598 PROCEEDINGS OF BOSTON MEETING. 



In the absence of the author the next paper was read by George H. 

 Williams. 



VOLCANITE, AN ANORTHOCLASE-AUGITE ROCK CHEMICALLY LIKE THE 



DA CITES 



BY WILLIAM H. HOBBS* 



[^Ahstracf] 



The eruption of Volcano f in 1888-'9 produced a peculiar type of volcanic projec- 

 tile which Dr H. J. Johnston- Lavis has characterized by the term, "bread-crust 

 bomb." J These projectiles consist of an acid pumice core surrounded by a thin 

 skin- of a vesicular glassy rock, which is warped and cracked by reason of the 

 strains to which it has been subjected in cooling. The structure of the projectiles ^ 

 has been discussed in the papers cited. 



The material of the bombs is porphyritic in structure with a glassy base, pheno- 

 crysts of felspar and augite being prominent in both core and rim. They are 

 often quite large, the felspars attaining a diameter of three-quarters of a centi- 

 meter. Inclusions of a more basic rock occur which are, with little doubt, fragments 

 of the dolerite so extensively developed at the south end of the island. An acid 

 enclosure obtained by Professor J. P. Iddings in a subsequent visit to the island 

 was found on examination to be quite certainly a partially fused fragment of lip- 

 arite. Dark colored inclusions having the outline of crystals, which are quite 

 abundant in the rock, have been shown to be magmatic pseudomorphs after augite 

 phenocrysts. 



The predominating felspar of the phenocrysts resembles sanidine, and generally 

 appears under the microscope as idioinorphic unstriated individuals, bounded by 

 the faces P, M, I and y. Cleavage pieces parallel to the base extinguish parallel, 

 and those parallel to the second cleavage yield an extinction angle of from 4 to 7 

 degrees. An obtuse positive bisectrix emerges from the latter. In most cases 

 there is no trace of twinning strige, but rarely an extremely fine microcline-like stria- 

 tion can be made out in the section. Simple twins, according to the Baveno law, 

 were observed. Parallel a,nd knotty or " knaiielformig " growths of felspar are 

 very common. A fragment of the unstriated felspar free from inclusions was found 

 to have a specific gravity of 2.559. A sufficient quantity of this felspar was sepa- 

 rated from the rock, with a slight admixture of magnetite and augite, by means of 

 the Thoulet solution. As these impurities do not compose entire grains of the rock 

 powder used in the separation, they were not easily removed. An analysis of this 

 material by W. F. Scoular is given under I in the following table : 



* I desire to acknoM'lt-dge obligation to Messrs Louis Kahlenberg, Leo C. Urban, and M. F. Seoular 

 for the chemical analyses furnished by them for this paper, and I am also indebted to Professor 

 George H. Williams, of Johns Hopkins University, for valuable suggestions. 



f Volcano is situated on one of the Lipari islands. 



X Johnston- Lavis : Proe. Geol. Assoc. London, vol. xi, 1890, p. 30O. Also Nature, vol. xlii, 1890, p. 78. 

 Hobbs : Trans. Wis. Acad., vol. ix, 1893, p. 21, pi. 1, fig. 2. 



g The present pa er is an abstract of a study of the material of the projectiles from a petrograph- 

 ieal standpoint. The full paper will appear in the Zeit-chrift der deutschen geologisclien Gesell- 

 scliaft. 



