Page:Popular Science Monthly Volume 44.djvu/172

162 of the geologic brevity of the time since the ice-sheets disappeared from North America and Europe make it clear, in the opinions even of some of the geologists who believe in a duality or plurality of Quaternary Glacial epochs, that not astronomic but geographic causes produced the Ice age.

Glacialists who reject Croll's ingenious and brilliant theory mostly appeal to great preglacial altitude of the land as the chief cause of the ice accumulation, citing as proof of such altitude the fiords and submarine valleys which on the shores of Scandinavia, and the Atlantic, Arctic, and Pacific coasts of North America, descend from one thousand to three thousand and even four thousand feet below the sea level, testifying of former uplifts of these continental areas so much above their present heights. But beneath the enormous weights of their ice-sheets these lands sank, so that when the ice attained its maximum area and thickness and during its departure the areas on which it lay were depressed somewhat lower than now and have since been re-elevated. This view to account for the observed records of the Glacial period is held by Dana, Le Conte, Wright, Jamieson, and others, including the present writer. It is believed to be consistent either with the doctrine of two or more glacial epochs during the Quaternary era, or with the reference of all the glacial drift to a single glacial epoch, which is thought by Wright, Prestwich, Lamplugh, Falsan, Hoist, Nikitin, and others to be more probable. To myself, though formerly accepting two glacial epochs with a long warm interval between them, the essential continuity of the Ice age seems now the better provisional hypothesis, to be held with candor for weighing evidence on either side.

The duration of the Ice age, if there was only one epoch of glaciation, with moderate temporary retreats and readvances of the ice border sufficient to allow stratified beds with the remains of animals and plants to be intercalated between accumulations of till, may only have comprised a few tens of thousands of years. On this point Prof. Prestwich has well written as follows: "For the reasons before given, I think it possible that the Glacial epoch—that is to say, the epoch of extreme cold—may not have lasted longer than from fifteen thousand to twenty-five thousand years, and I would for the same reasons limit the time of. . . the melting away of the ice-sheet to from eight thousand to ten thousand years or less."

From these and foregoing estimates, which seem to me acceptable, we have the probable length of Glacial and postglacial time together thirty thousand or forty thousand years, more or less; but an equal or considerably longer preceding time, while the areas that became covered by ice were being uplifted to great altitudes, may perhaps with good reason be also included in the Quaternary