CHEMICAL COMPOSITION OF RHYOLITES. 427 



VARIATIONS IX COMPOSITION AMONG THE RHYOLITES. 



The lavas which would be classed together as rhyolites on account of 

 their field relationships and similarity of habit are found to differ somewhat 

 in composition, both chemical and mineralogical. The chemical, composi- 

 tion of the commonest and also of the extreme varieties is shown by the 

 analyses in the accompanying table. With them have been placed for 

 comparison analyses of the most siliceous intrusive i-ocks from several locali- 

 ties in the Park. 



The range of silica in the rhyolite is from 70.92 to 75.89 per cent. 

 Four varieties contain between 74.70 and 75.89 per cent. They are the 

 flow forming Obsidian Cliff, in part obsidian, in part lithoidite; lithoidal 

 rock at Mount Sheridan; obsidian from the Elephant Back, and lithoidal 

 rhyolite from Madison Plateau north of the canyon. These represent 

 the normal rhyolite of the region. Another variety of rhyolite, which 

 appears to be a common form and carries abundant quartz phenocrysts, 

 has 71.85 per cent of silica. It is lithoidal and is part of the great mass 

 forming the table-topped spurs at the head of Tower Creek. A variety 

 of obsidian occurring north of Obsidian Cliff and apparently a portion of 

 the obsidian flow of that locality, though not certainly so, contains 72.59 

 per cent of silica. This rock is more of a pitchstone than obsidian, and 

 has a dull resinous luster. The variety lowest in silica, with 70.92 per 

 cent, constitutes the lava in the immediate vicinity of the Upper Geyser 

 Basin. It is lithoidal and dark colored, without noticeable phenocrysts of 

 quartz. 



Alumina varies but slightly in these rhyolites, from 12.27 to 14.11 per 

 cent. The alkalies are moderately high, being 6.65 to 8.50 per cent, with 

 soda in excess of the potash in most cases. The alkaline earths are the most 

 variable, lime ranging from 2.25 to 0.68 per cent, magnesia from 1.05 to 

 0.07 per cent, and ferric oxide from 3.54 to 0.42 per cent. Ferrous oxide 

 forms 1.55 to 0.08 per cent, the sum of the iron oxides ranging from 4.20 to 

 1.63 per cent. There is about 0.30 per cent of titanium oxide in three cases, 

 and none was found in three other cases. Phosphorous pentoxide was found 

 only in the less siliceous varieties. 



The chemical composition of the rhyolite from the head of Tower 

 Creek is not wholly in accord with the mineral composition and habit of 

 the rock, which has abundant glassy feldspars and quartzes, plagioclase being 



