A GEOLOGICAL ROMANCE. 



223 



ony of the ordinary geological work recorded in the terranes of the 

 plains. It may be said to consist of angular flakes of pumice, averag- 

 ing one sixteenth of a millimetre in diameter, and having a thickness 

 of about one three-hundredth of a millimetre. The most common 

 shape of the flakes is that of a triangle, or rather of a spherical tri- 

 angle, since the flakes are apt to be concave on one side and convex on 



Fio. 1. — Stratified Volcanic Ash near Meade, Kansas. 

 (From the University Geological Survey of Kansas, vol. ii. ) 



the other. In the miscroscope they sometimes appear like splinters of 

 tiny bubbles of glass, and this is really what they are (Fig. 2). 



The explosive eruptions which give rise to showers of this kind 

 of ash, or dust, are due to fusion and superheating of subterranean 

 masses of rocks charged with more or less moisture. A part of this 

 moisture escapes in the form of steam at the time of an eruption. But 

 the viscidity of the ejected material prevents much of the steam from 

 passing off, and such of the lava as cools most rapidly retains a cer- 

 tain quantity in solution, as it were. Obsidian is a rock which has 

 been made in this way. It often contains much of the original water, 

 which will cause it to swell up into a stony froth when fused. 



This volcanic dust has the same property. If one small particle 

 of it be heated on a piece of platinum foil it is seen to swell up into 



