth like 

 pumice. 



irly all crystalline rhyolites are full of l 

 cretions with a radiating structure, or alternations 



granular layers with spherulitic 



known as spherulites. I the 



mon form of quartz, another va: 



tridymite occurs, in h< '-tie 



ornblende may be difl 

 in tfa I he ' I Iritish volcanic n 



St ! and the W'rekin, are rhyol 



ashes often 0< md 



h they were mutually 



\ cent. 



\s the b the pei rentage 



with th( 



ilumnar 



