330 PRACTICAL LESSONS IN SCIENCE. 



to a depth of more than six or eight miles. Some think the crush- 

 ing of the rocks under the great horizontal pressure develops 

 heat enough to fuse the rocks and produce eruption. Others 

 ascribe the molten matter to chemical action between the unoxi- 

 dized interior and oxidizing agents, as air and water, from the 

 crust ; and again the action must be within reach of surface water. 

 The source must be local, as volcanoes near each other yield dif- 

 ferent kinds of material, and sometimes with lava standing in 

 their craters at widely different levels. And it frequently happens 

 that the same volcano yields different material at different times. 

 We know many things about the effects of earthquakes and vol- 

 canoes but very little about their causes. 



Rocks and lava cool slowly, thick masses retaining high tem- 

 perature for many years. It is said that cigars and sticks might 

 be lighted at fissures in a lava stream from Jorullo, Mexico, 21 

 years after the out-flow. Water percolating through such ma- 

 terial appears again as hot springs, and often with dissolved 

 gases and minerals, as, sulphur springs, iron springs, etc., and 

 doubtless the geysers of Iceland and Yellowstone Park are in 

 a sense secondary volcanic phenomena. 



A geyser appears to be a spring or pool of hot water, but when 

 its depth is measured it is found to be more like a funnel-shaped 

 well of hot water. At somewhat regular intervals eruptions of 

 water and steam occur. The phenomena of an eruption are about 

 as follows : Sounds like cannonading are heard below and bubbles 

 rise through the water, then the water rises a little, overflowing 

 the basin, and immediately afterward the water is thrown out as 

 from a fountain, followed by a great quantity of steam. It is 

 explained by supposing that water toward the bottom of the 

 tube is con verted into steam which throws out the water above it. 



