SOURCES OF IGNEOUS ERUPTIONS. 723 



craters on the globe ; and yet eruptions occur at the summit of the same mountain, 

 10,000 feet above the level of Kilauea, and so extensive that the lavas flow off for twenty- 

 tive to fifty miles, without any sign of sympathy in the lower crater. If the two are con- 

 nected, the siphon, in such a case, has the liquid 10,000 feet higher in one leg than in 

 the other. 



Connection without sympathy is possible only on two suppositions: (1) that the 

 junction of the two conduits is at such a depth that 10.000 feet is but a small fraction 

 of the whole length, and the additional pressure is more than counterbalanced by the 

 friction along the conduits; or (2) that, if the lavas rise in consequence of an inflating 

 process, the difference of length may not imply a corresponding difference of pressure. 



Even about Kilauea itself, eruptions sometimes take place through the upper walls of 

 the crater to the surface (as at P, Fig. 1109), when the lavas are boiling freely in the 

 bottom of the crater, undisturbed by the ejection. 



While the linear arrangement of the volcanic mountains of a group is evidence that 

 thev all originated in one grand breaking of the earth's crust, the several volcanoes in 

 a line may not stand over one prolonged fracture, but over a series having a common 

 direction, in the manner illustrated by the figures on page 19. This was, beyond ques- 

 tion, the mode of origin of the Hawaian Islands. 



The islands of Oahu and Maui (see Fig. 24, p. 31) consist each of two great volcanic 

 mountains, united at base, and Hawaii of three mountains. In the case of both Oahu 

 and Maui, the north western of the two volcanoes became extinct long before the south- 

 eastern, — as is apparent in the profound valleys of denudation that intersect its slopes 

 and almost obliterate its original features; while the lavas and parasitic cones of the lat- 

 ter look fresh and recent. In Hawaii, also, Mount Kea, the northern volcano, is the 

 extinct one. Again, in the whole Hawaian group, the only active volcanoes are in the 

 southeastern island, Hawaii, while the northwestern island, Kauai, shows in its features 

 that its extinction was among the earliest, if not the very earliest, of the whole number. 

 It appears, therefore, that each, Oahu and Maui, stands over a fissure which was largest 

 toward the southeast, since the fires of the southeast extremity of each were last ex- 

 tinguished; that Hawaii had a similar origin, but with probably a second more western 

 fissure as the origin of the volcano of Hualalai ; and that the whole Hawaian group 

 originated in a series of fractures, which increased in extent from the northwest to the 

 southeast; for Maui continued in eruption long after Oahu (a more western island in 

 the group); and Hawaii, the southeasternmost, is the only island now active, and the 

 one that through its prolonged activity has attained the greatest height above the sea. 



These facts illustrate a general principle, with regard to the fractures of the earth's 

 crust, as well as the origin of volcanic groups. 



2. Motion transformed into Heat. — Mallet, as stated on page 698, 

 has shown that the motion in the earth's crust, or its rocks, attending 

 mountain-making, is sufficient to generate great heat, and regards it as 

 sufficient to produce fusion, and to originate and sustain the volcanoes 

 of the globe. Many trachytic rocks have nearly the constitution of 

 fjranyte and gneiss ; and they may hence have come from such a 

 fusion. But the fact that eruptions of one epoch, along a country a 

 thousand miles or more in range (as over the 1,000 miles along the 

 Atlantic Border of North America, from Nova Scotia to South Caro- 

 lina, in the Triassico-Jurassic era), have ejected the same kind of 

 flolerytic rock, shows that the material of the fire-seas beneath was of 

 very uniform composition ; and this uniformity could not have come 

 from the fusion of the diversified sedimentary or metamor'phic rocks of 

 the region, or of its depths ; and, besides, scarcely any of these rocks 



