LAVA STREAMS 



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The front of a lava stream advances, not by gliding over the 

 ground, but by rolling, the bottom being retarded by the friction 

 of the ground and the top moving faster, so that it is continually 

 rolling down at the curved front end and forming the bottom. 

 Thus, the scoriae, though formed mostly on the top of the stream, 

 are rolled beneath it, and the whole is enclosed in a cindery 



Fig. 13. — Lava stalactites and stalagmites, in lava-tunnel, Kilauea. 

 graph by Libbey.) 



(Photo- 



envelope. Or the flow may be checked by the mass of cinders, 

 until the fluid lava bursts through them in a fresh stream. The 

 scoriaceous mass is a non-conductor of heat, and greatly retards 

 the cooling of the interior mass, which may remain hot for many 

 years. The arched surface of cindery blocks may become self- 

 supporting, and then the still fluid mass will flow away from 

 beneath it, leaving long tunnels or caverns. These tunnels are 

 especially well shown in Iceland and the Sandwich Islands. 



