606 



GEOLOGY. 



Even if the difference in the heights of the columns could be explained 

 by differences in specific gravity, the agitation of the one should be 

 communicated to the other, and an outflow of the one, particularly an 

 outflow by a breakage through its walls sufficient to lower its surface 

 hundreds of feet, as has repeatedly occurred in Kilauea, should change 

 the surface of the other proportionate^, if they were in hydrostatic 

 equilibrium. It seems a necessary inference, therefore, that the two 

 lava-columns have no connection with each other or with a common 

 reservoir. The tops of some lava-columns stand about 20,000 feet 

 above the sea, while others emerge on the sea-bottom far below sea-level. 

 The total vertical range is, therefore, probably between 30,000 and 



Fig. 463. — Surface of lava-flow of 1881, from Mauna Loa, as seen back of Hilo, Hawaii. 



(Photo, by Calvin.) 



40,000 feet, a difference which tells its own story as to their relative 

 independence. 



3. Unimportant coincidences. — Eruptions seem to be somewhat more 

 Uable to occur at times of high atmospheric pressure than at low, doubt- 



