112 



EARTH FEATURES AND THEIR MEANING 



ocean, was shivered like melted glass and thrown up in millions of 

 particles which darkened the sky and fell like hail over the sur- 

 rounding country. The light was so bright that at a distance of 

 forty miles fine print could be read at midnight. 



Protected from any extensive consolidation by its congealed 

 cover, the lava within a stream may all drain away, leaving behind 



an empty lava tunnel, which in the 

 case of the Hawaiian volcanoes 

 sometimes has its roof hung with 

 beautiful lava stalactites and its 



FIG. 107. Diagrammatic repre- . . . 



sentation of the structure of the floor studded with thin lava spines. 

 flanks of lava volcanoes as a re- Later lava outflows over the same 



suit of the draining of frozen lava of neighboring Courses bury Such 



tunnels beneath others of similar 



nature, giving to the mountain flanks an elongated cellular struc- 

 ture illustrated schematically in Fig. 107. These buried channels 

 may in the future be again utilized for outflows similar in char- 

 acter to that of Kilauea in 1840. 



While the formation of lava stalactites of such perfection and 

 beauty is peculiar to the Hawaiian lava tunnels, the formation of 

 the tunnel in connection with lava outflow is the rule wherever a dis- 

 sipation at the end has permitted of drainage. A few hours only 

 after the flow has begun, the frozen surface has usually a thickness 

 of a few inches, and this cover may be walked over with the lava still 

 molten below. At first in part supported by the molten lava, the 

 tunnel roof sometimes caves in so soon as drainage has occurred. 



Wherever basaltic lava has spread out in valleys on the surface 

 of more easily eroded 

 material, either cinder 

 or sedimentary forma- 

 tions, the softer inter- 

 vening ridges are first 

 carried away by the 

 eroding agencies, leav- 

 ing the lava as cappings 

 upon residual eleva- 

 tions. Thus are derived a type of table mountain or mesa of the 

 sort well illustrated upon the western slopes of the Sierra Ne- 

 vadas in California (Fig. 108). 



FIG. 108. Diagram to show the manner of forma- 

 tion of mesas or table mountains by the outflow 

 of lava in valleys and the subsequent more rapid 

 erosion of the intervening ridges. R, earlier river 

 valley ; R'R' t later valleys. 



