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POPULAR SCIENCE MONTHLY. 



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 mois- 

 ture-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 explo- 

 sion, 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 



Fig. 2. — Flakes of Volcanic Ash. Magnified 

 about loo diameters. A, liake with a branch- 

 ing rib ; B, fragment of a broken hollow 

 sphere of glass ; C, fragment with drawn- 

 out tubular vesicles ; D and E, plain frag- 

 ments of broken pumice bubbles. (From 

 American Geologist, April, 1893.) 



Fig. 3. — A Particle of Vol- 

 canic Ash swelled vv by 

 Fusion. Magnified UK) di- 

 ameters. 



why the little dust particles are thin, mostly triangular, and often 

 slightly concave flakes with sharp angles. Sometimes the angles ap- 

 pear 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 

 *w\\ 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 



