DALY. — THE NATURE OF VOLCANIC ACTION. 63 



batholith's roof has sunk along peripheral faults. As the sinking pro- 

 gressed, the luagiua was squeezed up, following the faults on nearly 

 every side of the sunken area. If the faulting had progressed still 

 further, it seems inevitable that foundering would have occurred. In 

 fact, the authors give a synthetic diagram which imjtlies foundering on 

 an enormous scale, though in no instance is the sinking crust-block 

 shown as having extended to the earth's surface.** 



Yellowstone Xational Pai-k. 



The possibility that foundering has played a part in comparatively 

 recent vulcanism, first became clear to the writer on a journey in the 

 Yellowstone Park. (See Figure 8.) The great rhyolite plateiiu, the 

 largest known on the earth, is cut by canyons reaching 600 meters in 

 depth. The canyon- walls of the Madison and Bechler rivers show 

 that, for nearly or (juite this depth and for very great areas, the rhyo- 

 lite is massive and is not divisible into a number of distinct Hows, as is 

 characteristic of fissure eruptions and central eruptions. The rhyulite 

 extends downward, below the river-levels, to unknown depths. 2° Does 

 it merge, directly beneath, into the true granite of a batholith ? 



Part I of the government survey Monograph is, unfortunately, not 

 yet published, and Part II states the field relations of the rhyolite in 

 too little detail that one can now assemble the facts bearing on this 

 problem. A few points may be noted to indicate the general need of 

 including foundering among the multiple hypotheses relative to the 

 mode of extrusion represented in the rhyolite plateau. 



1. Few geologists will doubt that this lava is the effusive equivalent 

 of a salic granite. The enormous volume of the rhyolite implies a 

 plutonic feeder of batholithic proportions, especially if we grant that all 

 Tertiary granite (or rhyolite) is a direct or indirect product of assimi- 

 lation. The solution of enough of the jire-Cambrian terrane and of the 

 locally thin, overlying sediments to yield, after differentiation, these 

 thousands of cubic kilometers of rhy<jlite, could oidy tiike place in a 

 very large plutonic chamber. General and independent theory sug- 

 gests, therefore, that a portion of the Yellowstone Park area is under- 

 lain by a granite batholith, which, like the rhyolite, is of Pliocene age, 

 and is one of the youngest bathoHths on record. 



2. The scale of the extrusion, \is topography, and topographic rela- 



" C. T. Clough, H. B. Maufc, and E. B. Bailey, Quart. Joum. Geol. Soc, 

 66. f,70 '1000). 



" J. I'. Iddinga, Monograj)!! 32, Tart 2, U. S. Gcul. Sur\-cy, 1899, pp. .'JW. 

 and 375. 



