88 J. D. Dana — Summit Crater of Mt. Loa. 



that of the Mt. Loa flow of 1880-81 being 2-98 ; that of ordi- 

 nary Kilauea lava, 2'97-3-05 ; an old lava, from Waldron's 

 ledge, excessively ehrysolitic, 3 '15. 



If neither of these explanations meets the case, we have only 

 the third to appeal to — a greater volume of elastic vapors. It 

 is, accordingly, probable that the cause which can produce occa- 

 sional jets of 100 to 200 feet in Kilanea is capable of producing 

 the prevailing high jets or fountains of the summit of Mt. Loa. 



But why should the volume of vapors in the lava-column be 

 greatest at the summit ! The amount of work done there is 

 ordinarily at least 100 to 1000 times greater than in Kilauea ; 

 for the jets are 5 to 10 times higher. 



This difference in amount could not be a fact if the vapors 

 within the slowly ascending lavas were from the profound 

 depths that supply the lava, or even from depths much below 

 the sea-level. For, under such circumstances, (lj the difference 

 in the amounts carried up to the two craters would be small, 

 since the rate of supply from below would be essentially uni- 

 form ; and (2) the difference in the height of the columns 

 would be more favorable to Kilauea, whose lava-column rises 

 above tide level but 3700 feet, than to Mt, Loa 9000 feet higher. 

 The area of the floor of Kilauea exceeds that of Mt. Loa. 



But if freshwater from precipitation over the island supplies 

 the vapors, then the difference in the heights of the conduit 

 lava-columns is greatly in Mt. Loa's favor. A section of its 

 lava column at the sea-level may receive moisture during the 

 whole time of its rise to the summit, a distance 3*8 times that 

 for Kilauea. The ratio 3*8 to 1 for the difference in supply of 

 moisture to the columns would be too large on account of the 

 less precipitation over the upper part of the mountain and the 

 much less extent of surface in this part ; but it may safely be put 

 at 2 to 1, if not 2£ to 1. The ascensive movement in the Mt. Loa 

 lava-column may be somewhat more rapid than in the shorter 

 conduit of Kilauea. provided the hotter central portion derives 

 any upward thrust from the pressure of the cooler lateral j)ortion, 

 (xxxiv. . . . ) ; and this cause would diminish the difference 

 between the two as to the supply of vapor received ; yet not 

 largely. 



The fact here apparently established — that only through 

 waters from the island-precipitation could Mt. Loa get its 

 larger supply — affords new evidence that the inland waters are 

 the chief source of the vapors concerned in Hawaiian volcanic 

 action. 



Is there any other source of the Projectile action f The lava- 

 fountains of the summit-crater are so marvelous in size consid- 

 ering the density of the lavas, so near the incredible, that we 

 naturally seek for other possible explanations. 



