T. A. Jaggar — Conditions affecting Geyser Eruption. 329 



It should be noted that exactly the reverse of Bunsen's predic- 

 tion takes place in our apparatus; an overflowing spring, when 

 confined by building up a basin about it (see page 327), finally 

 ceases to overflow : convectional currents are thus checked, the 

 boiling point is reached below and eruption begins. 



On this hypothesis it will be seen that the source of heat is 

 conceived to be fairly constant in the geyser region for like 

 depths, while the variations in the springs are dependent 

 wholly on delicately-balanced hydrostatic conditions ; this 

 seems consistent with the facts. A glance at the published 

 atlas sheets (U. S. Geological Survey) of the Yellowstone Park 

 will show that the three principal geyser basins are on drainage 

 lines, and represent the sources of the Gibbon and Firehole 

 Rivers, which unite- to form the Madison. These basins are 

 slight depressions in the rhyolite plateau, in a region of maxi- 

 mal rainfall " quite unlike that found in the adjacent country, 

 as is shown by the meteorological records."* On the opposite 

 side of the Continental Divide to the south, and of the Cen- 

 tral Plateau on the east, the headwaters of the Snake and 

 Yellowstone Pivers are represented by large lakes. No such 

 lakes exist on the Madison side, the fissured rhyolite there 

 holding its waters in hundreds of subterranean conduits, like a 

 huge hot sponge, and this water overflows in the depressed 

 areas in the form of sinter-building springs and geysers. The 

 Norris and Upper Geyser Basins are about 7400 feet, the 

 Lower Basin about 7200 feet above mean sea level ; while the 

 higher points on the divide rise to 8500 feet, affording drainage 

 into these basins of 1100 feet fall. It is further significant that 

 the highest of the geyser-basins is more than three hundred 

 feet below the level of that enormous water-surface, the Yel- 

 lowstone Lake, where hot springs still abound, and only a few 

 miles of uninterrupted rhyolite sheets stretch between the 

 geyser-basins and the lake. 



With a fairly constant source of heat, and springs variously 

 overflowing or confined according to the height of their orifices 

 relative to the local source in each case, it would follow from 

 the experiments that continuously overflowing geysers should 

 exhibit greater irregularity in their eruptive periods and inter- 

 vals than those so confined within their tubes that no discharge 

 is possible except by incessant eruption. This is borne out in 

 the types mentioned : Excelsior, the Great Geyser of Iceland, 

 and Potomahana are all known to have been quite irregular : 

 while Old Faithful, the Constant, the Minute-man and the 

 little Model, in the Yellowstone district, are all of the confined 

 type and erupt with great regularity. Excelsior, now recog- 

 nized as the greatest geyser in the world, was formerly called 



■■ Hague, 1. c. 



