328 T. A. Jaggar — Conditions affecting Geyser Eruption. 



off the superficial waters that accumulate from the abimdaut 

 rainfall of the Yellowstone Plateau. The " Excelsior " caul- 

 dron is stated by Hague* to discharge constantly into the 

 Firehole Kiver 4400 gallons of boiling water per minute, " and 

 there is no evidence that this amount has varied within the 

 last two or three years (1887)." Weedf has estimated, on the 

 moderate assumption that one third of the eruption-column of 

 Old Faithful is water, that 3000 barrels are thrown off at each 

 eruption. Here we have examples of continuous and spas- 

 modic drainage methods, both sending their waters eventually 

 to the Madison Kiver, and resupplied from a local source. 



The geyser basins are topographic hollows, which supply 

 vents for the meteoric waters accumulated in fissures of the 

 decomposed rhyolite. These waters are heated by vapors 

 escaping from the only partially cooled deeper lavas, and are 

 escaping in the form of springs and geysers. In the springs 

 the overflow is occasioned by hydrostatic pressure : in the gey- 

 sers it is permitted by occasional violent discharge. The tran- 

 sition from one phase to the other may readily be induced, as 

 shown in Experiment 2, by very slight changes in the hydro- 

 static pressure, i. e. variations in the mean level of ground- 

 water (Grundwasserspiegel), or in the local head for any specific 

 case. The head may be modified at either the source (supply- 

 reservoir") or the orifice of exit ; head is diminished by lower- 

 ing the reservoir through formation of new outlets or through 

 decreased supply, or by building up a cone around the geyser 

 tube. Conversely head may be increased by excessive supply 

 (rainfall) at the reservoir, by clogging of outlets, or by the 

 water finding a new vent at a lower level. 



Bunsen stated that the transition from a geyser to a tran- 

 quil "Laug" like those frequently found in Iceland, would be 

 occasioned by building up the geyser's bowl so as to raise the 

 water-level, and thus " the water in the depths below, owing to 

 the increased pressure, cannot attain its boiling point, and the 

 eruptions of necessity cease.":); While this may be true in 

 special cases, we are inclined to believe that the efficient cause 

 of cessation of eruption is generally to be found in an increase 

 in the amount of continuous overflow, due either to new out- 

 lets (which may in many cases be subterranean), or increase in 

 the local head. Probably many of the geysers whose eruption 

 intervals are irregular, are in a condition of equilibrium near 

 the critical point described in Experiment 2, so that very slight 

 changes in the local pressure bring about eruption or cessation. 



* Arnold Hague: Geol. History of the Yellowstone Nat'J Park, Trans. Am. 

 Inst, of Mm. Eng., vol. xvi, 1888. 

 fl. c. 

 \ Tyndall. 1. c. 



