I.KS30N8 IN GEOLOGY. 



193 



LESSONS IN GKol.txlY. V. 



I IIK ACTION OF THE OCEAN. 



THE watt'M of the ocean are never tranquil ; thoir surface U 

 < action of the winds, and U therefore in constant 

 tli- ti.lal wave affect* the lowest depth i ; whilst por- 

 uianvnt currents traverse almost every sea. 



ii of waves. Every ono who has stood on the sea-shore 

 and watched tin- breakers roll in and dash themselves to spray 

 ugiiinst th.' cliffs, must have felt that the wearing action of 

 water on the coast must be considerable. On shores which 

 are bounded by chalk cliffs, the eea margin is rendered milky 

 J.y |.:irt. .;,- t tho chalk which the waves have separated from 

 the rocks. This destroying action of the waves is visible on 

 \. rv coast. Whore the sea-barrier is a hard and resisting 

 rock, frequently cliffs stand out of the water some distance 

 from the shore, indicating the place to which the mainland for- 

 merly stretched. Example* of this are of frequent occurrence 

 along the Scottish coast and the west coast of England. If the 

 sea-board be of a soft species of deposit, the action is of course 

 much more rapid. Thus on the coast of Yorkshire, from Brid- 

 ling ton to Spurn, some thirty-six miles, the waves erode 2J yards 

 annually, so that the sea has encroached two milos since the 

 time of the Romans. Many old maps of Yorkshire indicate that 

 villages stood whore now the waves hold undisputed possession, 

 and ports mentioned in bygone history are no longer to be 

 found. 



The same destruction is taking place on the coasts of Norfolk 

 and Suffolk. The seaport towns are being driven back by the 

 encroaching waters. The sites they occupied years ago now form 

 thoir harbours. Between Cromor and Mundesley, according to 

 the Ordnance Survey of 1838, the cliff has receded at the rate 

 of fourteen feet a year. 



On the same coast, as in Yorkshire, many villages are only 

 historical remembrances. The church tower of Eccles is still 

 seen rising out of the sea-sand, but all other remnants of the 

 village have long since succumbed to the action of the waves, 

 or have been covered with the sand-hills which move along that 

 coast. Dunwich, on the Suffolk coast, offers another remarkable 

 instance of the destruction of the sea. What is now a small 

 village was once a large and flourishing seaport ; records of the 

 town are preserved even from Domesday-book, from which we 

 gather that the sea must have encroached on the land to the 

 distance of several miles. 



The Goodwn Sands are from three to seven miles distant from 

 the Kentish coast, nearly opposite Ramsgate. Tradition relates 

 that they were once the estates of the Earl Godwin. This is so 

 far confirmed, that when in 1817 the Trinity Board proposed to 

 erect a lighthouse on the sands, and for that purpose made 

 several borings, they found that the shoals were not all com- 

 posed of sand, but after a few feet of sand blue clay was 

 reached, and finally chalk. If this be the case and there is no 

 reason for doubting the tradition some idea may be had of 

 the eroding power of the waves. The same record of devas- 

 tation may be written of all the south coast, and for a detailed 

 description the reader is referred to chapter xx. of " LyelTs 

 Principles of Geology," and to the local histories of towns which 

 are built along these shores. All coast lines are thus acted 

 upon, the destructive operations being carried on with more 

 or less activity, accordingly as the coast is low, the sea-cliffs of 

 soft material, or of hard rock. We are not now considering the 

 gains of the sea, or we might allude to the many terrible inun- 

 dations which the histories of Holland and the adjacent low- 

 lying countries chronicle, of vast tracts of land suddenly swept 

 over by the sea, to the destruction of hundreds of villages, their 

 inhabitants, and their cattle. It is true the persevering industry 

 of the Dutch has raised dykes against their great enemy, and by 

 enclosing many of the meers with such walls, and then pumping 

 out their water, they have reclaimed from the devastator much 

 of his prey. This is not our object. We only mention the 

 action of the waves as they erode the shores they wash ; the 

 particles of matter they thus mix with their waters are swept 

 away by currents, and in tranquil spots, or along the path of 

 the current, the sediment reaches the bottom, and there forma a 

 new deposit. 



