170 



PHYSIOGRAPHY 



nothing to burn. There is, therefore, no smoke. What appears 

 as smoke is mostly condensed water vapor (cloud), often blackened 



by dust. 



The Products of Volcanoes 



Lava. This term is applied both to the liquid rock which 

 issues from a volcano and to the solid rock which results from its 

 cooling. It takes on various forms as it becomes solid. If it 



Fig. 170. Scoriaceous lava. The little cavities are of the general nature 

 of bubbles, filled with gas when the lava hardened. Cinder Buttes, Ida. 



hardens under little pressure, as at the surface, the gases and 

 vapors which it contains may expand, and it is converted into a 

 sort of rock froth (Fig. 170). Under other conditions it may be- 

 come volcanic glass (obsidian) or crystalline rock. 



Cinders, ashes, etc. Most of the fragmental materials blown 

 out of a volcano are pieces of lava which solidified before they were 

 blown out, or during their flight in the air. They include masses 

 of rock tons in weight, and smaller pieces of all sizes down to minute 

 particles of dust or "ash." It should be noted, however, that 

 "volcanic ash" is not ash in any proper sense of the term, for it is 

 not a product of burning. The dust is often shot up high into the 

 air, and, being light, is caught by the winds and scattered broad- 

 cast, some of it coming to rest thousands of miles from its starting- 

 point (p. 19). The fluid lava, and the larger fragmental materials, 

 on the other hand, stay near the vent. 



Gases and vapors. Gases and vapors of many kinds issue 

 from volcanoes. Some of them are poisonous, and, as in the case 



