96 J. D. Dana — Eruptions of Kilauea and Mt. Loa. 



only under the most extraordinary circumstances is the water 

 forced into the lava-column. The earthquakes of 1868 were 

 an exhibition of the power generated ; and hardly less so is the 

 noiseless fracturing for the greatest of eruptions, 



Some erupting action comes from hydrostatic pressure. But 

 the fact that the fissures first open quite near the summit of 

 Mt. Loa is evidence that pressure from this source is the least 

 efficient agent. 



Why southwestern Hawaii should be especially liable to 

 violent earthshocks in connection with its outflows is not 

 wholly clear. But there are three significant facts bearing on 

 the question. 



(1) The southern half of the longer diameter of the Mt. Loa 

 crater, and fissures from it down the mountain, point directly to 

 the place of outbreaks of 1868 and 1887, the probable localities 

 of the earthquake epicentra of those years. 



(2) The longer diameter of Kilauea, with a long line of fis- 

 sures, having the trend S. 52° "W., points nearly to the same 

 region of outbreak ; so that the two diametral lines, the Mt. 

 Loa and the Kilauea^ there intersect. (See map, p. 82 and Plate I.) 



(3) These lines have long been common directions of frac- 

 tures and eruptions, as shown by the old lavas of the 'surface 

 as well as by existing lines of fractures. 



This divergence between the courses of the longer diameters 

 of the craters of Mt. Loa and Kilauea comes up again for con- 

 sideration in the remarks on the relations of the two volcanoes. 



In the eruptions the ejecting force may be feeble or null ; 

 for the lava may flow out, when the source favors it, simply 

 through gravity ; but, in general, ejection is pushed forward, 

 (1) by the elastic vapors within the lava-column ; by vapors 

 generated outside, like those producing eruptions ; and by hydro- 

 static pressure. 



The first of these causes is the source of the high fountains 

 in the summit crater ; and the summit effects indicate that it 

 should have great propelling power at places of outflow. The 

 fountains at the outflows have hitherto been attributed to hydro- 

 static pressure ; but the two causes must here act together, and 

 it is impossible to say from present knowledge which prepon- 

 derates. 



Fountains attended the outbreak at the eruptions of 1852, 

 1859, 1868 and 1887 (pp. 19, 22, 21, 32,); and it is probably 

 that examination at other times would have added one or two 

 to the list. The lengths of the lava-column (A) above the jflace 

 of outbreak at these eruptions, and (B) the reported heights of 

 the fountains in feet, are as follows : 



