144 Washington — Submarine .Eruptions of 1831 and 1891. 



salt extracted by leaching. It is coaly black, with a slightly 

 brownish tinge and pitchy luster ; very highly vesicular, with 

 small, rounded vesicles, the size of which varies somewhat in 

 different parts of the specimen, as is also true of Foerstner's 

 piece. No phenocrysts are visible. 



Microscopic. — Judging from Foerstner's description and the 

 examination of my specimen there is considerable variation in 

 the microscopic character, though not more than is usual in 

 such glassy scorias. The most abundant and prominent pheno- 

 crysts are of a lime-soda feldspar, showing Carlsbad and less 

 often albite twinning, the extinction angles of which indicate 

 the composition Ab 1 An 2 , which is somewhat more sodic than 

 appears to be true of Foerstner's rock. These feldspars are 

 tabular and very thin in my specimen, measuring only up to 

 0.5mm j on g !^ T ^ ot more t | 1 ' au o-Ol — 0-05 mm thick; while in 

 Foerstner's they would appear to be larger and more euhedral, 

 and more nearly like those in my specimen of the Graham 

 Island lava. The phenocrysts of augite are few, up to about 

 l mm in diameter, subhedral and stoutly prismatic, of a brownish 

 or greenish gray. Colorless olivine is present, apparently in 

 less amount than the augite and in smaller, mostly anhedral 

 grains, though a few show euhedral rhombic sections. 



The groundmass in my specimen consists of a brownish, 

 clear glass, which is very thickly sprinkled with a rusty brown 

 or black dust, so much so as to be almost or quite opaque in 

 places. Foerstner notes two varieties of the glassy ground- 

 mass ; one highly vitreous, and the other less so, in which the 

 glass is present in small amount as a cement or mesostasis. He 

 describes these in great detail, which it is needless to quote 

 here. 



Chemical composition. — The material of the bombs ejected 

 by Foerstner Volcano has been analyzed by Foerstner and by 

 Perry, the results being given in II and III below. A new 

 analysis made by me is given in I, the rock powder having 

 been leached with water, as in the preceding case, by which 

 process only a very small amount of salt was extracted. Foerst- 

 ner states that his specimen contained about 25 per cent of sea 

 salts, so that his probably came from the exterior, and mine 

 from the interior, of a bomb. 



My analysis shows low silica and alumina, very low ferric 

 oxide but very high ferrous oxide, low magnesia and alkalis 

 and rather high lime and phosphorus pentoxide. The most 

 striking feature is the very large amount of titanium dioxide, 

 which is almost unparalleled elsewhere among volcanic rocks, 

 and is only exceeded in the melilite-basalts of the Hegau.* 



* U. Grubenmann, Inaug. Diss. Zurich, 1886. 



