ROCKS OF THE BASALTIC AKEAS 53 



railroad. Some of the vesicular lava weathers white and loses a part of 

 its substance, so that it seems almost like pumice. The cliffs by the side 

 of the railroad consist of thick horizontal basaltic beds cut in several 

 places by small vertical dikes, especially at Makua, which are chryso- 

 litic. All these dikes seem to trend in the direction of Kaala. 



On the Oahu plantation the vesicular lava is of two kinds, one with 

 large irregular cavities and the other with rather small spherical bubbles 

 of uniform size, equally distributed through the mass. Doctor S. E. 

 Bishop thinks these varieties are characteristic of the A a and Pahoehoe 

 lavas, as he has observed them at the typical localities on Hawaii. This 

 suggestion accords with what I have seen and is accepted for a working 

 hypothesis, and emphasizes the opinion that the volcanic phenomena 

 of Oahu, Hawaii, and all the islands are identical throughout. 



East of Bishop museum there is much chrysolitic basalt in ledges that 

 have been quarried. One opening shows an immense columnar mass 

 of a compact gray basalt, which represents the filling of an old lava 

 conduit, and would therefore be more modern than the surroundings. 

 These old tunnels must have been commonly filled up on Oahu. 



Near the head of the Palolo valley is a dike crossing the valley, nearly 

 60 feet thick, and running parallel with the Koolau axis. The constit- 

 uent minerals are more crystalline than usual, as the mass seems to have 

 cooled slowly, without any vesiculation. It has a specific gravity of 2.90, 

 according to W. L. Green, who has used this rock as the text for a dis- 

 cussion of the relations between basalt and clinkstone.* Its peculiar- 

 ities, according to him, are the absence of chrysolite, incipient crystal- 

 lization of the labradorite and augite, light color, low specific gravity, 

 and a subcolumnar structure. Mr Tassin finds a nephelite-melitite 

 basalt among the specimens brought from the Moiliili quarry cited 

 above, page 4(>. This is not the same as the nephelite basalt described 

 by Wichmann in " Ballast from Oahu," 1875. 



The most interesting subject concerning the basalts is their decompo- 

 sition. Some reference has been previously made to it. A genuine 

 basalt is by decay changed to a mass of nodules cemented by a reddish 

 clay substance. This might easily be mistaken for a lava flow of different 

 age and character, whereas it is only decomposition. Examples of it 

 are common, as at the Ewa church, Oahu plantation in the gulch, near 

 Oahu college, ascending the hill to Manoa valley, etcetera. After the 

 production of a clayey cement the transition to beds of sedimentary 

 origin is easilv consequent, since streams of watej* will remove and con- 

 centrate the silty material, and thus is formed the kaolin to which allu- 

 sion has been made. This subject has been treated in a masterful way 



* Vestiges of the Molten Globe, part II, p. 135. 



