Apkil 14, 1911] 



SCIENCE 



559 



shone Lake, measuring 12 square miles, to 

 say nothing of other picturesque sheets of 

 water of less imposing dimensions, all of 

 which lie upon the rhyolite from 500 to 700 

 feet above the Upper Geyser Basin, where 

 the greatest number of large geysers is 

 found and the activity and ovei'flow of 

 thermal waters displayed on a grand scale. 

 In time, much of the water from the 

 meadows and ponds naturally finds its way 

 to surface streams. Another portion is 

 taken up by the luxuriant vegetation or is 

 absorbed by the atmosphere. The remain- 

 ing water, which constitutes a very con- 

 siderable volume, is drawn down through 

 openings into underground reservoirs. In 

 other words, these descending waters 

 slowly percolate through the viscous lavas. 



PHYSICAL STRUCTUEE OP EHTOLITES 



Returning for a few brief moments to 

 rhyolite flows, let us consider certain phys- 

 ical features due to textural modifications. 

 No region surpasses the Yellowstone Park 

 in the varied phenomena of highly acid ex- 

 trusions. This is especially true of the 

 more glassy types, and in general a glassy 

 ground mass characterizes most of these 

 lava sheets. Mr. J. P. Iddings has sub- 

 mitted a large series of specimens of the 

 park rhyolites to a searching petrograph- 

 ical investigation, making a special study 

 of the mierogranular structure, and the 

 relations of the different microstructures 

 to one another, and pointing out the ab- 

 rupt 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, 

 presumably of water, and suggests that the water 



thus irregularly disseminated 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 exerted by such textural 

 modifications in the creation of fissures, 

 fractures and capillary openings for the 

 percolation of waters. Obsidians, perlites, 

 and pitch-stones were poured out over the 

 greater part of the central plateau and may 

 be found at the base of bold escarpments, 

 under accumulation 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 geological 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 crystalliza- 

 tion changed from time to time, being de- 

 pendent upon 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 different 

 physical properties, shrinkage cracks and 

 jointings in obsidian and perlite, over- 

 lapping of lava flows, all caused numerous 

 narrow capillary spaces in the rock. Some 

 of these openings for short distances lie par- 

 allel 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 eireu- 



'J. P. ladings, "Geology of the Yellowstone 

 National Park," Mon. XXXII., Part II., p. 425. 



