Page:The origin of continents and oceans - Wegener, tr. Skerl - 1924.djvu/138



HE displacement theory possesses the advantage over all other theories, with similarly far-reaching ideas, that it can be proved by accurate astronomical determinations. If continental displacements were active throughout such long periods of time, it must be assumed that they are continuing even at the present day. There thus remains only the question as to whether the movements are fast enough to be disclosed by astronomical measurements taken within a not too long period. We must go somewhat into the absolute duration of the geological periods in order to obtain a decisive opinion on this matter. The evaluation of these is, as is well known, very uncertain, but nevertheless, as a whole it does not render an answer to our question impossible.

The lapse of time since the last glacial period has been estimated by A. Penck, from his glacial studies on the Alps, at 50,000 years; by Steinmann, as at the least 20,000, and at the most 50,000 years; and by Heim, from his more recent calculations in Switzerland, and from the glacial geology of the United States, at only about 10,000 years. Milankovitch, by mathematical means, arrived at a climatic maximum cold of the last glacial period at about 25,000 years ago, and a climatic optimum (or climax of inter-glacial mild temperature, which the Swedish geologists claim to have existed), 10,000 years ago. De Geer concluded, from his