Page:Popular Science Monthly Volume 42.djvu/281

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power and light. Within the past hundred years the science of geology has sprung into existence, and only about fifty years ago its division known as glacial geology began with the grand work of Agassiz in his study of the glaciers in the Alps, of their former extension across the wide valley of western Switzerland to Mont Jura, and of the glacial drift in Great Britain which he at once saw to be due to the former presence of sheets of land ice. At nearly the same time, in 1841, Boucher de Perthes made the first collection of stone implements in the gravel terraces of the Somme Valley, by which geologists and archæologists were reluctantly convinced that the human race dates back to a remote prehistoric period, since which time very great changes of climate have taken place and the valleys of many rivers have been eroded far below their old flood plains. Because of the relationship of the earliest traces of man with the Glacial period, this latest part of the geologic record has attracted the interest of many observers and nearly all readers; new and important discoveries are being made every year, and a vast amount of literature in scientific journals and government reports is constantly accumulating; but many difficult problems in this field remain still under discussion, concerning measurements of the antiquity of man, the duration of post-glacial time and of the Ice age, the causes of its climatic changes, and whether it consisted of only one epoch of glaciation or of two or more separated by mild and warm interglacial epochs when the ice-sheets were melted away. Among these observers and writers none during recent years has traveled more extensively to gather information or been more successful in contributing to our knowledge than Prof. Wright, who in this book, as in his previous larger volume, treats this subject in a clear, vigorous, and entertaining style.

Agassiz reasoned, from the action of the Swiss glaciers in their wearing the rock surfaces over which they moved, and in their transportation of drift and formation of terminal moraines, that all countries bearing such marks or striæ on the bed-rocks and similar deposits of till, or intermingled bowlders, gravel, sand, and clay, have been overspread by ice. Prof. Wright similarly devotes fifty pages to descriptions of glaciers now existing, and of the ice-sheets of Greenland and the Antarctic continent, before considering the evidences of past glaciation. On the Sierra Nevada and Mount Shasta living glaciers are found, but are of very small size. Northward they occur in increasing numbers and abundance on the Cascade Range and in the Selkirk Mountains and the Coast Ranges of British Columbia and Alaska. About one hundred and fifty miles north of Sitka the Muir Glacier, which was explored and mapped by Prof. Wright in 1886, has an extent of about three hundred and fifty square miles; and the Malaspina Glacier or ice-sheet, lying between Mount St. Elias and the ocean, mapped by Russell in 1890 and 1891, covers some fifteen hundred square miles. These are very far surpassed, however, by the Greenland ice-sheet, explored by Rink, Nordenskiöld, Nansen, and Peary, which probably has an area of half a million square miles; and the Antarctic ice-sheet is ten times more extensive, occupying, indeed, a somewhat greater area than the northern half of North America, which was enveloped by ice during the Glacial period.

The terminal moraines of the ancient icesheet of this continent have been traced by Wright, Chamberlin, Salisbury, Leverett, Upham, and others, from Nantucket and Cape Cod, westward through Long Island, northern New Jersey and Pennsylvania, Ohio, and other States to Minnesota and North Dakota; and farther westward the glacial boundary crosses Montana, Idaho, and Washington to the Pacific south of Vancouver Island. Stone implements proving the presence of man here during the Ice age have been found in plains and terraces of modified drift deposited in valleys by streams flowing from the melting and receding icesheet in New Jersey, Ohio, Indiana, and Minnesota. Equal or greater antiquity must be also affirmed for the Calaveras skull, stone mortars, pestles, and spear-heads which have been obtained by Whitney, King, Becker, Wright, and others, from the gold-bearing gravels under the lava of Table Mountain in California.

In the chapter on the ancient glaciers of the Eastern hemisphere a very valuable contribution of more than forty pages is from the pen of Mr. Percy F. Kendall, relating to the glaciation of the British Isles, with a map