v 



218 DYNAMICAL GEOLOGY. 



the pressure in pounds and 64 the weight of a cubic foot of sea water), 

 '6083 X 32-2 ^ gg.32 fgg^_ rj^j^^ hydrostatic pressure due to a wave 20 feet 



high is over (1280 lbs.) half a ton to the square foot ; the rest of the force 

 comes from its velocity. Mr. Stevenson states that on one of the Hebrides a 

 mass of rock of about 42 tons weight was gradually moved in a storm five 

 feet; with each incoming wave it was made to lean landward, and the back 

 run uplifted it with a jerk, leaving it with little water about it. 



It is reported that at Unst, one of the Shetlands, walls were overthrown 

 and a door broken open at a height of 196 feet above sea level. Geikie attrib- 

 utes part of the effects of the impact to the compression of the air of cavities 

 by the striking waters, and then its sudden expansion, with tearing effects ; 

 and also to the rarefaction of air caused by the sudden withdrawal of the 

 waters after a broad stroke, this leading to displacement of blocks or masses. 

 He mentions the case of a securely fastened door at the Eddystone Light- 

 house forced outward by the stroke of the outside surface by a wave ; and 

 suggests that the principle may account for stones being started from their 

 places in a solidly built stone wall. The water driven into crevices has great 

 rending force. 



The heaviest waves exert little force against rocky cliffs, or the sea-bot- 

 tom, below a depth of 15 or 20 feet. Their abrading action cannot, therefore, 

 shear off cliffs, or wear away an island in the ocean, to a lower level. This 

 principle is recognized in making defense walls of masonry against breakers 

 by planting the wall out where the depth is 15 to 20 feet. 



Waves, as they march up a shore, sometimes throw stones to great heights. 

 Geikie cites the report that during northwesterly gales the windows of the 

 Dunnet Head Lighthouse, at the northern extremity of Scotland, 300 feet 

 above high water, are sometimes broken by stones from the enormous 

 breakers. 



In view of the force at work it is not surprising that, in regions like Cape 

 Horn, or the coast of Scotland, where storms are common, cliffs should under- 

 go constant degradation, be fronted by lofty castellated and needle-shaped 

 rocks, and that the land should be pierced at times for blow-holes where 

 layers of easy removal, or dikes or veins, face the breakers. 



The following figures from a memoir by Professor Shaler illustrate well 

 some of the results. They represent scenes on the coast of Maine, near 

 Mount Desert. 



Fig. 190 shows a detached rock on its march seaward ; and Fig. 191 

 a pile of displaced masses as it was left at the base of a cliff before an ele- 

 vation of the coast of 220 feet. All the processes of rock-destruction help 

 in this work of degradation, — the opening of rifts by intruding moisture, or 

 by oxidation, or by change of temperature, or by growing plants and the 

 decay of weak portions of the rocks. Under the incessant beating, every 

 stroke tends to slip out of place masses, however large, that rest on a sur- 

 face not perfectly horizontal. 



