of glass fragments which have riblike edges on their flat sides (Fig. 3, a).
The nature of the force which caused the eruption may thus be understood from the study of one little grain of the dust, but much more extended observations are needed in order to make out the place where the great convulsion took place. It will, perhaps, never be known what particular volcanic vent was the source of this ash. Different deposits may have come from different places. But it seems possible that it all came from the same eruption. There can be no doubt that the volcanic disturbances occurred to the west of the Great Plains. No recent extinct volcanoes are found in any other direction. This conclusion is corroborated by the fact that the dust is finer in eastern localities and coarser nearer the Rocky Mountains. In a bed near Golden, in Colorado, seventy-three per cent, by weight, of the dust consists of particles measuring from one fourth to one thirty-second of a millimetre, while some from Orleans, in Nebraska, contains seventy-four per cent of particles measuring from one six- teenth to one sixty-fourth of a millimetre in diameter. Still finer material comes from the bluffs of the Missouri River near Omaha. Evidently the coarser particles would settle first, and if the dust is finer toward the east, it must be because the wind which brought it blew from the west. Most likely the eruption occurred somewhere in Colorado or in New Mexico.
It may be asked how it can be known that the dust was carried this long distance by the wind. May it not as well have been trans- ported by water ? The answer must be, in the first place, that showers of the same kind of material have been observed in connection with volcanic outbursts in other parts of the world. One such shower is known to have strewn the same kind of dust on the snow in Norway after a volcanic eruption in Iceland, and after the great explosion on Krakatoa, in 1883, such dust was carried by the wind several hundred miles, and scattered over the ocean. If this ash had been transported by water, it would not be found in such a pure state, but it would be mixed with other sediments. There would, no doubt, also be found coarser fragments of the volcanic products. On the contrary, it appears uniformly fine. No particles have been found which measure more than one millimetre in diameter, and less than one per cent of its weight consists of particles exceeding one eighth of a millimetre in diameter. In seven samples taken from different places the proportions of the different sizes of the grains were about as follows:
| Diameter of grains in millimetres. | 1—124 |
1—148 |
1—1816 |
1—11632 |
1—13264 |
1—164128 |
1—1128256 |
| Percentage of weight of each size | 0.1 | 0.19 | 19 | 37 | 32 | 9 | 1 |
