‘ 
_ Parr Il. Sect. ii. §.6.] MARINE EROSION. ae 
It is evident, however, that changes analogous to those effected by fresh 
- water on the land must be in progress. Oxidation, and the formation 
of carbonates, no doubt continually take place. We may judge 
indeed of the nature and rapidity of some of these changes by 
watching the decay of stones and material employed in the con- 
struction of piers. Mr. Mallet—as the result of experiments with 
specimens sunk in the sea—concluded that from 2, to 4; of 
an inch in depth in iron castings 1 inch thick, and about 9 of an 
inch of wrought iron, will be destroyed in a century in clear salt 
water. Mr. Stevenson, in referring to these experiments, remarks 
that at the Bell Rock lighthouse, twenty-five different kinds and 
combinations of iron were exposed to the action of the sea, and all 
yielded to corrosion. In some of these castings the loss has been at 
the rate of an inch in a century. “One of the bars which was free 
from air-holes had its specific gravity reduced to 5°63, and its 
transverse strength from 7409 lb. to 4797 lb., and yet presented no 
external appearance of decay. Another apparently sound specimen 
was reduced in strength from 4068 Ib. to 2352 lb., having lost nearly 
half its strength in fifty years.”? Similar results were recently 
observed by Mr. Grothe, resident engineer at the construction of the 
ill-fated railway bridge across the Firth of Tay. A cast-iron cylinder 
(such as was employed in constructing the concrete basements for 
the piers), which had been below water for only sixteen months, was 
found to be so corroded that a penknife could be stuck through it in 
many places. An examination of the shore will sometimes reveal a 
good deal of quiet chemical change on the outer crust of wave-washed 
rocks. Basalt, for instance, has its felspar decomposed, and shows the 
presence of carbonates by effervescing briskly with acid. The 
augite is occasionally replaced by ferrous carbonate. 
B. Mechanical——lIt is mainly by its mechanical action that the 
sea accomplishes its erosive work. This can only take place where 
the water is in motion, and, other things being equal, is greatest 
where the motion is strongest. Hence we cannot suppose that 
erosion to any appreciable extent can be effected in the abysses of 
the sea, where the only motion possible is the slow creeping of the 
polar water. But where the currents are powerful enough to move 
grains of sand and gravel, a slow erosion may take place even at 
considerable depths. It is in the upper portions of the sea, however, - 
—the region of currents, tides, and waves,—that mechanical erosion 
is chiefly performed. The depth to which the influence of waves and 
ground-swell may extend seems to vary greatly according to the 
situation (ante, p. 423). A good test for the absence of serious abrasion 
is furnished by the presence of fine mud on the bottom. Wherever 
that is found, we may be tolerably sure that the bottom at that 
place lies beyond the reach of ordinary breaker action.” From the 
superior limit of the accumulation of mud up to high-water mark, 
and in exposed places up to.100 feet or more above high-water mark, 
1 'T. Stevenson on “ Harbours,” p. 47. , 2 Ibid. p. 15. 
