CHEMICAL COMPOSITION OF EHTOLITES. 429 



or pitchstone exhibit a gradual transition in the character and proportions 

 of the microscopic crystals or microlites from those of normal obsidian to 

 the abnormal basaltic facies (2164). It is to be noted that this facies is not 

 dacitic in its habit, although the silica percentage is 72.59. Such facies 

 are uncommon. The occurrence, as well as its transition into obsidian 

 with lithophysse, suggests that its magma is a transition form between 

 rhyolite and basalt. 



The slight variations in composition which are expressed by the min- 

 eralogical character of the rhyolite in different places do not appear to be 

 connected with any particular locality, although they are sometimes char- 

 acteristic of large areas of rhyolite. They recur in all parts of the region, 

 and appear to be oft-repeated modifications of the magma, dependent as 

 much on physical conditions as on chemical variability. 



From the table of chemical analyses (p. 426) it is seen that the most 

 siliceous facies of the Holmes bysmalith, which occurs along its margin at 

 Echo Peak, is almost identical chemically with portions of the lava flow at 

 Obsidian Cliff, while the main mass of the bysmalith has nearly the compo- 

 sition of the dacitic facies of the rhyolite at the Upper Geyser Basin, but is 

 lower in potash and higher in alumina. The fine-grained granite from the 

 volcanic core of the Crandall volcano has almost exactly the same composi- 

 tion as the rhyolite from the Upper Geyser Basin. The dacite-porphyries 

 of Bunsen Peak and Birch Hills have nearly the composition of the rhyolite 

 from Tower Creek and of the abnormal obsidian from the plateau east of 

 Willow Park, but they are higher in alumina and a trifle higher in lime, 

 with a little less silica. The quartz-mica-diorite-porphyry from the volcanic 

 core of Electric Peak, though nearly as high in silica as the rhyolite of the 

 Upper Geyser Basin, is distinctly higher in lime and lower in potash. On 

 the other hand, the trachytic rhyolite from the flanks of Sunset Peak, north 

 of the Park boundary, while agreeing with the rhyolite of the Upper 

 Geyser Basin in most of its chemical composition, is lower in soda and 

 higher in potash. 



The rocks here compared with the rhyolite are the more acid phases 

 of magmas that belong to much earlier periods of eruption. They show 

 the tendency of these magmas to differentiate into facies whose composition 

 approaches that of the rhyolite more or less closely. 



