PHYSICAL STRUCTURE OF RHYOLITE 111 



passes the Yellowstone Park in the varied phenomena of highly acid 

 extrusions. This is especially true of the more glassy types, and in gen- 

 eral a glassy groundmass characterizes most of these lava sheets. Mr. 

 J. P. Iddings has submitted a large series of specimens of the park rhyo- 

 lites to a searching petrographical investigation, making a special study 

 of the microgranular structure and the relations of the different micro- 

 structures to one another, and pointing out the abrupt transitions from 

 the glassy to the crystalline and from the pumiceous to lithoidal forms. 

 For further details the student is referred to this admirable work. In 

 conclusion, Mr. Iddings calls attention to the agency of water in bringing 

 about the varied products. He says: "The heterogeneity of the acid 

 lavas, so far as known, is confined to the distribution of vapors, pre- 

 sumably of water, and suggests that the water thus irregularly dissemi- 

 nated has not existed within the magma long enough to become uni- 

 formly diffused. It must therefore be looked upon as water absorbed 

 near the earth^s surface." ^ 



From the point of view of the present discussion, the cause of these 

 remarkable structural variations concerns us less than the influence ex- 

 erted by such textural modification in the creation of fissures, fractures, 

 and capillary openings for the percolation of waters. Obsidians, perlites, 

 and pitchstones were poured out over the greater part of the central 

 plateau and may be found at the base of bold escarpments under accumu- 

 lation of successive flows. Glassy forms present as marked a feature of 

 many of these earlier outpourings as they do of the more recent flows. 

 They were surface flows when ejected. They prove conclusively, on geo- 

 logical evidence, that similar physical conditions were identical from the 

 beginning to the close of the rhyolite phase of eruption. The liquidity of 

 the magma and its crystallization change from time to time, being de- 

 pendent on varying causes, such as the degree of temperature when 

 ejected from the point of discharge, the volume of the mass, and the 

 power of the lava to hold its contained heat. 



Banded and laminated lavas, contact surfaces between magmas of dif- 

 ferent physical properties, shrinkage cracks and jointings in obsidian and 

 perlite, overlapping of lava flows, all caused numerous capillary spaces in 

 the rock. Some of these openings for short distances lie parallel with the 

 lava flows ; others are vertical along planes of jointing, while still others 

 indicate great irregularity, broadening and contracting along a circuitous 

 course. Uniting below the surface they develop into wider channels, 

 affording free circulation of either descending or ascending waters. 



• J. p. Iddings : Geology of the Yellowstone National Park. Monograph XXXII, part 

 n, p. 425. 



