THE KATMAI REGION, ALASKA 581 



tinued into a stage not represented elsewhere, by which a plug of 

 viscous lava was extruded. 



Views of this sand-flow or sand-avalanche and of some of the 

 effects produced are shown in Figures 8-10. 



Considering further the origin of the sand-flow, we suppose that 

 rhyolitic magma, charged with dissolved gases, rose to the surface 

 in the newly formed vents. According to general observation 

 the usual course for such a magma is either to retain its gases and 

 form a flow of obsidian, or to evolve them with explosive violence 

 and scatter the disrupted particles to a great distance. In this 



Fig. 10. — Trees prostrated as if by wind accompanying baiid-iluw, though 

 beyond reach of the avalanche of sand itself. Photograph b}^ C. N. Fenner, 1919. 



instance, however, it apparently pursued an intermediate course, 

 and produced, by moderately forcible disruption, an outward- 

 spreading and forward-moving torrent of incandescent sand and 

 pumice, each particle of which was surrounded by and partially 

 suspended in gases which it continued to give forth during its 

 impetuous flow. 



An artificial reproduction of the properties that are believed 

 to have characterized this ashy material at the time of its extrusion 

 may be obtained by igniting the powder of basic magnesium car- 

 bonate. The substance boils in a manner extraordinarily like a 

 liquid, and the gases evolved buoy up the solid particles. In this 



