VOLCANOES. 739 



of Oahu, Maui, and Hawaii, which consist each of two or more united volcanic mountains. 

 At the southeastern end of the group occur the most recent lavas, and the only active 

 lires. The opening of Kilauea was in this direction, and the southeastern part of Hawaii 

 is the region of most of its recent eruptions. Fissure-eruptions began the group, and its 

 volcanoes mark where the rents remained open. (See author's Rep. Geol. Expl. Exp. 281.) 

 The Val del Bove is a gorge or valley, with precipitous sides, 1,000 to 3,000 feet high, 

 in the upper slopes of Mount Etna. Fresh-looking lavas cover the bottom ; and dikes 

 intersect the sides. It has been regarded as the result of subsidence. It is probable, as 

 suggested by the author in his Report on Volcanoes, that at its head was once a crater, 

 like Kilauea or the summit-crater of Maui. The conditions within and about the great 

 depression accord with this view. 



(7.) Slopes of Lava Cones dependent partly on the position over the 

 Mountain of the chief part of the Fissure-eruptions. — If such eruptions 

 are most common about the base they widen it, and thus southeastern 

 Hawaiian has been widened nearly a score of miles beyond the true 

 base of Mount Loa, this part having a pitch of only a degree or two. 

 But if fissures for large eruptions are opened continuously in the upper 

 part of the mountain, they tend to increase the pitch. 



(8.) Vibrations in the Rocks of a Volcanic Region. — These are of 

 three distinct sources. First, there are the tremors of a boiling caul- 

 dron, dependent on the escaping vapors of the open vent, which are 

 ordinarily light at Kilauea, but often appreciable in regions of viscid 

 lavas like Vesuvius. Secondly, vibrations come from those movements 

 of the volcano which produce fractures : either (1) the abrupt produc- 

 tion or condensation of steam ; or (2) hydrostatic pressure; or (3) un- 

 derminings of a region and subsidences. Thirdly, they may have their 

 source in former general movements of the earth's crust. See page 

 804. The microphone is now used for the study of the vibrations of 

 the volcanoes of Italy, and makes records even of the faintest mutter- 

 ings of the crater. 



(9.) Volcanoes situated in close proximity, independent in their Erup- 

 tions. — In the case of the summit crater of Mount Loa and its lateral 

 pit, Kilauea (20 miles distant), each is an active crater of the first 

 magnitude ; and, further, the two belong to the same mountain dome ; 

 and yet the lavas stand in one conduit, at the time of a summit erup- 

 tion, nearly 10,000 feet higher than they do in the other. The two 

 conduits are apparently branches of a common syphon ; but this ab- 

 sence of sympathy shows that the anion, if there is any at the present 

 time, is very deep below and much obstructed, or else that the lavas 

 in the two legs are of very unequal weight through the vapors present 

 or otherwise. 



Kilauea, although nearly four square miles in area, cannot prevent 

 eruptions at the summit of Mount Loa ; and if so, volcanoes, however 

 large, are very poor "safety-valves " A country is safer without them. 



