Vol. 57.] OF THE YELLOWSTONE AND GREAT BRITAIN. 217 



is brown and rather opaque, exhibits the mottled appearance of the 

 JS'orthern Jersey rocks, and traces of flow-structure exist. Porphy- 

 ritic crystals of quartz, unaltered orthoclase, and plagioclase are 

 common, the last predominating. In one slide an elongated cavity 

 is cut through, into which a porphyritic, absolutely unaltered 

 orthoclase projects. This cavity is lined by rather impure car- 

 bonates, while some discoloration of iron-oxide exists in the 

 neighbourhood, but no sign of replacement or of solution. This 

 indicates that the spherulite is more resistant to the action of hot 

 water charged with silica than are its surroundings. 



The cavernous hollows with a rough mammillated lining, found 

 in the altered rhyolites of the Upper Falls, and referred to above, 

 closely resemble at first sight some spherulites wdth irregular 

 vesicles from the obsidian of Obsidian Cliff. The concretionary 

 material which lines the cavities in the rock of the last-named 

 locality is easily powdered, and consists of branching rays of dusky 

 felspar and tridymite. In the altered rhyolite the similar material 

 is much harder, and no felspathic fibres appear. The encrusting 

 substance is probably largely tridymite, which mineral it resembles 

 in its index of refraction and polarization-tints ; but the characteristic 

 hexagonal outlines are not seen, perhaps owing to the fact that the 

 constituent scales are more closely aggregated. 



We may conclude that, the action of hot water charged with 

 silica may be to remove portions of the rock, or to permeate it 

 without destroying its characteristic structure ; we obtain, however, 

 no evidence, but rather the reverse, to show that the spherulites 

 are most easily attacked. 



Y. Conclusions in regard to tete Yellowstone. 



The theory formulated by Prof. Iddings ^ to account for the 

 structures exhibited by iithophysse at Obsidian Cliff and elsewhere 

 in the I^ational Park is, in my opinion, that which is most in 

 accord with the facts. It is based on the hypothesis that the 

 origin of lithophysse is due to the hydrous state of certain parts of 

 the magma. In an additional memoir, recently published,^ three 

 physical processes have been given as the means by which the 

 formation of lithophysse was carried out. These are, in the first 

 place, rapid crystallization ; in the second, ' a sudden liberation 

 of heat ' resulting therefrom ; and in the third place, a conse- 

 quent ' lowering of saturation of the surrounding mother-liquor.' 

 Thus he arrives at the ' spasmodic advance of crystallization ' 

 resulting in ' layers ' of varying coherence, and the ultimate forma- 

 tion of open spaces by * shrinkage.' In order to test the truth 

 of these hypotheses by independent data, I have consulted Ber- 

 thelot's ' Mecanique Chimique ' and kindred works, and have also 



1 7th Ann. Eep. U.S. Geol. Surv. (1885-86) p. 284 ; for an historical review 

 of the theories relating to lithophysse, see ibid. p. 287. 



2 Mem. U.S. Geol. Surv. Monogr. xxxii, pt. li (1899) p. 417 ; see also Bull. 

 Phil. Soc. Washington, vol. xi (1891-92; pp. 446-47. 



Q. J. G. S. No. 226. Q 



