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



Another group of specimens, differing in aspect widely from 

 those described, although not essentially so in composition, are 

 the highly vesicular kinds, sometimes coarsely vesicular and 

 again with very minute cavities. They have for the most part 

 a common character. Large grains of chrysolite are usually 

 present, often very large in comparison with the size of the 

 vesicles themselves, and with these also are sometimes large 

 crystals of augite and feldspar, often grouped together. The 

 ground-mass filling up the space between these first separated 

 constituents is a dark fine-grained mass of plagioclase and augite 

 with minute grains of iron sometimes so abundant as to render 

 the whole nearly black and opaque. The augite sometimes 

 shows a tendency to group itself in the radiating forms already 

 described. A fluidal arrangement of the feldspar is the excep- 

 tion though occasionally observed in indistinct form. Only 

 in rare cases is the whole mass of the rock made up of this fine- 

 grained mass without the large crystals. A specimen from the 

 source of the 1843 flow belongs here. 



A specimen (76) which is described as the " ordinary ancient 

 lava of the eastern brink of the crater" is a dark colored, 

 coarsely vesicular rock (G. = 3 - 00), with chrysolite abundant in 

 large grains, and augite and feldspar also in large individuals, 

 the amount of the fine-grained dark base of later formation 

 is relatively small and the augite is somewhat radiated. A pecu- 

 liar feature of the section is the inclusion by the augite of large 

 plagioclase individuals not regularly orientated and giving the 

 whole augite a peculiar mottled appearance. 



Specimens of glass. — The Mauna Loa collection includes a 

 large number of specimens of the scoria, many pumice-like 

 specimens, some of them of extreme lightness and also speci- 

 mens of glass. Several of the glassy kinds were examined 

 microscopically. One of them (103) was a dense black com- 

 pact mass unit' ormly glassy on one side, but on the other largely 

 devitrified ; the smooth surface of the glass was roughened by 

 minute projections due to the chrysolite crystals. Its specific 

 gravity is 2*91. Under the microscope the glass had a uniform 

 brown color, and amorphous character, except for numerous- 

 minute doubly refracting points scattered through it. Here 

 and there were clusters of small chrysolite crystals, having 

 sharp outlines and perfectly clear except for occasional inclu- 

 sions of the glass and minute black iron crystals. 



A section cut transversely showed with great beauty the 

 gradual transition from the amorphous glass to the largely de- 

 vitrified lava. The pale yellow-brown glass of the border 

 contained here and there elongated microlites, of dark brown 

 color due to the glass immediately surrounding them, and also 

 minute crystallites like those described below. In the inter- 



