VULCANISM 173 



of extinct volcanoes 500 to 800 miles from the sea in Arizona, 

 Colorado, and elsewhere. It cannot be said, therefore, that near- 

 ness to the sea or mountain ridges are conditions necessary for 

 volcanoes. Many of the active volcanoes lie near the line where the 

 continental plateau descends to the oceanic basins. This is per- 

 haps the most significant feature of their distribution. Volcanoes 

 on land are, in many cases, associated with lands which have been 

 recently raised or sunk. 



Historical. Volcanoes have existed throughout the history 

 of the earth, so far as this history is now known, even back to the 

 earliest ages; but volcanoes do not seem to have been equally 

 active at all times. There have been periods of great volcanic 

 activity, alternating with periods of less activity. There is no 

 knowledge, however, that vulcanism ever ceased altogether. 



After the volcanoes of a region die out, associated phenomena 

 are sometimes continued. Thus in the Yellowstone National Park, 

 where volcanoes once existed, there are numerous geysers, hot 

 springs, and other vents out of whi'ch hot vapors issue. 



Topographic Effects of Volcanoes 



By the making of cones, volcanoes become an important factor 

 in shaping the surface of the lithosphere. The early stages of 

 growth have sometimes been observed. Thus in 1538 a small 

 volcano appeared on the north shore of the Bay of Naples, and built 

 up a cone 440 feet high and half a mile in diameter at its base in a 

 few days. Its crater was more than 400 feet deep. 



In 1770 the volcano Izalco in Central America broke out in the 

 midst of a plain which was then a cattle-ranch. Since that time 

 it has built up a cone about 3,000 feet high, with steep slopes. In 

 the earlier part of its history, lava-flows were frequent, but for many 

 years no lava has flowed out, though the volcano has remained 

 active, discharging explosively. 



Early in the last century a volcanic island (Graham Island) 

 arose in the Mediterranean, between Sicily and Africa, where the 

 water had been 800 feet deep. In 1831 a ship near the place felt 

 earthquake shocks. In July a sea captain reported that he saw 

 a column of water 60 feet high and 800 yards in diameter rising 



