THE AGE OF ICE. 



649 



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 

 ice filled with water in later times. Such are the basins of the great 

 American lakes. The dimensions of the lakes are areally proportion- 

 ate to the extent of the drainage-system in which they occur. 



Fig. 7. 



Till overlaid with Bowlder-Clay, Kiver 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 lat- 

 ter 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 hun- 

 dreds 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 mast-head. 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. 



