THE OCEAN AND ITS WORK 203 



work. (2) Moreover, the air in the fissures, even above the reach 

 of the waves, is suddenly compressed and forced into the minute 

 cracks as the waves dash against the cliffs. Upon the withdrawal of 

 a wave the pressure is suddenly released, and the air and water 

 rush out with a suction which, when frequently applied, may loosen 

 and dislodge large blocks of rock. An often-quoted example is that 

 of the Eddystone lighthouse, England, in which a securely fastened 

 door was driven outward as a result of the partial vacuum produced 

 by the withdrawal of a wave during a storm in 1840. Blocks of stone 

 in well-built sea walls are sometimes started from their places, partly 

 at least in this way. (3) The rocks which are broken or quarried 

 from sea cliffs by the impact of the waves and in other ways become 

 tools with which the waves are able to accomplish their greatest 

 work of erosion. As these are lifted by the waves and hurled against 

 the cliffs, they act as hammers which beat to fragments the rocks 

 against which they strike. A high cliff is affected in the same way as 

 a lower one, but is usually cut back more slowly, because as the waves 

 undercut it, the talus (p. 29) falling from above may accumulate in 

 quantities greater than the waves can quickly remove. Under such 

 conditions the energy of the waves may be largely expended in grind- 

 ing to pieces and removing the talus. Sea cliffs, however, weather 

 back more rapidly than cliffs inland, as they are wet with spray and 

 are usually undermined by springs and are comparatively free from 

 talus. When a cliff descends precipitously into deep water the waves 

 merely wash up and down and, having no tools with which to cut, wear 

 it back very slowly, (4) The spray thrown up by the waves also has 

 an erosive effect upon certain rocks, since it washes away the weaker 

 ones and dissolves others which it can affect chemically. In this 

 latter way silicates are broken down and limestones are dissolved. 



Shore Ice. — In high latitudes shore ice protects the shore during 

 the winter months, and even when loosened by the summer thaw it 

 prevents the waves from breaking against the coasts with their full 

 force. Shore ice, nevertheless, is important in the erosion of coasts 

 in regions where it forms. During the winter a broad shelf of ice 

 develops, whose thickness is usually much greater than that which 

 would result from the direct freezing of the sea, which even in polar 

 regions seldom exceeds 8 or 10 feet. The thickness of 30 to 60 feet 

 to which this shore ice or ice foot forms is the result partly of the direct 

 freezing of the ocean water, partly of the accumulation of snow on 

 the ice, which is converted into ice by the water from the waves, and 



