24 
AMERICAN GEOLOGY. 
down, and the soil is washed away, and carried to other points 
upon the coast. Portions of Long island, Martha’s Vineyard, 
are undergoing changes of this kind. Points of land where 
fortifications and lighthouses have been built, have often been 
undermined by sea action. The sea encroaches on the land. 
But the constructive action has greatly exceeded the destructive 
on our own coast. 
§ 20. Ice ar.d semifluid ice is an instrument of change 
which should be noticed in this connection. Water congeals, 
and remains so the whole of the year, upon mountains which 
rise above 15,000 feet above the sea under the equator, and at 
still lower levels at points north or south of it. In consequence 
of this, there are vast accumulations of snow and ice upon the 
tops and in the high valleys of many mountains. These beds 
of snow and ice are called glaciers. This ice, as it approaches 
the lower parts of the mountain, softens, and becomes movable 
upon the inclined plane upon which it rests. The glaciers 
freeze, however, during the night, and become stationary; but 
becoming softened, or partially thawed, during the day, they 
ao'ain move on at a certain rate. The middle moves faster 
o 
than the sides. The glaciers are now regarded as instruments 
of change upon the rocks over which they move. The position 
of these glaciers is such that they receive all the rocks and 
debris of the mountains where they are formed. They exist 
more or less throughout their icy beds, and hence it often hap¬ 
pens that rocks stand out from their inferior surfaces. Owing 
to this circumstance, the glacier, as it moves, forces these rocks 
over the rocks in place beneath: they are therefore abraded 
and loosened, and the melted ice, as it flows away, is charged 
with mud, which is merely the matter worn off from the rocks. 
The glaciers then work mechanically, and with great power; 
and thus they aid in the process of leveling the mountains and 
filling up the valleys at their base. Glaciers, when they reach 
the sea in high latitudes, carry directly the abraded matter to 
the ocean. Glaciers in high latitudes jut over the sea, and 
the sea edge being left unsupported, break off from the.main 
