Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/299

Rh wom; on THE SE.-L] temperature of the tracts into which they ﬂow ; those from cold regions lower it. The ocean, in short, is the great distributor of temperature over the globe. As an illustra- tion the two opposite sides of the Atlantic may be taken. 'l‘he cold arctic current ﬂowing southward along the nortl1—east coast of America reduces the mean annual tem- perature of that region. On the other hand, the Gulf-stream b1'ings to the shores of the north-west of Europe a te1npera- ture much above what they would otherwise enjoy. Dublin an-.1 the south-eastern headlands of Labrador, lie in the same parallel of latitude, yet differ as much as 18° in their mean annual temperature, that of Dublin being 50°, and that of Labrador 32° Fahr. Dr Croll has calculated that the Gulf- stream conveys nearly half as much heat from the tropics as is received from the sun by the entire Arctic regions} II. EROSION. The chemical action of the sea upon the rocks of its ed and shores has not yet been properly studied.‘-’ It is evident, however, that changes analogous to those effected by fresh water on the land must be in pro- gress. Oxidation, and the formation of carbonates, no doubt continually take place. Ve may judge indeed of the nature and rapidity of some of these changes by watching the decay of stones and material employed in the construction of piers. 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. Mr lIallet——as the result of experiments with specimens sunk in the sea——con- cluded that from —,%ths to T4Uths of an inch in depth in iron castings 1 inch thick, and about 3‘-’Uths 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 he has in his possession specimens of iron which show even a more rapid rate of decay. In castings used at the Bell Rock 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 to 4797 lb, and yet presented no external appearance of decay. Another apparently sound specimen was reduced in strength from 4068 lb to 2352 lb, having lost nearly half its strength in ﬁfty years.”3 Similar results were recently ob- served by Mr Grothe, resident en- GEOLOGY 285 very slow erosion may take place even at considerable depths. It is in the upper portions of the sea, however, where, owing to currents, tides, and waves, the water suffers most disturbance, that the main mechanical erosion goes on. The depth to which the inﬂuence of waves and ground-swell may extend seems to vary greatly according to the situation. The astronomer-royal states that ground- swell may break in 100 fathoms water.4 It is common to ﬁnd boulders and shingle disturbed at a depth of 10 fathoms, and even driven from that depth to the shore, and waves may be noticed to become muddy from the Working up of the silt at the bottom when they have reached water of 7 or 8 fathoms in depth.-‘3 Gentle movement of the bottom water is said to be sometimes indicated by ripplc—marks on the ﬁne sand of the sea-ﬂoor at a depth of 600 feet. A good test for the absence of serious abrasion is furnished by the presence of ﬁne mud on the bottom. Vherever that is found, we may be tolerably sure that the bottom at that place lies beyond the reach of ordinary breaker action.“ From the upper limit at which the accumulation of mud is possible to high-water mark, and in exposed places up to 100 feet or more above high-water mark, 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 :—(l) the enormous force of the breakers suffices to tear off fragments of the solid rocks; the alternate compression and expansion of the air in the crevices of rocks exposed to heavy breakers dislocates rocks even above the limits of wave—action ; (3) the hydraulic pressure of those portions of large waves which enter ﬁssures and cavities forces asunder masses of rock; (-1) the waves make use of the loose fragments within their reach in battering down the cliffs exposed to their fury. 1.) Abundant examples of the dislodgement of huge blocks of rock from their parent masses are furnished by the precipitous shores of Caithness, and of the Orkney and Shetland Islands. It sometimes happens that demonstra- Fig. 7. 5

@- gineer at the railway bridge across _ ,-_  3 J’; ' /‘E7777/2)»? the Firth of Tay. A cast-iron cylin- W '7 K35’   E 3 E ,5”), der which had been below water for. .-- '-.§*'- ; '15‘ ""'“;‘." 5"“'".'.‘.T'°“i' ii‘ 3 3 in ‘D :_ only sixteen months was found to 5'w' _ "'5' be so corroded that a penknife co11ld R3‘ 8' 5” be stuck through it in many places. '55’ An examination of the shore will _; _, I ,.,=,, ,.,,.,E..',_. SL .92, sometimes reveal a good deal of quiet 5.2. G " haw“ ' J chemical change on the outer crust of rocks exposed to the waves. Such ,3+__s9v _,_g '0? =30 FEET rocks as basalt have their felspar decomposed, and show the presence of carbonates by effervescing briskly with acid. One of their minerals, augite, is occasionally replaced by pseudo- morp/rs of carbonate of iron. It is mainly by its mechanical action that the sea accom- plishes 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 that slow creeping of the polar water along the bottom already referred to. But where the currents are powerful enough to move grains of sand and gravel, 1 See papers by Dr Croll on “ Gulf-stream and Ocean-currents,” in (/col. Jllrt//. and Phil. .’|[a_r/. for 1869, 1870-74, and Climate and Time. 2 See Bischof’s Clzemical Geology, vol. i. chap, vii, 3 T. Stevenson on Ilarbours, p. 47. FIGS. 7 and 8.—Sections of the Bound Skerry of Whalsey. Shetland (from Stevenson's Harbours, p. 32) : a, b, c, and d, positions ot blocks moved by the sea. tion of the height to which the effective force of breakers may reach is furnished at lighthouses built on exposed parts of the coast. Thus, at Unst, a door was broken open at a height of 195 feet above the sea, and at the Bishop Rock lighthouse a bell was wrenched off at a level of 100 feet above high-water mark.7 Some of the most remark- able instances of the power of breakers have been observed by Mr Thomas Stevenson among the islands of the Shetland group. O11 the Bound Skerry he found that blocks of rock up to tons in weight had been washed together at a height of nearly 60 feet above the sea, that blocks weighing from 6 to 13-._‘,~ tons had been actually quarried out of their original bed, at a height of from 70 to 75 feet, and that a 4 Encyclopocrlia Jlldropolitana, art. “ Vavcs.’ 5 T. Stevenson on Harbours, p. 15. °Ibid. 7151.41-1 P- 31-