836 ELEMENTARY GEOLOGY 
instead of a lava flow, we have an eruption of true 
rock ash. This may be blown high into the air; and 
in the case of the smaller particles, even to elevations 
of several miles above the surface (Fig. 203). Much of 
it, particularly the heavier portion, falls back to the 
earth near the crater, the place of ejection. Since 
Fic. 203. 
Photograph of eruption of Vesuvius in 1872, showing the steam and ash 
rising high in the air. 
more falls here than at any other point, this gradual 
accumulation causes the mountain to rise in the form 
of a cone, built around the point of outlet ; but much of 
the ash, particularly the light dust, drifts away in the 
air, fallmg miles from the place of ejection, possibly 
after being suspended in the air for months, or even 
years. 
