DALY. — THE NATURE OF VOLCANIC ACTION. G5 



tions, and the relative insignificance of pjToclastic phases, are all 

 opposed to the view that the rhyolite issued from one or more vents of 

 the ordinary central-eruption kind. The unprecedented thickness of 

 the lava in a single, extended mass — more than G< )0 meters thick for 

 hundreds, if not thousands, of Sfjuare kilometers together — is tiuite 

 unparalleled in masses known to have been emitted during true hssure 

 eruption. Moreover, Iddings points out that rhyolite dikes (feeding 

 fissures) are almost entirely lacking in the Park. 21 The superposition 

 of distinct rhyolite flows in the regions surrounding the main rhyolite 

 body may be explained as due to successive outHows from the area of 

 foundering. Thus, so far as the facts of the field declare against the 

 central-eruption and fissure-eruption h^'potheses, they appear to favor 

 some other explanation. As yet the writer has conceived uo other 

 except that of foundering. 



3. The geysers of the Park are practically confined to the area 

 covereil by the Pliocene rhyolite. In a similar way the other two great 

 geyser-fields of the world, in Iceland and New Zealand, are located in 

 regions where magma of liparitic (granitic) composition has been 

 poured out at the earth's surface. The " fossil " geysers, described 

 by Freeh in the Kremnitz district of Hungary, are confined to areas of 

 highly acid lavas.22 Following the general argument outlined above, 

 the liparitic lava in each of these four districts has been differentiated 

 from one or more relatively and absolutely large magma chambers, 

 i. e., in abyssal injections big enough to assimilate acid rocks in 

 notable amount. 



According to Holmes, the Yellowstone geysers were in operation long 

 before the Pleistocene period and have been active ever since.^^ By an 

 ingenious method Schlundt and Moore have calculated that about 

 '2(),(H)() years have elapsed since the Glacial period in the Park. 2* This 

 figure agrees well with other recent estimates of post-Glacial time. 

 Twenty thousand years is clearly much less than the total tiuie during 

 which the geyser activity has been sustained in the Park. Competent 

 observers agree that the geyser waters are wholly or chiefly of meteoric 

 origin. The thermal energy required to superheat this cold water to 

 explosion-points during so many millenniums must be enormous. 

 Th(jugh the spring waters are radioactive, the heat set free by radio- 

 activity is not to be considered as of any direct importance in the 



»» MonoKraph :}2, Part 2, U. S. Gcol. Survey, 1S*)9, p. 3S1. 

 " V. Freeh, .\u3 der Natur, Vol. 1, Heft S, lOO.'). 



»» W. H. Holmes, 12th Ann. Report, U. S. Geol. and Goor. Survey of the 

 Territories, Part 2, ls.s:{, p. 2i). 



" H. Schlundt and II. B. Moore, IJull. 305, U. 8. Gcol. Survey, lOOl), p. 31. 



VOL. XLVII. — 5 



