92 J. D. Dana — Eruptions of Kilauea and Mt. Loa, 



S. 20° "W., as marked on the upper and lower margin of 

 Plate II. Four of the largest lava-streams of Mt. Loa, those 

 of 1843, 1852, 1855 and 1880, and two others to the south, 

 those of 1868 and 1887, have their places of outbreak nearly in 

 the line of the respective halves of the longer diameter. Again, 

 three of the eruptions, those of 1851. 1859 and 1877, broke out 

 on the west side of the summit, nearly in the line of the shorter 

 diameter, or between the summit and Hualalai. 



There is here probable evidence of a dependence of the 

 eruptions to some extent on the two great fissure-lines upon or 

 about which the mountain's foundations were laid. 



The direction of the longer diameter of Kilauea is about 

 N. 50° E.-S. 50° W. The chief course of eruptions, as on 

 Mt. Loa, is marked by a line of fissures and ejections running 

 west-south west ward in the direction of the longer diameter. 

 But the large outflow of 1840, and the fissures leading to it, 

 instead of pointing toward the crater, have a course nearly 

 parallel to the longer diameter, but fifteen miles south of the 

 Kilauea line. This is seen on the map, page 82, but better 

 on Plate I, the stream being the one near the east cape. 



2. Causes of Eruptions. 



1. State of readiness for an Eruption. — The ordinary quiet 

 work of the craters has been shown to be carried on by — 



(1) The ascensive force of the conduit lavas ; this force pro- 

 ducing (1) a slow rise in the liquid rock from depths below ; 

 and (2) a raising of the crater's bottom. 



(2) The elastic force of rising, expanding and escaping 

 vapors ; producing jets and fountains in the lava-lakes ; over- 

 flows or ejections spreading the lavas over the crater's bottom ; 

 vesiculation of the lava and consequent increase of its bulk. 



Other causes have been mentioned as occasionally in action 

 (xxxv, 228), but as not essential to the chief results. 



After a season of this ordinary activity, with more or less 

 gradual increase of intensity, a state of readiness for an erup- 

 tion and its determining conditions have been reached. This 

 has happened when the lava has risen, through these agencies, 

 to what might be called high-lava mark; a level some hundreds 

 of feet above low-lava mark or the low level occasioned by the 

 preceding .discharge. 



2. Action needed for an Eruption. — After this preparation 

 nothing is needed for an eruption but an agency of sufficient 

 force to break the lava-conduit ; for if broken seriously the 

 lava will run out, and therein is an eruption or discharge. 



Neither of the agencies carrying on the ordinary quiet work 

 of the volcano has shown itself capable, during historic time, 

 that is, since 1822, of breaking the lava-conduit for a discharge. 



