Page:Popular Science Monthly Volume 54.djvu/238

224 a compound bubble of glass (Fig. 3). It is evident that this is due to the expansive force of the heated included moisture, to which the viscid half-molten glass readily yields. At the time of the eruption which produced this dust, subterranean heat was applied to the moisture-bearing rock until this was superheated to such an extent that the weight of the overlying material was insufficient to hold the water from expanding into steam. Then there was a tremendous explosion, and the molten magma was thrown up with such a force that it was shattered into minute droplets, in the same way as water does when it is thrown forcibly into the air. Being thus released from pressure, the steam inside of each little particle of the heated glass caused it to swell out into a tiny bubble. As this kept on expanding it was cooled, the thin glass wall of the bubble congealed, and finally burst from the pressure of the steam within. This is the reason

why the little dust particles are thin, mostly triangular, and often slightly concave flakes with sharp angles. Sometimes the angles appear rounded, as if the fragments had been viscid enough to creep a little after the bubble burst. The study of one single little grain of dust, barely visible to the naked eye, thus makes clear the nature of a catastrophe which must have shaken a whole mountain, and which left its traces over a quarter of a continent.

That the dust was produced in this way is quite evident from other circumstances. If a handful from the dust of this place be thrown into water and gently stirred, it nearly all will settle after a while. But some rather large particles remain floating on the surface. If these are removed and examined under the microscope, they are seen to be hollow spheres (Fig. 2, b). These are some of the original bubbles that never burst, either because they contained too little steam or else because the steam was cooled before it had time to break the walls open. It is evident that not every droplet of the molten magma would form a single sphere, but that many also would swell up into a compound frothlike mass of pumice. A few such pieces may sometimes be observed in the deposit at this place, and that many more were made and broken is evident from the great number