50 



the region of 'the National Park is the vast amount of igneous rock that 

 has been poured out in the form of molten lava over the surface in all 

 directions. The whole upper mass of the mountains to the east of the 

 Yellowstone is composed of this material, thousands of feet in thickness. 

 The liver cuts its way through canons of it, and it extends to the west- 

 ward for hundreds of miles. These lava beds are known to be of com- 

 paratively modern date, because they are found resting on Tertiary 

 strata, which shows that they were ejected after the deposition of all 

 the underlying series. It is not unreasonable to suppose, therefore, 

 that at this point the great fissures in the earth's crust through 



which this lava escaped may still contain molten matter at no great 

 depth below the surface, and that the heat from this is transmitted up- 

 wards through the rocks sufficiently near to be reached by the percolat- 

 ing waters from above. 



The water. — It is one of the indispensable conditions to the existence of 

 an active geyser that the water shall contain silica in solution, and this 

 to the exclusion of more than a very limited amount of lime, soda, potash, 

 or magnesia, as the tube is to be prepared by silicious deposit. Analy- 

 ses of water and deposit from active geysers, wherever these have been 

 found, invariably show that the solid ingredient is silica with but a trace 

 of other minerals, and in order to dissolve silica the water must be 

 slightly alkaline in addition to the heat. The necessity for these nice 

 conditions may explain the great rarity of geysers; hot springs in which 

 they are not demanded are common enough. 



The formation of the tube. — The waters from the surface having found 

 their way down through the strata to the heated rocks below and having 

 received the requisite amount of alkali and silica, the process of build- 

 ing a .suitable tube through the deposition of silicia from the water 

 begins and may continue for years or centuries, during which time the 

 future geyser is merely a hot spring. The tube may be irregular, 



crooked, or branching, but it must be sealed up sufficiently at its lower 

 ramifications to give effect to the expansive power of the steam upon 

 the column of water to be raised. If too lung, the resistance of the column 

 would be greater than the force of steam could overcome, and, when too 

 short, the difference between the boiling points at top and bottom will 

 not be sufficient to form much steam when the pressure is relieved. If 

 the tube is too wide, circulation of the water up and down will equalize 

 the temperature, allow the steam to escape gradually, and spoil the 

 eruption. The boiling point at the place where the heat is applied 

 must be considerably higher than that at the outlet, and this condition 

 must be maintained in order that a large amount of steam may form 

 there at the moment when the crisis comes, and the greater this differ- 

 ence, within certain limits, the moreejective force the geyser will have. 

 If the outlet be proportionately wide the jet will be lower, and rice versa, 

 but when by silicious deposit it becomes too contracted, the geyser may 

 cease to be eruptive, or force a new one. 





i 



