118 A. HAGUE THERMAL WATERS IN YELLOWSTONE PARK 



gases may be contained in surface waters. I think it has been shown that 

 under the peculiar conditions in which these waters occur and their lack 

 of uniformity of composition they must be considered as absorbed by 

 vadose waters. 



One of the most marked characteristics and one of geological signifi- 

 cance is the frequent variation shown in temperature, flow, and salinity 

 of the thermal waters where they issue from the rhyolite plateau. The 

 solvent power of water holding mineral matter in solution is, as is well 

 known, far greater than that of pure water. Now the downward perco- 

 lating waters gather material from the disintegrated rhyolite soil and in 

 some measure from the soluble salts previously brought up from below. 

 There is also a certain amount of carbon dioxide derived from the atmos- 

 phere. It is a fair assumption, therefore, that in percolating downward 

 the waters carry with them to the water level below no inconsiderable 

 amount of material. 



The ascending superheated waters, under pressure, exert a far greater 

 influence. The work done by these waters, and that which is still going 

 on, is self-evident even to the most casual tourist. It is shown by the 

 broad areas and ridges of altered and bleached rhyolite. Nowhere is this 

 more in evidence than in the escarpments along the Grand Canyon of the 

 Yellowstone, where the gorgeous coloring is due to the oxidation of the 

 ferruginous minerals. The potent influence of such waters under exist- 

 ing conditions can hardly be questioned. They readily attack both the 

 glassy groundmass and crystalline feldspars of the rhyolite, and when 

 the metasomatic changes are complete they leave behind an impure kaolin- 

 ized material mixed with quartz and held together by colloidal silica. 



Development op Springs and Geysers 



The ascending waters, in their circuitous course, penetrate fresh seams 

 and cracks in unaltered rock,- which slowly widen under the disintegrat- 

 ing influences of aqueous vapors. Finally the thermal waters, following 

 these cracks, issue at the surface as hot springs and pools. The early 

 waters are usually acid in composition and deposit ferric and aluminous 

 salts. Occasionally they set free sulphur, derived from the decomposition 

 of hydrogen sulphide. In time the openings through which they flow 

 become broader, the waters themselves, free from hydrogen sulphide, 

 become clearer and neutral, and at last issue as siliceous alkaline waters. 

 Underground reservoirs are excavated and become sources of hot springs 

 and, under favorable conditions, ge^^sers. The geyser itself is simply a 

 stage in the development of geological processes. In time geysers them- 

 selves become extinct. New geysers break out and, given the essential 

 physical conditions, may develop eruptions quite as fine as any in action 



