688 DYNAMICAL GEOLOGY. 



the sides or their rocks, or cause fractures and ridges over the surface. 

 But freezing usually begins about the shores, and in the expansion 

 there, the ice slips over the water, leaving a central portion only to be 

 thrown into a strain. 



2. Ice of Rivers and Lakes. 



Ice, forming along streams in which there are stones, envelops the 

 stones in shallow water, even to a depth of two or three feet, or more 

 in the colder climates. Other stones and earth fall on the ice from 

 the banks. When the floods of spring raise the stream, and break up 

 the ice, both ice and stones often float down stream with the current, 

 or are drifted up the banks high above their former level, or are 

 spread over the river-flats. 



Ice sometimes forms about stones in the bottom of rivers, when the 

 rest of the water is not frozen, and is then called anchor-ice. In this 

 condition, it may serve as a float to raise the stones, and to transport 

 them, with the aid of the current. 



The same modes of transportation are exemplified in lakes as in 

 rivers, except that there is less current ; and the stones are mostly set 

 back up the shore. Large accumulations of stray stones far above 

 the ordinary level of the lake are in some places thus made. 



3. Glaciers. 

 I. General Features ; Formation, and Movement of Glaciers. 



1. Nature of Glaciers. — Ordinary glaciers are accumulations of 

 ice, descending along valleys from snow-covered elevations. They 

 are ice-streams, 200 to 5,000 feet deep or more, fed by the snows and 

 frozen mist of regions above the limits of perpetual frost. They 

 stretch on 4,000 to 7,500 feet below the snow-line (limit of perpet- 

 ual snow), because they have such magnitude that the heat of the 

 summer season is not sufficient to melt them. Some of them reach 

 down between green hills and blooming banks, into open cultivated 

 valleys. The extremities of the glaciers of the Grindelwald and Cha- 

 mouni valleys lie within a few hundred feet of the gardens and houses 

 of the inhabitants. Each glacier is the source of a stream, made from 

 the melting ice. The stream begins high in the mountains, from the 

 waters that descend through the crevasses to the ground beneath ; 

 finally, it gushes forth from its crystal recesses, a full torrent, and hur- 

 ries along over its stony bed down the valley. 



2. Glacier Regions. — The best known of glacier regions is that of 

 the Alps. West of the head-waters of the Rhone, the chain is divided 

 into two nearly parallel ranges, a southern and a northern. The 

 former includes, besides minor areas, two large glacier districts, — the 



