J. D. Dana — Summit Crater of Mt. Loa. 87 



flow. Then begins (2) a rising of the lava of the column 

 until it again shows part of its fiery top in the bottom of the 

 crater engaged in its usnal projectile work, and until finally it 

 has reached a maximum height ; and then follows (3) a new 

 discharge, and another time of inactivity for the crater. 



2. The projectile action loithin the crater. — Projectile action 

 in the Mt. Loa crater is in strong contrast with that of Kilauea. 

 Instead of the Kilauea feature of low jets suggesting ordin- 

 ary ebullition, with only occasional throws to a height of 100 

 to 200 feet, the descriptions of the summit action tell solely of 

 fountains of clustered jets 75 to 600 feet, and even 900 feet 

 high, as if the height of the jets or the intensity of the action 

 was proportional to the height of the lava-column. The four 

 accounts of this activity, one in 1872, two in 1873, and one in 

 1880, are alike in this respect. One of the two in 1873 de- 

 scribes the crater when the summit-light appeared feeble from 

 below, and the other when brilliant, and the former is scarcely 

 less marvellous in its fountains. The evidence is almost con- 

 clusive that such fountains are of ordinary occurrence. This 

 was the opinion of Mr. Coan ; and Mr. W. L. Green, in view 

 of his summit observations in 1873 (p. 28) and the reported 

 facts of others, ascribes to all the periods of summit illumina- 

 tion " great fountains." 



3. Causes of the Ordinary movements within the Crater. 



1. The rise of the lava in the conduit. — The rise of the 

 conduit lava may be safely attributed in part, probably a large 

 part, as in Kilauea, to the quietly-acting ascensive force in the 

 lava-column. 



The other volcanic agency of greatest prominence, as admitted 

 for other volcanoes, is that of the rising, expanding and escap- 

 ing vapors. The vesiculating effects of the vapors as regards 

 the Mt. Loa flow of 1880-81 have been already described 

 (xxxv, 222) : and it remains to consider — 



2. The cause of the high projectile action in the summit 

 crater. — Higher projectile action in Mt. Loa than in Kilauea 

 through the escape of elastic vapors might come (1) from 

 greater viscidity in the lava ; or (2) from less specific gravity 

 of the material ; or (3) from a larger supply of vapors. The 

 first of these causes cannot be the right one, for greater vis- 

 cidity should lead to high cinder ejections ; on the contrary, 

 the lavas show that they are as mobile as the Kilauea lavas by 

 the velocity of the lava streams and all the attending phe- 

 nomena, and more by the free play of the fountains. The 

 second is set aside by the near identity of the lavas in density : 



