

IGNEOUS AGENCIES. 147 



Great Geyser. This is a low mound,, with a basin- 

 shaped depression at top, from the bottofti of which de- 

 scends a tube or well to unknown depth, but may be 

 sounded to eighty feet or more. The basin is fifty feet 

 across, and the tube or throat ten feet in diameter at the 

 top but narrowing downward. Both the basin and the 

 throat are lined with silica deposited from the water, and 

 doubtless the mound itself was built up by similar deposits. 

 In the intervals between eruptions the basin is filled to 

 near the brim with water at 180. 



Phenomena of an Eruption. As the time for the 

 eruption approaches, the first thing observed is a series of 

 explosions in the bottom of the throat like subterranean 

 cannonading ; then bubbles of vapor are seen to rise and 

 burst on the surface ; then the water of the surface bulges 

 up and overflows. Immediately thereafter the whole of 

 the water in the throat and basin is ejected with violence 

 one hundred feet into the air, forming a fountain of daz- 

 zling splendor, followed by the roaring escape of steam. 

 As the water falls back, it is again ejected, and the foun- 

 tain continues to play several minutes until the steam has 

 all escaped and the water partly cooled ; then all is quiet 

 again until another eruption. The interval between erup- 

 tions is irregular. An eruption may be brought on pre- 

 maturely by throwing large stones down the throat of the 

 geyser. 



Yellowstone Geysers. But in splendor of eruption 

 the Icelandic geysers are far surpassed by those of Yellow- 

 stone Park. This, like Iceland, is a volcanic region, but, 

 unlike Iceland, primary volcanic phenomena are all ex- 

 tinct. The geyser phenomena here occur in a narrow 

 valley surrounded on all sides with volcanic rocks of great 

 thickness, of comparatively recent origin, and doubtless, 

 therefore, still hot in their interior. In this little valley 

 there are no less than 10,000 vents of all kinds, hot springs, 

 boiling springs, mud-volcanoes, lime-depositing springs, 



