XII.] EARTHQUAKES AND VOLCANOES. 193 



when the matter is in a pasty condition ; just as the porous 

 texture of a piece of bread is due to the presence of 

 bubbles of gas evolved by the fermentation of the yeast. 

 The stone largely used for scouring paint under the name of 

 pumice^ is a lava of very porous texture ; its name recalling 

 its origin as the froth or scum of lava. Sometimes, the 

 masses of lava, which are tossed into the air, are rotated 

 during their flight, and fall as more or less rounded bodies, 

 known as volcanic bombs. Occasionally a very liquid lava may 

 be caught by the wind, and drawn out into delicate fibres, like 

 spun glass ; this beautiful form is very abundant at Kilauea, 

 a volcano in Hawaii, one of the Sandwich islands, where it 

 is known as Pete's hair, its name being borrowed from that 

 of an old goddess who was supposed to reside in the crater. 

 Other lavas again are vitreous, and strongly resemble dark- 

 coloured bottle-glass, when they pass under the name of 

 obsidian. This kind of lava was largely used by the ancient 

 Mexicans for making rude knives and other cutting instru- 

 ments ; and a hill in northern Mexico, formerly worked for 

 this material, is still known as the Cerro de Navajas (Spanish 

 '■• Hill of Knives.") 



It often happens that the lava which wells up in the pipe 

 of a volcano, breaks by its sheer weight through the rim of 

 the crater, or even breaches one side of the conical hill. 

 Thus Fig. 52 represents a group of small extinct volcanoes 

 in Central France, showing cones which have been broken 

 through in this way. In some cases the flanks of the cones 

 are rent, and lava is then injected into the cracks, forming, 

 when cold, huge rocky ribs known as dyi^es. In other cases, 

 the chimney gets choked by a plug of hard lava, and new 

 vents may then be opened on the side of the cone. Fig. 53 



1 Pumice, from the Lat. puviex, formerly spnnex, allied to spiana, 

 " froth." 



