J. D. Dana — Summit Crater of Mt. Loa. 89 



Hydrostatic pressure is out of consideration, inasmuch as the 

 fountains are at the summit of the dome and at times throw 

 their jets 50 to 100 feet above the mountain's top — over 14,000 

 feet above the sea-level. 



Another source of projectile action has been suggested by 

 Mr. Green, as briefly mentioned on a preceding p'age (xxxv, 

 216). In opposition to other writers on volcanoes, he sets aside 

 the idea that vapor of water is concerned effectually in the 

 projectile action even of Kilauea. The feeble amount of 

 vapors observed by him over the fountain of the summit-crater 

 in 1873, and the general absence of vapors from the flowing 

 lava-streams of 1859 and 1880-81, besides other similar facts, 

 have led him to his position on this point. He recognizes the 

 fact* that great heaps and columns of clouds form over an 

 active crater, and rise at times to a height of many thousands 

 of feet ; but accounts for these on the assumption that the 

 heated current ascending from the active crater derives rapid 

 accessions of air from either side, and this air, by being carried 

 up to cold heights, yields the moisture by condensation, and so 

 forms the column of clouds. Further, he finds a cause of some 

 projectile action for the Kilauea lava-lakes and others in 

 atmospheric air carried down by the descending lavas of the 

 jets into the lava-lakes — as the crests of waves carry down air 

 into the sea; and for the rest of it, or that producing the 

 crater-fountains like those of Mt. Loa, he holds that the ascen- 

 sive action in the conduit, after a time of quiet, suddenly over- 

 comes resistances or stoppages that have come to exist in the 

 conduit at depths below, and, as a consequence, the lavas, sud- 

 denly released, are thrown up in fouu tains, like the jets of min- 

 eral oil from an artesian boring. 



I have already met part of the argument as to the absence of 

 vapors of water, in my remarks on vesiculation, by showing (1) 

 how extremely little moisture is needed to produce vesiculation 

 (xxxv, 226), and (2) how much moisture hot air will dissolve 

 and make invisible. It has also been stated (3) that if a Mt. 

 Loa lava-stream has but a single fountain-head, as is generally 

 supposed, though not proved, nearly all the vesiculation must 

 occur at the source, so that for this reason and the heated air 

 above it, the lava-stream should be vaporless, or appear so, ex- 

 cept where there are fissures below for additional supply. + 



Farther (4), direct observation proves that the vapors come 

 up out of the crater. They often rise directly from the 



* Yestiges of the Molten Globe, pp. 75, 1G2-1G7, 175, 272-278. 309. 314. 



f Mr. Green states, as an exceptional case, that at one place on the Mt. Loa 

 flow of 1880-81, the lavas spread into a large lake, and vapors rose from it in 

 great amount. This is good evidence of the existence there of a local supply of 

 lavas through a fissure. 



