XII.] EARTHQUAKES AND VOLCANOES. tgi 



be an illusion, due to illumination of the vapours, partly by 

 the sparks and red-hot stones and ashes shot out at the 

 same time, and partly, by reflection from the glowing walls 

 of the pipe and from the surface of the molten matter 

 below. In the early stages of an eruption, huge fragments 

 of rock may be ejected ; for when, after a period of repose, 

 the pent-up steam and gases at last gain vent, they violently 

 eject the materials which have accumulated in the throat of 

 the chimney, and choked its opening. Masses of rock, some 

 weighing as much as twenty tons, are said to have been cast 

 forth from Mount Ararat during the eruption of 1840, and 



Fig. 51. — Diagrammatic Section of a Cinder Cone. 



in other cases stones have been hurled to a distance of 

 more than thirty miles from a volcano. 



During an eruption, ashes are commonly ejected in great 

 quantity, but it must be borne in mind that the materials 

 so-called are very different from the partially-burnt fuel of 

 the domestic hearth. Volcanic ashes are, in fact, nothing 

 but fragments of lava, or partially-fused rocky matter. When 

 jets of this lava are shot forth from the volcano, the liquid is 

 broken up by the air, and so splashed about that it falls in 

 drops, which harden into small spongy fragments, resembling 

 ashes and cinders. In some cases, the lava is broken into 

 such fine particles that it is known as volcanic dust or sand ; 



