ic,o6.l SEE— THE CAUSE OF EARTHQUAKES. 373 



siderable. Besides the honeycombed pumice which underHes vol- 

 canoes, it may be supposed that they frequently contain a certain 

 number of passages out of which the pumice has been blown, some 

 of which may become real caverns when the lava subsides after an 

 eruption. When a new eruption begins these old passages might 

 offer little resistance till the lava column came within a short dis- 

 tance of the surface, and hence the shallowness of the shocks wit- 

 nessed in eruptions. The shallowness of these shocks does not 

 prove the superficial character of the lava erupted; on the contrary 

 the earthquake shocks felt over the whole region around every 

 active volcano shows that the subterranean disturbances arise in a 

 layer which acts as fluid just beneath the crust. The forces de- 

 veloping in this layer find their relief in the eruption of steam and 

 lava from the volcano. 



§ 44. The origin of volcanic ashes due principally to the break- 

 ing and grinding up of pumice. 



In his useful w^ork on the " Volcanoes of North America," the 

 late Professor Russell, of the University of Michigan, makes the 

 following explanation of the origin of volcanic ashes (pp. 75-76) : 



"Sheets of Volcanic Sand and Dust. — In the case of volcanic eruptions 

 of the explosive type, the steam occluded in the lava expands as external pres- 

 sure is relieved; this expansion is frequently so violent that the rock is dis- 

 integrated and the fragments projected high in the air. Besides this priniary 

 mode of reducing the lava to fragments, and much of it to the condition of 

 dust, the larger fragments as they are shot upwards with a velocity in some 

 instances even greater than the initial velocity of shells fired from modern 

 rifle-cannon, strike against one another and against falling fragments, and 

 are shattered, thus tending to increase the quantity of fine dust-like particles 

 produced. While much fine material originates thus, and is carried away 

 by the wind, many of the fragments that escape comminution fall into the 

 the crater from which they were thrown and are again violently ejected, 

 thus multiplying the chances of their being reduced to powder. An erup- 

 tion of the explosive type thus tends to form much fine dust, which is car- 

 ried high into the air by the upward rushing steam and falls most abundantly 

 near the place of discharge. Should a strong wind be blowing, the dust is 

 carried to leeward of the volcano, and on reaching the earth forms a sheet, 

 which, owing to the winnowing action of the wind, is composed of finer and 

 finer fragments, the greater the distance from the volcano." 



Professor Russell appreciated more fully than many geologists 

 the necessity of explaining the enormous clouds of dust which 

 arise from volcanoes, but it is difficult to escape the impre'ssion 



