242 The Realm of Nature CHAP. 



heated from below to begin to boil ( 72) ; this relieves the 

 pressure on the layers beneath, which flash into vapour in 

 a series of explosions, throwing up a column of water and 

 steam with a terrific roar. The geyser remains quiescent 

 until it fills up again, when the same process is repeated. 

 In the Yellowstone region of North America ( 364) the 

 Giantess Geyser throws up a stately column of steam and 

 water 250 feet high in each outburst, after which several 

 weeks of tranquillity elapse ; and " Old Faithful," throwing 

 a column of 150 feet, explodes with wonderful regularity 

 at intervals of about an hour. 



317. Caverns. Since the masses of tufa or sinter 

 formed round hot springs have been taken from the rocks 

 beneath, hollows or caverns must be left in the Earth's crust. 

 These are usually enlargements of the natural crack or fault 

 which allowed the spring to reach the surface. In limestone 

 regions caverns are very numerous and often of great size, 

 on account of the solvent action of rain-water charged with 

 carbonic and other acids on the joints and faults of the 

 strata. The roofs of caverns sometimes sink in, leaving a 

 funnel-shaped hollow on the surface called a sink or swallow- 

 hole, in which, if rubbish blocks up the outlet below, small 

 isolated lakes may form. Part of a cavern roof may 

 remain standing as a natural tunnel or bridge after the 

 debris of the fallen portion has been carried away by rivers. 

 Caverns are usually very picturesque on account of the 

 formation by the dripping water of fantastic stalactites, 

 white or tinted icicle-like appendages of carbonate of lime, 

 hanging from the roof. Where the water-drop falls from 

 the stalactite to the floor more carbonate of lime is de- 

 posited, and a stalagmite grows upward, and the two 

 ultimately form a natural pillar. Small stalactites formed 

 by the, percolation of rain-water through the mortar may be 

 seen hanging from the arches of bridges. The most 

 extensive limestone caverns are those of Adelsberg in 

 Austria, the Mammoth Cave in Kentucky (which comprises 

 more than 150 miles of passages), and the Jenolan Caves 

 in New South Wales. Some of these caverns contain 

 lakes tenanted by blind fish, and underground rivers 



