AKT. 17 MINERALS OF OBSIDIAN CLIFF FOSHAG 7 



in tlu'iii. Von Riehthofen"' first ex})l;iine{l tlieni as a result of ox- 

 paiuliiiL' gas bubbles Avhile the rock Avas in a molten or })lastic state, 

 ihe disengagement and expansion of the gas l>eing brought about by 

 the release of pressure due to the eruption of the magma. 'J'his hy- 

 pothesis has found supporters in Von Hauer,^^ Zirkel.^- AVeiss,^^ 

 Cross,^* and IddingsJ"' A second hyijothesis is that of Szabo ^'' v.dio 

 considered the lithophysae to be the remains of solid spherulites 

 from which nuich of the material had been removed by chemical 

 means, leaving the lithophysal minerals as an insoluble remainder. 

 This theory was supported by Cole.^' A chemical examination of 

 these bodies and of the accompanying glass by Tenne,^'^ however, 

 demonstrated that they were identical, and, therefore, that no 

 remoA^al of material is involved. The lithophysae are rather a special 

 type of crystallization of obsidian glass. 



MINERALS OF THE LITHOPHYSAE 



The minerals identified in the cavities and lithophysae of the Ob- 

 sidian Cliff lavas are. in order of their abundance, feldspar, tridy- 

 mite. cristobalite. fayalite, and quartz. Numerous minute needles 

 associated with tlie feldspars may be hornblende. 



Feldspar. — In numerous spherulitic lavas, glassy phenocrysts of 

 feldspar are abundant, but this is not the case in the flows of Ob- 

 sidian Cliff proper. Here the feldspars are largely confined to the 

 spherulties and lithophysae. In the glassy obsidian, the cavities are 

 lined with, or sometimes partially filled by a white, semifibrous coat- 

 ing, which, in the larger cavities becomes more pronounced and 

 under the microscope resolves itself into an aggregate of prismatic 

 orthoclase crystals arranged in parallel position. The constituent 

 groups are sometimes straight, sometimes curved, while occasionally 

 they show delicate branching forms. These feldspar groups are 

 beautiful objects under the microscope (pi, 4, fig. 1), but only 

 in the very coarsest ones can the arrangement be discerned with the 

 naked eye. Twinning is sometimes developed but the twinning law 

 could not be determined with any amount of certainty. The rodlike 

 crystals aie terminated by a single glassy crystal that seems to be 

 untwinned. The feldspars in the more glassy portions of the flow 

 contain small white pellets of cristobalite disseminated throughout 



"F. von Riclithofen. .Lihib. k. k. peol. RoicIiKanstalt, vol. 11, 18G0, p. 180. 

 ^' Karl von Ilaner, Vorhandl. k. k. jreol. Rciclisan.'^talf, 1800, \i. !t8. 



^2 Ferdinand Zirkel, IT. S. Geol. Expl. 40th Far., vol. 6, Microscopical Fetrography. 

 1876. p. 212. 



'^Ch. E. Weiss. Zcits. d. I)<'ut!^c]i. Geol. Gopcll., vol. 20. 1877, \^. 418. 

 "Whitmsin Cro.«s. .\mcr. .Tourn. Sci., ser. ,^, vol. 31, 1886, p. 4.'?2. 

 "J. V. Iddings, 7th Ann. Rep. U. S. Geol. Surv., 1888, p. 279. 

 ".Joseph Szabo, .Tahrb. k. k. {reol. Reichs^aiistalt, vol. 10, 1880, p. 80. 

 "Grcnville A. J. Cole, Quart. .Tourn. Geol. Soc. London, Mux. 188.5, p. 162. 

 '«C. A. Tenne, Zeits. d. Deutsch. Geol. Gesell., 18S5, p. 610. 