Of the various oceanic currents we shall first speak of 

 The tides. All bodies attract each other; the power of the 

 force exerted depending upon the weight of the bodies and 



65 N.E. 



tin -ir distance from each other. The weight of any body is, in 

 fact, the force with which the earth attract* that body to itself. 

 The celestial bodies are all chained together by this force of 

 attraction. The son and the moon both exert an attractive in- 

 fluence on the earth, inducing our planet to approach to them ; 

 this attraction being counterbalanced by the centrifugal fore*, 

 we describe a curve, which is the resultant of those two force*. 

 Hut the surface of the earth consist* of fluid and solid; the 

 former, owing to it* mobility, exhibit* a greater tendency to 

 obey the attractive influence, and therefore rues to meet the 

 sun or the moon. 



The sun, on account of his enormous bulk, exercise* a much 

 greater attractive force on the earth than the moon, but the 

 solar tide is much less than the lunar tide, for this reason that 

 the moon, being nearer the earth, attract* the surface of the sea 

 far more than its solid bed, and therefore the water rise* in a 

 heap underneath the satellite. The sun, on the other hand, 

 being so distant, exerts nearly as much force on the surface a* 

 on the ocean-bed beneath, and therefore lifts np the water but 

 very little. Identically the same effect is produced on that part 

 of the earth most distant from the sun or moon, only in this 

 case the ocean-bed is drawn towards those bodies more rapidly 

 than the water, which is, in fact, left behind. 



When the sun and moon are either in conjunction or oppo- 

 sition that is, when the line joining them passes in the neigh- 

 bourhood of, or directly through, the earth then their attrac- 

 tive forces being united, the tidal wave will be at a maximum, 

 forming " spring tides." If they be in " quadrature" that is, 

 if the lines drawn from their centres to the earth's centre form 

 a right angle then the tides will be at a minimum, or " neap 

 tide" will result. It will be evident, then, that if the earth 

 were a world of waters, each tidal wave would pass com- 

 pletely round the earth in twenty-four hours. The existence of 

 continents materially modifies its transit, and it is driven from 

 its course, and consequently retarded. 



The great tidal wave takes its rise in the Antarctic Ocean. 

 As it traverses the ocean the water is not raised more than 

 a few feet; but when it enters a shallow sea, or an estuary, where 

 the tide finds itself in a sort of funnel, then the rise is some- 

 times as much as seventy feet, as is the case in the Bay of 

 Fundy. The wave is not a wave of transmission, but one of 

 motion, and if the particles of water were destitute of all co- 

 hesion or friction among themselves, they would only rise and 

 fall into the same place after the attraction had passed. A 

 wave of this nature is illustrated by throwing a stone into a 

 pond ; the wavelets expand from the point of disturbance, but 

 do not carry to the shore anything which floats on the surface 

 of the water, such bodies only rising as it were to allow the 

 wave to pass beneath them ; this proves that the water had 

 only an upward and downward movement as it formed the 

 wave. We shall allude to this wave of motion in speaking of 

 earthquakes. In mid-ocean the water under tidal influence 

 moves only about twelve miles, but when the wave from 

 a deep sea acts on the water of a shallow sea, or an estuary, 

 then the tides are high. Local causes have great effect on the 

 tides, so that at two seaports not many miles distant a difference 

 of many feet is found in the high-water mark. For example, at 

 the extremity of the Wash, between Norfolk and Lincolnshire, 

 the tide rises twenty-four feet, at Lowestoft only seven or 

 eight, at Cromer sixteen. 



The rapidity of the flow is sometimes very great. It is said 

 that in the Solway Frith the rising tide can overtake a man on 

 horseback. As these tidal waves beat against the shores of 

 every continent and island in the world, except those which 

 bound inland seas, twice in each day, the wearing action on the 

 shores must be immense; the ebb tide carries out the eroded 

 matter, which is either deposited in the deep sea or swept away 

 by currents, to subside at a distance. 



Cm-rents. It is impossible to draw the limit which bounds the 

 effect that ocean currents have in the re-distribution of the 

 material of the earth's surface. We cannot conceive that vast 

 volumes of water traverse the oceans without taking a very 

 active part in geological work. Of their direct action on the 

 bed of the seas through which they pass, we know little or 

 nothing ; at present they are chiefly looked upon as means oi 

 transportation. 



The system of ocean currents may be thus described : The 

 trade winds, as is known, are caused by the heated air rising 



