GROUND-WATER 35 



matter which is conspicuous either because of its color, odor, or 

 taste. Many mineral springs are thought and sometimes rightly 

 - to have healing properties, and so are known as medicinal springs. 

 Many of the famous watering-places and resorts for invalids are 

 at hot mineral springs. The Hot Springs of Arkansas, of South 

 Dakota, and of Carlsbad (Bohemia) are examples. Many springs 

 which are charged with gases are called mineral and medicinal, even 

 though their waters are worthless for healing purposes. 



Geysers. In some parts of the world, the water of hot springs 

 is forced out violently from time to time. Such springs are called 

 geysers (Fig. 1, PI. XI, p. 32). There are more than 100 geysers in 

 the Yellowstone National Park, and there are geysers in Iceland 

 and in New Zealand. All geysers are in regions of recent volcanic 

 activity. Some geysers send up boiling water and steam to a height 

 of 200 feet or more, though this is quite above the average. Some 

 geysers erupt frequently, and others infrequently. The eruptions 

 of some occur at regular intervals, and those of others do not. One 

 of the geysers in the Yellowstone Park is named "Old Faithful/ 7 

 because it discharges its waters at nearly regular periods of about 

 an hour. 



The features which may be seen generally at a geyser are the 

 following: (1) An opening leading down to unknown depths. This 

 is called the geyser tube. (2) A shallow basin about the opening, 

 often filled with water. The basin is sometimes, though not always, 

 in the top of a mound. In some cases there is an irregular mound 

 perforated by openings, instead of a basin about the top of a tube 

 (Fig. 2, PL XI). Both basins and mounds are composed of mineral 

 matter (commonly silica) which has been deposited by the water 

 which has issued from the geyser. (3) At the time of discharge, 

 much steam as well as liquid water issues. 



The eruption. It seems certain that it is steam which ejects 

 the water from a geyser. It is believed (1) that ground-water enters 

 the geyser tube much as it enters a well ; (2) that the walls of some 

 part of the tube are hot; (3) that the water in the tube is brought 

 to the boiling temperature at some point in the tube below the top of 

 the water', and (4) that when this takes place, the water which is 

 converted into vapor forces out all the water above. 



