Page:Popular Science Monthly Volume 35.djvu/580

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as engraved illustrations in this volume. The author also presents very fully the results of the labors of others, both in the United States and in Canada, as Agassiz, Dana, E. and C. H. Hitchcock, Newberry, Le Conte, Lesley, White, Chamberlin, Salisbury, Todd, Gilbert, McGee, Shaler, Davis, Stone, Russell, Upham, A. and N. H. Winchell, Claypole, Spencer, Whitney, Sir William and G. M. Dawson, Bell, Chalmers, and many more, often quoting from their reports and memoirs, and reproducing their illustrations and maps. The work is thus a compendium, well brought up to date, of the already voluminous literature of this wonderful geologic winter of our globe.

Glaciers now exist, as described in this volume, on the Sierra Nevada, on Mount Shasta, in the Selkirk Range, and in great numbers and extent northward to Mount St. Elias and Unalaska. In the chapter on the glaciers of Greenland, a map shows the route of Nordenskiöld in 1883, and of Dr. F. Nansen last year upon the ice-sheet that covers its interior, extending in a vast monotonous expanse which rises gradually to elevations in its central portion six thousand to ten thousand feet above the sea. The further description of glaciers in other parts of the world, and of the antarctic ice-sheet, prepare the reader for the discussion of the signs of former glaciation in the now temperate regions of North America and Europe.

The striation of the bed-rocks, the striated pebbles and bowlders of the drift, sections of till and of stratified drift and loess, the characteristic topography of kames, terminal moraines, and the oval hills of till called drumlins, are very clearly described, with excellent illustrations from photographs. The boundary of the glaciated area from the Atlantic to the Mississippi is shown in a series of six maps; and a general map showing the glacial geology of the United States delineates, besides this southern limit of the North American ice-sheet and drift, the successive terminal moraines formed at times of halt or readvance of the ice during its retreat and final melting, the courses of the glacial striæ and transportation of bowlders, the driftless area of southwestern Wisconsin and portions of adjoining States, the modified drift deposited in valleys of southward drainage from the ice-sheet, and the boundary of the glacial Lake Agassiz which was held in the basin of the Red River of the North and of Lake Winnipeg by the barrier of the ice while it was being melted away.

Important changes in the drainage of the country, caused by the ice-sheet and its drift deposits, are noticed in considerable detail. In the same way that Lake Agassiz was formed, outflowing by the glacial River Warren along the course of the Minnesota and Mississippi Rivers, the Great Lakes of the St. Lawrence were held by the receding ice-barrier at levels much higher than now, similarly outflowing over the lowest points in their southern water-shed to the Mississippi; and these ancient lake-levels are still found distinctly marked by beach ridges and deltas of gravel and sand. Another very interesting glacial lake was formed in the basin of the Ohio River by the temporary dam of the ice-sheet, which at its time of maximum area extended across this river at Cincinnati, carrying its morainic drift into the northern edge of Kentucky. "These glacial deposits south of the Ohio," according to Prof. Wright's observations, "are such as to make it certain that the front of the continental glacier itself pushed, at some points, seven or eight miles beyond the Ohio River; and it is altogether probable that for a distance of fifty miles (or completely around the eastern, northern, and western sides of the Kentucky peninsula formed by the great bend of the river) the ice came down to the trough of the Ohio, and crossed it so as completely to choke the channel and form a glacial dam high enough to raise the level of the water five hundred and fifty feet—this being the height of the water-shed to the south." Traces of the former existence of this Lake Ohio are found along a distance of about four hundred miles in the valleys of the Ohio, Alleghany, and Monongahela Rivers and their tributaries. At the present time the abundant lakes, and the waterfalls on streams, throughout the glaciated area, so remarkably contrasted with their general absence farther south, are due to irregularities in the deposition of the drift and to its obstructions of the preglacial drainage.

A chapter is devoted to the flight of plants and animals during the Glacial pe-