VULCANISM 177 



Mt. Rainier is another splendid mountain built up by a former 

 volcano. Various features of the mountain show that a second 

 period of activity followed a long period of quiet in the history of 



Fig. 175. The summit of Mount Rainier, Washington. 



this snow-capped mountain. Hot vapors still issue from some 

 small vents in the mountain, though the discharge of lava ceased 

 long ago. The mountain is snow-covered, and has several glaciers. 



The Marysville buttes. This circular cluster of hills (Fig 176), 

 10 miles in diameter, rises 1,700 to 2,000 feet above the level of the 

 Sacramento River in California. The buttes are composed of lava 

 with an outer layer of fragmental material (or tuff). The volcanic 

 cone, which probably once rivaled Vesuvius, has been dissected 

 into a group of hills with jagged and fantastic outlines. 



Volcanic necks. When a volcano becomes extinct, the throat, 

 or passage from the interior, may be filled with hardened lava. 

 This may be of rock much more resistant than the rest of the cone, 

 and as the cone is worn away, the plug, transformed into a hill, may 

 still mark the site of the former volcano. These volcanic necks, 

 or plugs, are sometimes conspicuous. East of the Mt. Taylor 

 plateau, in central New Mexico, a number of them rise by precipi- 

 tous slopes 800 to 1,500 feet above their surroundings. Massive 

 intrusions of lava may have a similar effect (Fig. 1, PL XXI, p. 84). 



