GLACIERS. 



45 



the scouring action of the tides, are 

 gradually filled up with sediments 

 brought down by the rivers, leaving 

 only narrow passages for the flow of 

 the tide. In this manner were 

 formed the sea-islands all along our 

 Southern coast, separated from the 

 mainland only by narrow tidal in- 

 lets. These tidal inlets may become 

 filled up, and the whole coast-line 

 transferred farther seaward. 



A large portion of the coasts of 

 the world is thus bordered by wave- 

 formed islands. We have already 

 seen, however, that on some coasts, 

 e. g., Norway, Scotland, etc., islands 

 are formed by the destructive ac- 

 tion of waves. Bordering islands, 

 so common along all coasts, are 

 therefore of two classes, and formed 

 by two opposite effects of waves — 

 the one land-destroying, the other 

 land-forming. The islands of one 

 class are high and rocky, of the other low and sandy or muddy ; the 

 former are the scattered remains of an old coast-line, the latter the 

 commencing points of a new coast-line.* 



Section 3. — Ice. 

 The agency of ice will be considered under the heads of Glaciers 

 and Icebergs ; the effects of frost in disintegrating rocks having been 

 already treated of under Atmospheric Agencies. It is only compara- 

 tively recently that the great importance of ice as a geological agent 

 has been recognized. To Agassiz is due the credit of having first 

 fully recognized this importance. 



Glaciers. 

 Definition.— In many parts of the earth, where the mountains reacli 

 into the region of perpetual snow, and other favoring conditions are 

 present, we find that the mountain-valleys are occupied by masses of 

 compact ice, connected with the snow-cap above, but extending far 

 below the snow-line into the region of cultivated fields, and moving 

 slowly but constantly down the slope of the valley. Such valley-pro- 



* Coast islands are, however, often formed by subsidence of continental margins. 



Fig. 36.— Coast of North Carolina. 



