VULCANISM 



371 



beds of ash, cinders, or other pyroclastic material. The inference 

 is, therefore, that the lavas welled out quietly, and flowed over 

 the surrounding country. For the most part these wide-spreading 

 flows are composed of basic material, which is more easily liquefied 

 and more fluent at a given temperature than the acidic lavas. The 

 latter are more disposed to form thick bodies near the point of 

 extrusion. 



Massive outflows of this class are the greatest examples of 

 extrusions, though they are not now the dominant type. It has 



Fig. 290. Lava-flows of the northwestern part of the United States. 



been thought that the volcanic type of extrusion followed the 

 fissure eruptions as a phase of decline; but this view has not been 

 substantiated. 



b. Volcanoes 



A volcano is a circumscribed vent in the earth's crust, out of 

 which hot rock, gases, and vapors issue. The material is generally 

 built up into mounds or cones (Figs. 291-293). These cones are 

 often called volcanoes, though they are really the results of vol- 

 canic activity. So long as a volcano is active there is likely to be 

 a depression, or crater (Fig. 294), in the summit of its cone. The 

 crater connects downward with the source of the lava at an un- 

 known depth. Craters may be a mile or more across, but they 

 are usually smaller, some much smaller. After sufficient erosion, 



