THE ORIGIN AND DESCENT OF ROCKS. 



405 



chiefly built. In these larger fragments, crystals are not infrequently 

 found, and the same is even true of the volcanic ash. These crystals 

 are undoubtedly such as had already been formed in the lava before it 

 exploded, and their formation, as suggested above, may have con- 

 tributed to the explosion. 



Fragments too large to be borne far away by the air, but still small, 

 are known as lapilli, especially if they are somewhat rounded and gravel- 

 like. A finer variety, of the nature of sand, much used in making Port- 

 land cement, is locally known as puzzolana. 



Fig. 337. — Volcanic bomb of unusual form, 13 feet long. 

 (Russell, U. S. Geol. Surv.) 



Cinder Buttes, Idaho. 



The rougher, irregular fragments of a clinker-like nature ejected by 

 volcanoes are known as scorm or cinders. They are more or less dis- 

 tended by gas-bubbles and are hence fight and pumiceous. 



The larger masses of lava ejected into the air are often caused to 

 rotate by the unequal force of the projection, or by the unequal fric- 

 tion of the air, and to assume spheroidal forms, the internal gases at the 

 same time often expanding and rendering the mass vesicular. These 



