560 



SCIENCE 



[N. S. Vol. XXXIII. No. 850 



lation of either descending or ascending 



waters. 



MINERAL AND CHEMICAL COMPOSITION OF 

 EHYOLITE 



As I hope to be able to show that the 

 mineral matter brought to the surface in 

 solution by ascending thermal waters cir- 

 culating through rhyolite is mainly de- 

 rived from these lavas, it is necessary to 

 examine in detail both their chemical and 

 mineral composition. The chemical com- 

 position of the rhyolite appears remarkably 

 uniform when the enormous bulk is con- 

 sidered and the different physical condi- 

 tions under which the lava streams were 

 extravasated. 



In the subjoined table will be found 

 analyses of eleven specimens of rhyolite, 

 arranged according to their tenure of 

 silica. They were made in the laboratory 

 of the United States Geological Survey, but 

 are here brought together in tabular form. 



ties, but, curiously, analysis fails to show 

 the presence of both in the same flow. 

 Traces of manganese have been detected in 

 many specimens from widely separated 

 parts of the tableland, which is interesting 

 from the fact that in one or two localities 

 solfataras have deposited manganese oxide 

 as dendritic incrustations. Considering 

 the rhyolite as a homogeneous mass the 

 composition of the molten magma is prob- 

 ably best shown in the specimen from 

 Madison Canyon. Here the silica per- 

 centage was 75.2 per cent., the alumina 

 13.77, and the combined potash and 

 soda 7.16. 



As regards mineral composition the 

 rhyolite is by no means as simple; owing 

 to textural modifications that range from 

 semitransparent, amorphous obsidian, to 

 liparite with relatively little ground mass. 

 Nevertheless, the species that have crystal- 

 lized out from the magma are few in num- 

 ber, the only essential rock-making miner- 



ANALTSES OP EHTOLITES FEOM TEIiOWSTONE NATIONAL PAEK 



VII. 



vm, 



l^IX. 



Upper Geyser Basin 



Head of Tower Creek 



Plateau east of Willow Park 

 Clilf east of Excelsior Geyser 



Obsidian Clifif. 



North Madison Plateau 



Elephants Back 



Obsidian Cliff. 



Obsidian Cliff. 



Sheridan Volcano 



Sherman Volcano 



SiO, 



70.92 

 71.85 

 72.59 

 73.84 

 74.70 

 75.19 

 75.34 

 75.50 

 75.52 

 75.89 

 77.65 



Ti02 AI2O3 FejOs FeO MnO 



.52 



None 

 None 

 None 

 None 

 None 

 .50 

 .14 



13 24 

 13.17 

 13.47 

 12.47 

 13.72 

 13 77 

 12.51 

 13.25 

 14.11 

 12.27 

 11.60 



Trace 

 Trace 

 Trace 

 Trace 



.07 

 None 

 None 

 None 

 None 



CaO NasO K„0 LijO P2O5 SO3 HjO 01 Total 



.57 



100.31 

 99.83 

 100.04 

 100.05 

 100.49 

 100.06 

 99.84 



In five analyses the range in silica falls 

 within seven tenths of a per cent. Lime 

 and magnesia show the greatest variation, 

 while the alkalies do not appear to be 

 higher than in many other localities where 

 rhyolite has been extravasated in the form 

 of massive eruptions. Titanic oxide has 

 been determined in small amounts, but was 

 not detected in the obsidians or in any of 

 the extreme glassy rocks. On the other 

 hand, both sulphuric acid and chlorine occur 

 in small quantities in the fresh glassy varie- 



als being quartz and sanidine. In certain 

 lavas quartz, in irregular crystals, occurs 

 abundantly disseminated as megascopic 

 phenocrysts while in others it is wholly 

 wanting. Plagioclase stands next in 

 order, being easily recognized in many 

 thin sections under the microscope, al- 

 though being seldom recognized by the 

 naked eye. This is probably owing to the 

 small amount of alkaline earths present. 

 Small flakes of biotite have been detected 

 here and there, but in the typical rhyolite 



