ART. IT MINERALS OF OBSIDIAN CUFF — FOSHAG 13 



described by Wright^' from Hrafntinnuhryggiir, Iceland. It has a 

 double significance in demonstrating that the crystals formed while 

 the glass was still somewhat viscous, while its distinctive fish-hook 

 shape suggests that it was forced into the cavity by pressure. Thus, 

 though still rather viscous, the preservation of the delicate structure 

 of the lithophysae shows that the mass must have been essentially 

 at rest. The temperature of formation then must have been some- 

 where near the point of solidification of the glass. Brunn found 

 that obsidian from Lipari no longer flowed at 800° C. For a fresh 

 magma still charged with volatile constituents, the temperature 

 must be somewhat lower. In some specimens of lithophysae, tra- 

 pezohedral quartz takes the place of the more common tridymite as 

 an apparently primary cr^^stallization. These facts would suggest a 

 temperature of formation of the minerals as somewhere in the neigh- 

 borhood of 500° C. This is, of course, much below the quartz-tri- 

 dymite and tridymite-cristobalite inversion points,-" and the last 

 named minerals must have formed as unstable phases. 



ORIGIN OF THE MINERALS 



The first mode of origin that suggests itself is the transportation of 

 the materials in aqueous solution from the outside into the cavity, 

 much after the manner in which zeolites are formed. Because of the 

 relatively slight solubility of these minerals, even at somewhat ele- 

 vated temperatures, a considerable volume of solvent would be re- 

 quired. Glass is easily susceptible to hot aqueous solutions and if 

 the origin of the minerals had actually occurred^in this manner, a 

 decided alteration would be apparent. The glass, however, is per- 

 fectly fresh to the very walls of the lithophysae. If aqueous solu- 

 tions were inadequate, then perhaps the materials were transported 

 in gaseous solution whose mineral content was in the form of easily 

 volatile compounds. But such a process involves an abundant sup- 

 ply of fluorine, chlorine, or other mineralizers, and, although sought 

 for, no evidence of these constituents in any large quantity was 

 found. 



It is difficult to conceive any origin whereby the materials were 

 introduced into the cavities from outside, or to escape the conclusion 

 that the materials making up the minerals originated within the 

 cavity itself. Iddings -^ has already pointed out that " these lithophy- 

 sae, composed of prismatic quartz, tridymite, soda-orthoclase, fayal- 

 ite, and magnetite (to which should be added cristobalite) , are of 

 aqueo-igneous origin and have been produced by the action of the 



19 F. E. Wright, BuU. Geol. Soc. America, vol. 26, 1915, p. 255. 

 «• Clarence N. Fenner, Amer. .Tourn. Scl., vol. 36, 1013, p. 331., 

 =" J. P. Iddings, Tth Ann. Rep. U. S. Geol. Surv., 1888, p. 283. 



