286 DYNAMICAL GEOLOGY. 



soon made it evident that it was produced out of the falling blocks by the lifting of the 

 bottom owing to the ascensive action of the lavas beneath, like Lyman's ridge described 

 on page 284. (F. S. Dodge.) 



The recent eruptions of Mount Loa, the summit crater, have been vastly 

 more extensive than those of Kilauea. Situated topographically within the 

 same mountain mass, as the following diagram (Fig. 248) shows, the two 

 have yet gone on with their preparations and eruptions simultaneously, but 

 in general independently ; the loftier crater unaffected in its lifting and its 

 eruptive forces by the great opening at the lower level. Kilauea has none of 

 the virtues of a safty-valve for Mount Loa, though probably as much of a 

 safety-valve for the mountain as any volcanic vent ever is. The recent erup- 

 tions of Mount Loa occurred in the years 1843, 1852, 1855, 1859, 1868, 1880, 

 1887 ; and excepting the two on the southern slopes, those of 1868 and 1887, 

 the place of outbreak was at heights of 10,500 to 13,000 feet, and the lengths 

 of the streams 20 to 35 miles. At the place of outbreak in several in- 

 stances, there have been great fountains of lava, 300 to 700 feet in height, 



Section of Mount Loa and Kilauea. 



that played for a few days, as the stream gushed forth — a consequence 

 either of the projectile force of escaping vapors, or of hydrostatic pressure 

 from the lavas in the Mount Loa lava-column, or from both causes combined. 

 In contrast with Mount Loa, the famous Hecla of Iceland, about 5000 feet 

 in height, has had only five eruptions since 1700, viz., in 1728, 1754, 1766, 

 1845, 1878. 



(c) Earthquakes not an essential feature of volcanic eruptions. — The great 

 eruptions of Mount Loa, excepting those of 1868 and 1887, have been unat- 

 tended by noticeable earthquakes. The rupturings must have caused vibra- 

 tions, but they have usually been unperceived at the villages of the island. 

 "A fire on the mountain" has been the first announcement of the outbreak. 

 When the outflow has begun, the liquid lava in the bottom of the summit 

 crater has disappeared, and the crater has lost at the same time its activity. 



In 1868 and 1887, however, there were violent earthquakes ; but otherwise the circum- 

 stances were not different. In 1887, two days intervened between the appearance and 

 fading of the light at the summit and the exit of the lavas, and, in 1868, four days, owing 

 apparently to the distance of the place from the discharging conduit ; but when once out 

 the lavas rose into a fountain of 100 to 200 feet, showing that they were under great 

 pressure, and then the shakings ceased. At the eruption of 1868, Kilauea was discharged 

 at the same time as Mount Loa, — Mount Loa forces evidently producing this remark- 

 able result by breaking first the Mount Loa conduit, and then four days later, before the 

 earthquakes ceased, that of Kilauea. In other words, the fracturing of the mountain made 

 by vapors generated by the Mount Loa fires finally extended to the Kilauea conduits. 



