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increase the length of the greater in elevation, or, in other words, to scoop out material from the ledges, in a part of the course, and, after the strength of the graver has exhausted itself, the ice will move up a slope with little energy. This process will excavate hollows that may be filled with water in later times. Such are the basins of the great American lakes. The dimensions of the lakes are areally proportionate to the extent of the drainage-system in which they occur.

Fig. 7.
PSM V04 D669 Till overlaid with boulder clay.jpg
Till overlaid with Bowlder-Clay, River Stinchar. r, Rock; t, Till; g, Boulder-clay; x, Fine Gravel, etc.

Bowlder-Clay.—There is a distinction to be drawn, in Scotland, between the "till" and "bowlder-clay." The two deposits pass into each other on the Highlands, and Mr. Geikie proposes to limit the latter to the maritime districts. The bowlders of the clay are more rough and angular than those found in the till. The annexed section shows where the two deposits come into juxtaposition. This clay has not been met with more than 260 feet in vertical height above the sea. It contains an abundance of shells of Arctic mollusca. Possibly it is the "Champlain clay" of America.

Antarctic Ice-Sheet.—From a study of the ice of the Antarctic Continent, it is possible to understand the origin of icebergs, and the transportation of large blocks of stone, in "erratics." The water is deep, and thus buoys, of enormous size, may float northerly for hundreds of miles.

Sir J. C. Ross attained the highest southern latitude on record, but found all his attempts to penetrate farther frustrated by a precipitous wall of ice, frequently 180 feet in height. For 450 miles he found this cliff unbroken by a single inlet. While coasting along this barrier his ships were often in danger from stupendous icebergs and thick pack-ice, extending in masses too compact to be penetrated. At one point the ice descended sufficiently low to allow Ross to look down upon it from the masthead. The upper surface appeared to be a smooth plain, shining like frosted silver, and stretching away as far as eye could reach into the illimitable distance. In principle, the sheet is the same with that figured in the north, but more extensive. Like this must have been some portions of the glacial sheet in Scotland, when the land was mantled in ice-covering, filling up the intervening straits and channels of the sea, and terminating far out in the Atlantic Ocean, in aflat-topped vertical cliff of blue ice.