E. S. Dana — Petrography of the Sandwich Islands. 467 



iuwaa Valley is a very compact, nearly black basalt, unusual in 

 showing occasional grains of pyrite. The feldspar is fresh, 

 but the augite is more or less altered and its place taken by a 

 serpentinous substance, while occasional cavities are filled with 

 a light colored radiating zeolitic mineral showing feeble double 

 refraction. Besides the usual magnetic iron, which is scattered 

 through in grains or octahedral crystals, there are also curious 

 aggregations of iron ore in very slender rod-like forms, some- 

 times crossing each other at right angles, but usually matted 

 together with a confusedly reticulated structure, sometimes 

 forming nearly opaque spherical aggregates. Specific gravity 

 2-90. 



Chrysolite is present very sparingly in the remaining rocks, 

 the hand specimen showing only here and there an isolated 

 grain, and sometimes close search is needed to detect it. They 

 are all light bluish gray basalts, with specific gravity ranging 

 from 2*86 to 2*91. No very close study has been made of 

 these specimens, but with a number of them their aspect, their 

 highly felspathic character, and the microscopic structure, made 

 it seem as if they might more properly belong to the andesites ; 

 a silica determination of one of them (36, G. = 2'86) by Mr. 

 Wheeler gave, however, only 50*55 p. c. Si0 2 . Several of these 

 rocks show more or less alteration, and in one of them (36) the 

 occasional crystals of chrysolite have entirely passed into serpen- 

 tine. For the most part they are highly crystalline, but one 

 of the group (45, G. = 2'88) is exceptional in showing numerous 

 patches of a dark brown glass ; this specimen is the most highly 

 chrysolitic of the number, it being present in minute grains 

 among the feldspar and augite, each grain having its orientated 

 fringe of titanic iron. ~No nepheline basalt was detected among 

 the specimens in hand. 



Resume. — The chief points brought out and discussed in the 

 preceding pages are, the characters of the clinkstone-like basalt 

 with its novel forms of feather-augite, and also of the heavy 

 chrysolitic basalt, each from the summit crater ; the general 

 similarity between the lavas of Mauna Loa and of Kilauea and 

 the crystalline character of both, even of the recent forms ; the 

 structure and origin of the stalactites in the caverns of the 

 Mauna Loa lava stream, like those in the caverns of Kilauea, 

 offering new problems in the formation of igneous rocks. It 

 is shown further that the lavas in hand from Maui — with the 

 exception of the andesite from the western part of the island — 

 and also those from Oahu, belong to the basaltic type, though 

 often approaching andesite in appearance ; furthermore, that 

 the lavas of Hawaii show extensive alteration but only so far as 

 the iron oxidation is concerned, while some of those of Maui and 

 Oahu, especially the latter, are much more profoundly changed. 



