It is generally supposed that the change is a phenomenon of climate, and this hypothesis includes harmoniously the increase of streams with the increase of lake surface. By some it is thought that the climate of the district is undergoing, or has undergone, a permanent change; and by others that the series of oscillations about a mean condition which characterizes every climate has in this case developed a moist phase of exceptional degree and duration. The latter view was my own before I became aware of the features of the ancient storm line, but it now appears to me untenable. That a variable surface of evaporation, which had for a long period recognized a limit to its expansion, should not merely exceed that limit, but should maintain an abnormal extent for more than a decade, is in a high degree improbable. It is far more probable that one of those gradual climatic changes, of which geology has shown the magnitude and meteorology has illustrated the slowness, here finds a manifestation. The observed change is apparently abrupt, and even saltatory; but of this we cannot be certain, since it is impossible from a record of only thirty years to eliminate the limited oscillation. It is quite conceivable that were such elimination effected, the residual change would appear as a continuous and equable increase of the lake. However that may be, a certain degree of rapidity of change is necessarily involved, for the climatic change which is able in a decade to augment by a sixth part the mean area of evaporation cannot be of exceeding slowness. If we can ascertain how great a change would be demanded, it will be well to compare it with such changes as have been observed in other parts of the country, and see whether its magnitude is such as to interfere with its assumption. The prevailing winds of Utah are westerly, and it may be said in a general way that the atmosphere of the drainage basin of Great Salt Lake is part of an air current moving from west to east. The basin having no outlet, the precipitation of rain and snow within its limits must be counter-balanced by the evaporation. The air current must on the average absorb the same quantity of moisture that it discharges. Part of the absorption is from land surfaces and part from water, the latter being the more rapid.