456 II. S. Washington — Rhyolites of Lipari. 



posed the term " dacitoide. ' ' Although some such term 

 seems to be highly desirable, yet it is questionable if 

 "dacitoide" is quite the most appropriate. Etymolog- 

 ically, the name implies a rock that resembles a dacite 

 (that is, according to the original definition, one that 

 contains modal quartz), while in reality the rock is one 

 that, while it has the chemical composition of a dacite, 

 resembles modally an andesite. The term hyalo dacite 

 was introduced by Rosenbusch long ago, implying that 

 modal quartz is present. Some years ago, in describing 

 the rocks of Pantelleria, I suggested 16 the term "hyalo- 

 pantellerite" for the very glassy forms of the pantel- 

 lerites, in which feldspar phenocrysts are abundant, but 

 those of pyroxene are scarce, though the rocks have a 

 chemical composition practically identical with that of 

 the noncrystalline pantellerites. Following the same 

 idea, I would suggest here that the term hyalo-dacite 

 be used for lavas of dacitic chemical composition, that is, 

 which contain a considerable amount of excess silica, 

 though modally they may be free from quartz. 



Rhyolite Obsidian. Island of Milos. 



Since writing the foregoing pages I have made an 

 analysis of a specimen of the obsidian of Bombarda, on 

 the Island of Milos, which I obtained from Kranz of 

 Bonn many years ago. The analysis shows that it is a 

 sodi-potassic rhyolite, but slightly higher in silica and in 

 lime than the Lipari obsidians. A description of it is 

 given here, for comparison with the Lipari rhyolites, and 

 because no analyses or detailed descriptions of the rocks 

 of Milos have yet been published. 17 



The obsidian is a quite typical one, perfectly glassy, of 

 a grayish black color, not as pure a black as the Lipari 

 obsidians, and with a slightly duller vitreous luster. It 

 shows perfect conchoidal fracture, and also a distinct 

 flow structure. There are no phenocrysts. In thin sec- 

 tion it is seen to be an almost pure, colorless glass, with- 

 out spherulites, but with numerous, extremely minute 

 (about 0.01 mm. long) black, baculitic microlites, which 



1C H. S. Washington, Jour. Geol., 22, 704, 1914. I called highly silicic, 

 glassy but quartz-free, rocks from Asia Minor, dacites many years ago 

 (This Journal, 3, 43 and 47, 1897). 



17 For a description of Milos, see K. Ehrenburg, Die Inselgruppe von Milos, 

 Leipzig, 1889. 



