146 GEOLOGICAL HISTORY OF THE YELLOWSTONE PARK. 



the building-up process was under the most favorable conditions; yet, 

 in a. number of instances, I can see no appreciable change in three or 

 four years. Ile-visiting hot springs in out-of-the-way places after 

 several years' absence, I am surprised to see that objects that I had 

 noted carefully at the time remain unchanged. Taking the entire 

 area of the Ui)per Geyser Basin covered by sinter, I believe that the 

 development of the deposit does not exceed one-thirtieth of an inch a 

 year, and this estimate I believe to be much nearer the maximum than 

 the minimum rate of growth. The thickness of the geyserite has 

 never been ascertained; the greatest thickness measured is 70 feet, 

 the depth reached in the conduit of Old Faithful geyser, without 

 meeting any obstruction. Supposing the deposit around the Castle 

 geyser to have been built up with the same slowness as observed 

 to-day, and assuming it to grow at the rate given — one-thirtieth of 

 an inch a year--it would re(|uire over twenty-flve thousand years to 

 reach its i)resent development. This gives us a great antiquity for the 

 geyserite, but I believe that the deposition of the siliceous sinter in 

 the Park has been goi/ig on for a still longer period of time. It is 

 certain that the decomposition of the rhyolite of the plateau dates 

 still further back. 



From a geological j)oint of view, there is abundant evidence that ther- 

 mal energy is gradually becoming extinct. Tourists re-visiting the Park 

 after an absence of two or three years occasionally allude to the springs 

 and geysers as being less active than formerly and as showing indica- 

 tions of rapidly dying out. It is true that slight changes are con- 

 stantly taking place, that certain springs become extinct or discharge 

 less water, but this action is fully counterbalanced by increased activity 

 in other localities. Close examination of the source of the thermal 

 waters fails to detect any diminution in the supply. Moreover, it stands 

 to reason that if the flow of these waters dated — geologically sY)eaking — 

 far back into the i)ast, the few years embraced within the historical 

 records of the Park would be unable to indicate any i)erceptible change 

 based upon a gradual diminution of the heat.* Accurate descriptions 

 of the region go back only to 1871, the year of the first exploration by 

 Dr. F. V. Hay den. 



The number of geysers, hot springs, mud-i3ots, and paint-pots scat- 

 tered over the Park exceeds o,.")00, and if to these be added the fumaroles 

 and solfataras from which issue in the aggregate enormous volnmes of 

 steam and acid and sulphur vapors, the number of active vents would 

 in all in-obability be doubled. Each one of these vents is a center of 

 decomposition of the acid lavas. In the four principal geyser basins the 

 geysers in action — or known to have been active within the past sixteen 

 years — nnmbered 84. The following list comprises all the geysers that 

 are known in the Norris, Lower, Midway, and Upper Geyser Basins. 



