Were these changes sudden or gradual? Experience has no answer, for no similar events have occurred in historic times, and though there are records in the geological history of many mutations in the elevation of the land, we have no information as to their rate of progress, and we know little of their causes. The changes of this kind known to us in modern times are merely local, not general, and in regard to their rate are of two kinds. Some are abrupt and accompanied with earthquake shocks. These are very local, and usually occur in regions of volcanic activity. Others are so slow and gradual as to be scarcely perceptible, and are often of wider distribution. It is evident, however, that these slight and local phenomena furnish but little clue to the mutations of past periods. These were on a far grander scale and affected vast areas. We have no modern instances of these almost world-wide depressions of continents under the sea, though we know that these have occurred, one of them within the human period, and it is idle to speculate as to their rate or duration in the absence of facts. We know pretty certainly, however, from the gauges of time which can be applied to the close of the glacial period, that this latest subsidence must have occurred within six thousand years of our time. With reference to the particular movement in question, we know that the close of the palanthropic period was accompanied by a movement at least equal to the difference between the wide lands of the second continental period and the shrunken dimensions of the present lands. Besides this we find on the surface of the land modern raised beaches, deposits of loess and plateau gravels, intrusions of mud into caves of considerable elevation, and evidences, as in Siberia, of large herds of animals perishing on elevated lands on which they seem to have taken refuge.[26] In short, no geological fact can be better established than the post-glacial subsidence. [26] Prestwich, 'Evidence of Submergence of Western Europe,' _Trans. Royal Society_, 1893; 'Possible Cause for the Origin of the Tradition of the Flood,' _Trans. Vict. Inst._, 1894; Dawkins, _Journal Anthrop. Inst._, February 1894. Kingsmill and Skertchly (_Nature_, November 10, 1892) report the Asiatic loess to be marine, and to extend far upward on the Caspian plain and the Pamirs, so that all Asia must have been submerged within a very recent period. See also _Fossil Man_, by the author, 1880.