Under special circumstances, opportunity is afforded for rough determinations of the distance and height to which wind-blown dust is transported. Snow taken from snow-fields in high mountain regions is found to contain a small amount of earthy matter. Its particles are often found to be in part volcanic, even when the place whence the snow was taken is scores or even hundreds of miles from the nearest volcano. There is probably no snow-field so high, or so far from volcanoes, but that volcanic dust reaches it. If this be true of all snow-fields, it is probably true of all land surfaces. In the great Krakatoa[4] eruptions of 1883 large quantities of volcanic ash (pulverized lava) were projected to great heights into the atmosphere. The coarser particles soon settled; but, caught by the currents of the upper atmosphere, many of the finer particles were transported incredible distances. Through all their long journey, the particles of dust were gradually settling from the atmosphere, but not until the dust had traveled repeatedly round the earth did its amount become so small as to cease to make its influence felt in the historic red sunsets which it occasioned.[5] Some of this dust completed the circuit of the earth in 15 days. In various parts of Kansas and Nebraska[6] there are very considerable beds of volcanic dust, locally as much as 30 feet thick, which must have been transported from volcanic vents by the wind, though there are no known centers of volcanic action, past or present, within some hundreds of miles of some of the localities where the dust occurs. These beds of volcanic dust, so far from its source, may serve as an illustration of the importance of atmospheric movements as a geological force. Volcanic dust is shot into the atmosphere rather than picked up by it. Dust picked up by the wind is perhaps transported not less widely than volcanic dust, but, after settling, its point of origin is less readily determined. It would perhaps be an exaggeration to say that every square mile of land surface contains particles of dust brought to it by the wind from every other square mile, but such a statement would probably involve much less exaggeration than might at first be supposed. Examples of extensive deposits of dust other than volcanic are also known. In China there is an extensive earthy formation, _the loess_, sometimes reaching 1,000 feet in thickness, which von Richtofen believes to have been deposited by the wind.[7] This conclusion has, however, not passed unchallenged.[8] The loess of some other regions has been referred to the same origin, and some of it is quite certainly eolian.[9]