Page:Popular Science Monthly Volume 42.djvu/787

 Rh swept across the field and wiped out the records written as with an iron pen on the rocks, and has engraven in their stead a palimpsest of its own. The Ice age is now a familiar topic, and its massive ice-sheet a reality to all. The continental glaciers which covered a great part of North America and Europe with ice thousands of feet thick, and enduring for thousands of years, literally swept from the face of the country the monuments of preceding life, leaving in their place its own memorials which the geologist is now learning to interpret.

Here is the unexpected barrier which meets the archæologist and the geologist in their investigations. They can follow the trail of man back into the Pleistocene era almost or quite to the edge of the ice. There it either becomes exceedingly faint or is lost altogether. In the tangled maze of glacial history the previous confusion is worse confounded, and the thin thread of evidence for man's existence is broken or lost.

The nature and date of the Glacial era and man's relation to it thus become important problems in the main issue, and it is these with which Prof. Wright's book deals. To some geologists the Ice age was single, to others it appears to have been double, triple, or even more complex. Some believe that man was contemporary with the later and even with the earlier stages of the era. Space will not allow us here to do more than mention these divergences of opinion, but so much was necessary in order to understand the scope of the work.

The appearance of Prof. Wright's little book has been the signal for a renewal of the controversy with fresh energy, not to say with acrimony, yet in it the ordinary reader would scarcely find any cause for commotion. It is for the most part merely a condensation of the same writer's larger work on the Ice Age in North America. Its aim is to lay before the general reader a short sketch of the present state of our knowledge of the Glacial era, and to briefly state the evidence bearing on man's existence during it or any part of it. The book is not sensational; it contains little or nothing that is new; it publishes no startling facts; it propounds no novel or strange theories, scientific or unscientific; it is simply, as it professes, a summary view over the field of glacial geology.

The author is well known to geologists by his share in the epoch-making work of tracing the southern limit of the ice-sheet across the North American continent. This was accomplished by him in connection with Lewis, Upham, Smock, Chamberlin, Cook, Leverett, etc., and, as far as the western Illinois State line, may now be considered definitely known. In this great work Prof. Wright may fairly claim a place among the first, having commenced his studies on the drift hills of Andover,