J. D. Dana — History of the Changes in Kilauea. 285 



ah the floor of the pit ; 5'c, the surface of the black ledge. Let 

 now the falls from the wall above e make the talus deb with a 

 slope of 45°, causing thereby the 

 wall (and limit of the black 



ledge) to retreat to g. If the a' d> 



floor be now lifted 400 feet, to 

 the position a'b', the debris of 

 the talus deb would make an 

 elevation at top equal to d'e'b', 

 besides filliDg up efb', {efb'= 

 d'e'b'=\deV)\ the interval b'fg n $/ 



would represent the canal, and 

 d'e'b', 100 feet high, the ridge. 



If the floor were raised 50 feet higher, the ridge would be 

 lowered, say 25 feet, owing to material that would slip down 

 into the canal ; and consequently, the height of the ridge 

 above the floor over the center of the crater would then be 25 

 feet less than before, while 25 feet more than it was above the 

 black ledge. If no talus had been formed at the foot of the 

 wall, an uplift of the floor of 500 feet would have made a 

 precipice of 100 feet fronting toward the black ledge, the falls 

 from which would have produced a steep talus. These are two 

 conditions in the different parts of the ridge mentioned in Mr. 

 Lyman's paper. 



(2) Fault-planes about Halema , uma , u. — In Halema'uma'u 

 at the eruption of 1886, there was a circumferential fault plane ; 

 this seems to be implied in the fact that the return of lava was 

 mostly through vents toward the walls, little coming up at 

 the .center; and the fact that even a year and a half after- 

 ward, the action was greater outside of the cone than at its 

 center. At the discharge, the debris from the tumbling walls 

 fell beyond the fault-plane and made an accumulation of 

 blocks, like the talus of the lower pit of 1840, and this, as Mr. 

 Dodge's description shows and the photographs illustrate, was 

 the material that became the cone as the lifting went forward. 



(3) Conclusion. — By the above facts, it is proved that the 

 conduit lavas of the volcano not only keep up the supply of 

 heat, and carry on, by means of the vapors, projectile action 

 and vesiculation, but also that they furnish power for lifting, 

 in a quiet, unperceived way, the floors of craters with whatever 

 is upon them, and thus raising the level of volcanic activity ; 

 and that this goes forward as part of the ordinary operations 

 of the crater. The action has long been recognized as a means 

 of supplying heat and lavas, but not as a mechanical agent to 

 the extent here indicated. The force at work in making the 

 Gilbert laccoliths must be the same, and Mr. Gilbert, in his 



