112 a. hague thermal waters in yellowstone parit 



Mineral and chemical Composition of Ehyolite 



As I hope to be able to show that the mineral matter brought to the 

 Burface in solution by ascending thermal waters circulating through 

 rhyolite is mainly derived 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 considered and the different physical conditions 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. 



In five analyses the range in silica falls within seven-tenths of one 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 varieties, 

 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 homo- 

 geneous mass, the composition of the molten magma is probably best 

 shown in the specimen from Madison Canyon. Here the silica percentage 

 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, amor- 

 phous obsidian, to liparite, with relatively little groundmass. Neverthe- 

 less, the species that have crystallized out from the magma are few in 

 number, the only essential rock-making minerals being quartz and sani- 

 dine. In certain lavas quartz, in irregular crystals, occurs abundantly 

 disseminated as megascopic phenocrysts, while in others it is wholly 

 wanting. Plagioelase stands next in order, being easily recognized in 

 many thin sections under the microscope, although being seldom recog- 

 nized 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 it may be said to be absent. Py- 

 roxenic minerals are rare and only in microscopic crystals. Magnetite is 



