THE TRAVELS OF THE UNDERGROUND WATER 191 



pumping to raise it from some lower level. Ideal conditions are 

 furnished where the geological structure of the district is that of a 

 broad basin or syncline. The water which falls in a neighboring 

 upland is here impounded between two parallel, saucer-like walls 

 and will flow under its head if the upper wall be perforated at 

 some low level (Fig. 201, 3). 



FIG. 201. Schematic diagrams to illustrate the different types of artesian wells, 

 (1) A non-flowing well ; (2) flowing wells without basin structure caused by 

 clogging of the pervious formation ; (3) flowing wells in an artesian basin. The 

 dotted lines are the water levels within the pervious layers (after Chamberlin). 



A monoclinal structure may furnish artesian conditions when 

 the generally pervious layer has become clogged at a low level so 

 as to hold back the water (Fig. 248, 2). Pumping wells may be 

 used successfully even when such clogging does not exist, for the 

 slow-moving underground water flows readily in the direction of 

 all free outlets (Fig. 201, 1). 



Hot springs and geysers. Thermal springs whose temperature 

 approaches the boiling point of water are known as hot springs. 

 A geyser is a hot spring which intermittently ejects a column of 

 water and steam. Both hot springs and geysers are to be found 

 only in volcanic regions, and appear to be connected with uncooled 

 masses of siliceous lava. In two of the three known geyser regions, 

 Iceland and New Zealand, the volcanoes of the neighborhood are 

 still active, and the lavas of the Yellowstone National Park date 

 from the quite recent geological period which immediately pre- 

 ceded the so-called " Ice Age." 



Wherever found, geysers are in the low levels along lines of drain- 



