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progress is transverse to the bounding highlands. It may also be presumed that the wind retards its velocity, when going down an inclined plane. The greater depth of the atmospheric ocean in these instances ought to have the same influence on the general current as the widening or deepening of a river channel. If this be the case with extensive continental depressions, valleys of rivers and smaller depressions of the earth's surface ought to produce somewhat similar effects in retarding the passing wind and inducing it to give up a part of the dust it may happen to carry along. On the other hand, when the wind passes over land covered by a growth of timber or only tall grass, its lowest part will be held comparatively still and will drop its load. Did the same air remain among the vegetation all the time this unloading process would stop with the first deposit, but as the eddies no doubt keep up a slow but constant exchange with the air above, the accumulation continues as long as there is any dust left.
Several important deductions can be drawn from the foregoing considerations.
The velocities in the atmosphere being so much greater than those obtaining in rivers, lakes, and seas, the distances over which materials may be transported in it will be correspondingly greater. In the sea sediments are carried out 200 miles and even farther. In the atmosphere, where the velocities often are 100 times greater than those in the sea, dust may, no doubt, be transferred a distance of several hundreds, if not a few thousands of miles. The very finest particles may be borne round the earth, as shown by the dust of Krakatoa, or may, indeed, circle about it for some time.
The greater depth of the aerial ocean renders it but little dependent in its movements on smaller elevations of the land. In a sea five miles deep an elevation of the bottom 8,000 feet high would interpose no serious obstacle to a general forward movement of the whole body of the fluid. Few of our mountain ranges exceed this height, and it would not seem impossible, therefore, that dust in some notable quantities should be carried