GASEOUS PRODUCTS 53 



their characteristic texture and appearance, so as to be readily 

 recognizable, though perhaps only with the microscope. The 

 great bulk of these materials consists of lava shattered by the 

 steam explosions and quickly chilled. The coarser fragments 

 display the frothy and vesicular nature of scoriae, while the finer 

 particles are glassy or crystalline. Mere comminution of the mass 

 does not change its essential texture. 



It will be readily imagined that lavas do not contain fossils. 

 Though the flows often overwhelm living beings, the intense heat 

 at once destroys them, leaving not a trace behind. In tuffs, on 



FlG. 17. — Mauna Loa, seen from a distance of 40 miles. (Photograph by Libbey.) 



the other hand, fossils, especially those of plants, are frequently 

 well preserved, and tuffs formed under water have fossils as 

 abundantly as any other aqueous rocks. 



(3) The Gaseous Products are important as agents of the 

 eruptions, in promoting the crystallizing of the lavas, and in alter- 

 ing the rocks with which they come in contact. The most abun- 

 dant is steam. Carbon dioxide is common, especially when the 

 action is failing, and often continues after all other signs of activity 

 have died out. Sulphurous acid (S0 2 ) is very characteristic and 

 is the source of many other compounds. Sulphuretted hydrogen 

 (H 2 S) is a common volcanic gas, as is also hydrochloric acid 

 (HC1). Several solids are vapourized, such as the chlorides of 

 ammonium, iron, calcium, etc., but these are of little significance. 



