428 DYNAMICAL GEOLOGY. — [Boom II. 
lies the zone within which the sea does its work of abrasion. To — 
_ this zone, even where the breakers are heaviest, a greater extreme 
vertical range can hardly be assigned than 300 feet, and in most cases 
it probably falls far short of that extent. 
The mechanical work of erosion by the sea is done in four ways. 
a. The enormous force of the breakers suffices to tear off 
fragments of the solid rocks. Abundant examples are furnished b 
the precipitous shores of Caithness, and of the Orkney and Shetland 
Islands. It sometimes happens that demonstration of the height to 
which the effective force of breakers may reach is furnished at light- 
houses built on exposed parts of the coast. Thus, at Unst, the most 
northerly point of Shetland, walls were overthrown and a door was 
broken open at a height of 196 feet above the sea. At the Bishop 
Rock lighthouse, on the West of England, a bell weighing 3 ewt. 
was wrenched off at a level of 100 feet above high-water mark.’ 
Some of the most remarkable instances of the power of breakers 
have been observed by Mr. Stevenson among the islands of the 
Shetland group. On the Bound Skerry he found that blocks of 
rock up to 93 tons in weight had been washed together at a height 
of nearly 60 feet above the sea, that blocks weighing from 6 to 134 
tons had been actually quarried out of their original bed, at a 
height of from 70 to 75 feet, and that a block of nearly 8 tons 
had been driven before the waves at the level of 20 feet above 
the sea, over very rough ground, to a distance of 73 feet. He 
likewise records the moving of a 980-ton block by the waves at 
Barrahead, in the Hebrides. At Plymouth, also, blocks of several . 
tons in weight have been known to be washed about the breakwater 
like pebbles.? 
8. The alternate compression and expansion of air in 
crevices of rocks exposed to heavy breakers dislocates large masses 
of stone, even above the direct reach of the waves. It is a fact 
familiar to engineers that, even from a vertical and apparently 
perfectly solid wall of well-built masonry exposed to heavy seas, 
stones will sometimes be started out of their snes) and that when 
this happens a rapid enlargement of the cavity may be effected, as if 
? T. Stevenson, op. cit. p. 31. D. A. Stevenson, Min. Proc. Inst. Civ. Engin. xlvi. 
(1876), p. 7. 2 T. Stevenson, op. cit. pp. 21-37. . 
* The student will bear in mind that the relative weight of bodies is greatly reduced 
when in water, and still more in sea-water. ‘The following examples will illustrate this 
fact (T. Stevenson’s “ Harbours,” p. 107) :— 
Specific | No. of cubic feet No. of feet to a ton 
eo eae in sea-water of spe- 
Gravity. to a ton 1n alr. cific gravity 1:028. 
—_— 
——_—_——_——_—_—_ 
BEE? in) 4k Na Ay VG ‘99 : 18°26 
Red granite .. . 7 3° 21°30 
Sandstone. .. . : : 26°00 
Cannel Coal . . . , ; 70°00 



