374 SEE— THE CAUSE OF EARTHQUAKES. [October 19, 



that his explanation is somewhat labored. It would be more natural 

 to say that a volcanic mountain is underlaid and filled with pumice 

 and when the explosions become violent some of this porous ma- 

 terial is ground up and blown out as dust. Much of it may already 

 be in the form of powder from former earthquake shocks when the 

 mountain was packed with pumice, and simply requires to be blown 

 out; and hence the vast clouds which obscure the sun and darken 

 the earth for hundreds of miles ! If the rock broken up by the 

 explosion were solid, as supposed by Russell, it would be less easy 

 to account for the enormous outpourings of ashes, observed in 

 such volcanoes as Hecla, ^tna, Vesuvius and Conseguina. Solid 

 rocks would produce lapillae and sand rather than fine ashes, which 

 result from the breaking up of pumice with very thin bubbles. 



When lava is forced up in the throat of a volcano some of it 

 may run out, relieving the pressure which raised it, and the rest 

 then sinks back into some of the passages which lead to the throat 

 of the volcano. There may be, and in general probably are, several 

 of these passages, unequally opened at different times, and the lava 

 is forced up from some of them. After the lava is poured out and 

 subsides, other passages formerly closed may be opened and eject 

 vast quantities of volcanic ashes without encountering any molten 

 rock whatever. It would be a great mistake to suppose that all 

 ashes which pour from a volcano are forced through a layer of 

 liquid lava before ejection. If the ashes were forced through a 

 layer of liquid they would be red hot when cast out, and such heat 

 would give the particles a ruddy glow. As a general rule, such a 

 glow is not observed, and hence the theory that the ashes are ejected 

 through a layer of liquid is untenable. The outpouring of lava is 

 only a part of the operations of a volcano; the ejection of vapor, 

 ashes and pumice being, perhaps, even more important. Steam is 

 the one force which has to be relieved, and the other substances 

 ejected are incidental thereto. 



In his well-known on "Geology" (fourth edition, 1903), p. 



173, Sir Archibald Geikie says: 



"... The finest dust is in a state of extremely minute subdivision. 

 When examined under the microscope, it is sometimes found to consist not 

 only of minute crystals and microlites, but of volcanic glass, which may be 

 observed adhering to the microlites or crystals round which it flowed when 



