OF THE HAWAIIAN ISLANDS. 439 



If a break occurs near the base the lavas will contain more augite, as the lava-stream which 

 formed Aliapaakai on Oa.hu, where crystals of augite are quite common. Intermediate 

 vents will pour out a lava of a mixed composition, and of this nature are most of the 

 Hawaiian lavas. But this is the rule only when an eruption has been preceded by a long 

 period of rest. 



It is a mistake to adopt the theory that formerly trachytic, and now basaltic lavas are 

 ejected. In many volcanoes basaltic underlie trachytic, and lavas of all gradations between 

 the two are found at different ages of the same volcano. 



There is another cause of the variety of eruptions from the same mountain, in the sep- 

 aration of minerals of different fusibility. Ferruginous minerals may be volatilized, leaving 

 in one place a lava destitute of iron, while in another the lava is highly ferruginous ; or 

 they may line the cavities of the upper mass with specular iron ore, as at Elba. 



M. Daubree has shown that the concurrent influence of heat and pressure will form crys- 

 tals of augite, feldspar, quartz and mica from water containing alkaline silicates in solution 

 with common clay ; * and it is not at all improbable that the frequent melting and hardening, 

 the great heat and pressure on the mass of lava, may cause the crystallization of its elements 

 into new mineral forms. It is certain that such crystals are formed in the interior of the 

 mountain and ejected in an undecomposed condition. Mr. Darwin saw in Albemarle Island 

 a stream of black lava thickly studded with large fractured crystals of albite, many of them 

 half an inch in diameter, which he says were evidently enveloped and penetrated by the 

 lava and rounded by friction as the stream flowed on. MM. Monticelli and Covelli describe 

 a lava ejected from Vesuvius in 1822 as containing leucite in the proportion of six to one 

 of the other ingredients. The granules were melted on the surface. 2 



The Hawaiian lavas contain much olivine, a very refractory mineral ; and where the stream 

 has issued from a considerable depth, as in the eruption from Kilauea in 1840, it is in large 

 granules and very abundant. Its specific gravity, it will be remembered, is 3.33 - 3.5. In 

 the ejections from the summit it is in minute particles, as if broken and carried up by the 

 currents in the melted mass. M. Von Buch has remarked of the basaltic lavas of Lancerote 

 (and Mr. Scrope has observed the same in those of the Eifel and the Vivarais), that while 

 the nodules of olivine are large near the source of the current, they dwindle away towards 

 the extremity so as to be scarcely visible. 3 



In the lava-stream of Aliapaakai, before mentioned, the eruption took place after a long 

 period of rest, and from a considerable depth ; and the lava contains large nodules of olivine 

 five or six inches in diameter. The same is seen in the lava of Koko, a similar formation, 

 where crystals of augite also occur. 



The separation of the less fusible portions of melted lava is well shown in the formation 

 of a-a which is simply a sort of imperfect crystallization of the parts of the lava first cooled, 

 from which the mother liquor, the still liquid lava, has been suddenly drained by the re- 

 moval of the dam which blocked the stream. 



When the volcanic action ceases or becomes extinct, the conical mass of lava cools slowly, 

 and forms a mass wholly destitute of stratification. This fact was noticed in the volcanic* 

 islands of the Pacific by Prof. Dana, and it is in these islands that the inner core of the 



1 See Daubree, Etudes sur le Mctamorphisme, Paris, 1859. 3 Scrope on Volcanos, p. 119, (2d ed.) 



2 Monticelli e Covelli. Sloria de' Fenomeni del Vesuvio, 

 Napoli, 1823. 



