GEYSERS OF THE FIREHOLE RIVER. 195 



its mouth being on a level with the surface of the ground. The 

 Great Geyser only ejects water once in twenty or thirty hours, throw- 

 ing up a column 60 feet high, accompanied with clouds of vapour. 

 The Strokr may be caused to erupt at any time by artificially block- 

 ing its throat, and its outbursts have lasted for half an hour at a time. 

 But the phenomena change from time to time. In the middle of 

 the eighteenth century there were three or four eruptions in a day, 

 and some reached a height of 300 feet. Their activity is influ- 

 enced too by earthquakes ; and it is remarkable that no mention of 

 geysers is found in the old Icelandic writings. When the geyser 

 basin is full, the water is clear and in a state of ebullition, the 

 temperature being about the boiling point, but Bunsen found that in 

 the descending tube the temperature increased to 266 F. It would 

 hence seem that the water draining in from the neighbouring hills 

 has its temperature augmented by contact with heated rock, or water 

 coming from heated rock, until a portion of the mass is so far raised 

 in temperature that the water there, having boiled off the gases held 

 in solution, overcomes the pressure of the column above, and bursts 

 into vapour, so as to throw up the column above it in a fountain. 

 The fact that Bunsen was able by experiment to reproduce periodic 

 eruptions from a tube by heating its middle portion, goes far to 

 demonstrate the accuracy of his interpretation, though the details of 

 nature's mechanism beneath the surface are necessarily unknown. 



Geysers of the Yellowstone. All over the great lava district of 

 the far west of North America geysers are numerous, though many 

 are extinct. They are specially instructive, since they are often sur- 

 rounded by mud volcanoes, and associated with calcareous springs, 

 which have deposited limestone terraces, so that all the constituents 

 of water-formed rocks are here differentiated, and poured out from 

 neighbouring vents, though the materials are in some cases mixed. 

 On the Yellowstone Lake the geyser called " Fishpot" is so close to 

 the water that, without moving, the fisherman has caught trout in 

 the lake, and cooked them in the boiling geyser. The Grotto arid 

 Giant Geysers of the Yellowstone throw up columns of water from 

 one to two hundred feet high, and the eruptions last from one to two 

 hours. A mud geyser in this district, at Crater Hill, has a small 

 basin, 60 feet in diameter, formed chiefly of layers of clay and silica, 

 situate in a larger basin with a higher rim, which measures 200 feet 

 by 150 feet; and on one side of the outer basin is a ravine with 

 holes in the banks, which are lined with sulphur. The geyser basins 

 on the Firehole river are upwards of 7000 feet above the sea; they 

 are rarely conical, more frequently globular, and often have overhang- 

 ing ledges. The Grand Geyser throws up a column 6 feet in diameter 

 and 200 feet high. 1 The temperature of the water is usually below 

 the boiling point, and the water itself is green. Many mineral sub- 

 stances have been detected in these geyser ^yater3, and among others 

 small crystals of gold. 



1 See Professor F. V. Hayden, U.S. Geol. Survey of Montana, 1871, and 

 Montana, Idaho, Wyoming, and Utah, 1872. 



