ony of the ordinary geological work recorded in the terranes of the plains. It may be said to consist of angular flakes of pumice, averaging 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 triangle, since the flakes are apt to be concave on one side and convex on
(From the University Geological Survey of Kansas, vol. ii.)
the other. In thethey 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 certain 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