138 DYNAMICAL GEOLOGY. 



like pitch, and therefore runs with less and less speed, 

 until it becomes rigid and stops. Being a bad conductor 

 of heat, lava cools and forms a crust on the surface while 

 it is still liquid and flowing within. The liquid finally 

 flowing out, often leaves a hollow tube or gallery. Again, 

 since all lava contains more or less of gas and vapor, the 

 crust is a sort of concreted lava-foam. This vesicular, 

 spongy lava is called scoria. Sometimes, in very stiffly 

 viscous lava, the vapor-bubbles run together and form 

 huge blisters, which, by hardening, form caves. Thus, 

 nearly all lava-beds are full of galleries and caves. It 

 was in the galleries and caves which honey-combed the 

 ancient lava-flows of southern Oregon that a handful of 

 Modocs defied so long the power of the United States 

 Army. 



Again, the liquidity of lava, and its appearance after 

 solidifying, depend much upon the kind of fusion. Lavas 

 are often in a state of hydr other mal fusion (page 133), 

 i. e., half fusion, half solution in superheated water. Such 

 a semi-fused mass, on concreting, makes a kind of earthy 

 stone. Sometimes, in fact, the ejecta are little more than 

 hot mud, and concrete into tufa. 



Cinders, Sand, and Ashes are only different forms 

 of hardened lava. The liquid lava, before ejection, may 

 be so largely mingled with gas and vapors that it is liter- 

 ally a rock-foam. Masses of this rock-foam, ejected with 

 violence into the air, cool and fall as cinders. Often 

 the greater part of the ejections is of this kind, and thus 

 are formed cinder-cones. Sometimes the violence of the 

 explosions is so great as to break up the liquid mass into 

 rock-spray. This falls again as sand or ashes., according 

 to its fineness ; or else the rock-fragments and cinders 

 are thrown up, and, falling again repeatedly, may be 

 triturated into dust or ashes. The finest rock-dust hang- 

 ing in the air is called smoke; the same, fallen to the 

 earth, ashes. Volcanic ashes, wet with water and con- 



