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 transported 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 | 1⁄2-1⁄4 | 1⁄4-1⁄8 | 1⁄8-1⁄16 | 1⁄16-1⁄32 | 1⁄32-1⁄64 | 1⁄64-1⁄128 | 1⁄128-1⁄256 |
| Percentage of weight of each size | 0.1 | 0.1 | 19 | 37 | 32 | 9 | 1 |
Flaky particles of this size are easily carried along by a moderate wind. In some places it appears as if the dust were resting on an old land surface where no water could have been standing when it fell. There is really no room for doubt that it was carried several hundred miles by the wind. It must have darkened the sky at the time, and it must have settled slowly and quietly over the wide plains, covering extensive tracts with a white, snowlike mantle several feet in thickness. What a desolate landscape after such a shower! What a calamity for the brute inhabitants of the land!
Fig. 4.—Tracks in the Volcanic Dust, probably made by a Crawfish.
Reduced to 2⁄8 diameter.
Right here in McPherson County there was either a river or a lake at the time of the catastrophe. This is plainly indicated in several ways. In one place the dust rests on sand and clay, with imbedded shells of fresh-water clams. It is assorted in coarse and fine layers like a water sediment. Lowermost is a seam of very coarse grains. These must have settled promptly through the water, while the finer material was delayed. In another place it lies on higher ground, and here marks of sedges and other vegetation are seen extending up about a foot into the base of the deposit, from an underlying mucky clay. Bog manganese impregnates a thin layer just above the clay, indicating a marshy condition. Here also the material is somewhat sorted, but in a different way. It is ripple-bedded. The water was evidently shallow, if there was any water at all. A burrow like that of a crawfish extended down into the old clay bottom. On a slab of the volcanic ash itself some tracks appeared (Fig. 4). These were probably made by an individual of the same race in an effort to escape from the awful fate of being buried alive like the inhabitants of Herculaneum and Pompeii.
Fig. 5.—Ripple Marks in the Volcanic Dust.
Reduced to ¼ diameter.
The shower must have lasted for a time of two or three days. I infer this from the nature of the wind changes, which are indicated by the ripples in the dust. These still lie in perfect preservation (Fig. 5), and may be studied by removing, inch by inch, the successive layers from above downward, for it is evident that as the direction of the wind changed, the ripples were also turned. The deciphering of this record must be made backward. The bottom layers were deposited first, and the excavation must begin on top. Otherwise the record is as perfect as if it had been taken down by an instrument when the shower occurred. It may be only local in its significance, for it shows the direction of the wind at this particular place alone. The wind may have been somewhat deflected from the general direction by local topographic peculiarities, though these appear to have been of small importance. In any case, the old legend is quite interesting to read, being, I believe, the only geological record ever found of the passing of a cyclone over the United States.