Page:Popular Science Monthly Volume 20.djvu/253

Rh to its original level. Later authors suggest that the formation of ice-caps at the poles would draw away enough water from the lower latitudes to increase the amount of land in the north. Either view would afford conditions adequate to produce the results; but the evidences of such an oscillation, from the nature of the case, are very difficult to obtain. The most obvious are derived from the existence of ancient river-channels now submerged beneath the ocean. Upon the south sides of Long Island and Cape Cod there are several ravines channeled out of the till and modified drift, too large to have been excavated by the present drainage system. They are fiords, and many of them are filled by linear fresh-water lakes, kept in position by bars of beach-sand near the ocean's level. Similar facts have been reported in connection with the former entrances of tributary streams into the Great Lakes. In all these cases the excavations must have been made in connection with the disappearance of the ice-sheet.

A more extensive series of excavations occurred in the larger rivers, though it is not so easy to fix their date. By studying the submarine contours off the mouths of the Connecticut and Hudson Rivers, we can follow the outlines of their valleys for many miles out to sea. The first named flowed around the east end of Long Island, leaving the Housatonic to join the Hudson just below Manhattan Island, as shown by Professor Newberry. The Hudson continued southerly for seventy or eighty miles, as first pointed out by Professor Dana. As no deltas appear in connection with the present mouths of these streams, their submergence must have been comparatively recent, while they may have existed as channels of erosion for millions of years.

A further examination of coast-charts reveals the fact that there is a belt of shallow land bordering the continent from New Jersey to Newfoundland, and that it is as wide as the extensions of the Hudson and Connecticut. Hence, if there has been a submergence off New York, the same oscillation occurred along the whole coast; and thus a tract of land, as large as Pennsylvania, New York, and the maritime provinces combined, has been lost to the continent, probably since the glacial period. The corresponding area in Northern Europe, which seems to have been elevated at the same time, may be found indicated in Geikie's "Ice Age."

More impressive proofs of a former elevation of the continent appear from a careful study of the lower Mississippi. The contour-line of one thousand feet depth suggests the continuance of the river-bed for a distance of forty miles into the Gulf of Mexico. The Tertiary rocks of the river-basin have been excavated to the depth of two