Page:Popular Science Monthly Volume 44.djvu/290

280 side of the Rocky Mountains in Alberta. The stratified beds, some of them fossiliferous, and others containing layers of lignite, which are found in Canada between deposits of till, may probably be explained by moderate advances of the ice-sheet interrupting its general recession, not so prolonged nor important as to be called interglacial epochs.

Prof. T. C. Chamberlin reviewed the history of the Ice age in the United States, concluding that it has probably a threefold division. Two long glacial epochs had preceded the chief time of deposition of the loess, which was followed by the principal interglacial epoch with retreat of the ice border perhaps generally to the northern line of the United States. The last great ice advance and stages of its retreat were attended by the formation of the remarkable marginal moraines, ten to twenty in order from south to north, which have been mapped across the northern United States and portions of Canada, while others doubtless remain to be traced in regions farther north.

Mr. Warren Upham noted the uniqueness of the climatic conditions of the lee age, and the absence of glacial periods from the far longer Tertiary and Mesozoic eras. So exceptional climate during the Quaternary era must have resulted from very unusual causes, which could not be astronomic, for in that case records of frequently recurring general glaciation would be found in the long preceding eras. Great uplifts of glaciated countries to such altitude that they received snowfall instead of rain during all the year are regarded as the cause of the ice accumulation; but the vast weight of the ice-sheets finally depressed the land, bringing on a warm climate by which the ice was at last rapidly melted away. Only one epoch of glaciation, with fluctuating advance and recession of the ice, is held to be a sufficient explanation for the observed glacial phenomena of both North America and Europe; and the Glacial period in each of these continents appears to have ended only some six thousand to ten thousand years ago.

Mr. Frank Leverett described the diverse deposits of the older drift in northwestern Illinois, showing on a map of that State the courses of the glacial boundary and retreatal moraines which he has traced. Comparison of the depths of stream erosion in the older and newer drift indicates that their times of formation were divided by a much longer interval than the time from the end of the Ice age until now.

In the discussion following these papers, Prof. G. r. Wright spoke of the rock gorges eroded by the Delaware, Susquehanna, and upper Ohio Rivers below the highest drift-gravel terraces. This erosion has been referred to an interglacial epoch, but he finds evidence that it was preglacial, and that the valleys were filled with the early drift gravels from their present bottoms to the level of the high terraces. The general parallelism of the drift boundary and the successive retreatal moraines is thought to imply the formation of all the drift during a single epoch.

Prof. R. D. Salisbury cited the much deeper oxidation and leaching of the older than of the newer drift as proof of their widely different ages, separated by a long interval of ice departure and mild climate.

Major C. E. Dutton objected to the extension of theories beyond the warrant of facts observed. He thought it too early at the present stage of investigations to decide the causes of the Ice age; but he doubts the astronomic theory, and looks rather to geographic conditions.

Lack of time prevented the consideration of the subject assigned for special discussion, on the correlation of glacial formations in opposite hemispheres, which, however, had been more or less touched upon by several of the papers. The prevailing view seemed to be that the glaciations of Europe and America were nearly or quite at the same time, and that there was a close agreement in the sequence of events constituting the Ice age on both continents.

The World's Fair Model Library.—The model library of five thousand volumes shown by the American Library Association at the World's Fair is to be sent to the Bureau of Education at Washington for use and exhibition. This library marks a noteworthy step in advance in the choosing of books in each department the selection was committed to an authority in his field. In the sections of electricity, photography, general political economy, and American government, lists were printed, each title being followed by a note of description and appraisal