QE26 v. 2 PRINCIPLES OF GEOLOGY. Differences in the rise of the tides Rennell's Account of the currents Action of the sea on the British coast (p. 12.)— Shetland Islands - Large blocks removed- Effects of light- ning Isles reduced to clusters of rocks - Orkney Isles East coast of Scotland (p. 19.)-East coast of England. Waste of the cliffs of Holderness, Norfolk, and Suffolk Silting up of estuaries (p. 27.)- Origin of submarine forests - Yarmouth estuary - Suffolk coast - Dunwich (p. 31.)— ALTHOUGH the movements of great bodies of water, termed tides and currents, are in general due to very M362304 changes which are objects of geological interest. These forces may be viewed in the same manner as we before considered rivers, first, as employed in destroying portions of the solid crust of the earth, and removing them to other places; secondly, as reproductive of new strata. Tides. It would be superfluous at the present day to offer any remarks on the cause of the tides. They are not perceptible in lakes, or in most inland seas; in the Mediterranean even, deep and extensive as is that sea, they are scarcely sensible to ordinary observation, their effects being quite subordinate to those of the winds and currents. In some places however, as in the Straits of Messina, there is an ebb and flow to the amount of two feet and upwards; at Naples and at the Euripus, of twelve or thirteen inches; and at Venice, according to Rennell, of five feet. In the Syrtes, also, of the ancients, two wide shallow gulfs which penetrate very far within the northern coast of Africa, between Carthage and Cyrene, the rise is said to exceed five feet.† In islands remote from any continent, the ebb and flow of the ocean is very slight, as at St. Helena, for example, where it is rarely above three feet. In any given line of coast, the tides are greatest in narrow channels, bays, and estuaries, and least in the intervening tracts where the land is prominent. Thus, at the entrance of the estuary of the Thames and Medway, the rise of the spring-tides is eighteen feet; but when we follow our eastern coast from thence northward, * Geography of Herodotus, vol. ii. p. 331. + Ibid. p. 328. Romme, Vents et Courans, vol. ii. p. 2. Quart. Journ. of Science, March, 1829. Rev. F. Fallows, towards Lowestoff and Yarmouth, we find a gradual diminution, until, at the places last mentioned, the highest rise is only seven or eight feet. From this point there begins again to be an increase, so that at Cromer, where the coast again retires towards the west, the rise is sixteen feet; and towards the extremity of the gulf called "the Wash," as at Lynn and in Boston deeps, it is from twenty-two to twenty-four feet, and in some extraordinary cases twenty-six feet. From thence again there is a decrease towards the north, the elevation at the Spurn Point being from nineteen to twenty feet, and at Flamborough Head and the Yorkshire coast from fourteen to sixteen feet.* At Milford Haven in Pembrokeshire, at the mouth of the Bristol Channel, the tides rise thirty-six feet; and at King-Road near Bristol, forty-two feet. At Chepstow on the Wye, a small river which opens into the estuary of the Severn, they reach fifty feet, and sometimes sixtynine, and even seventy-two feet. A current which sets in on the French coast, to the west of Cape La Hague, becomes pent up by Guernsey, Jersey, and other islands, till the rise of the tide is from twenty to fortyfive feet, which last height it attains at Jersey, and at St. Malo, a seaport of Brittany. Currents.-The most extensive and best determined system of currents, is that which has its source in the Indian Ocean, under the influence of the trade winds; and which, after doubling the Cape of Good Hope, inclines to the northward, along the western coast of Africa, then crosses the Atlantic, near the equator, and is lost in the Caribbean Sea, yet seems to be again *The heights of these tides are given on the authority Captain Hewett, R.N. † On the authority of Captain Beaufort, R.N. revived in the current which issues from the gulf of Mexico, by the straits of Bahama, and flows rapidly in a north-easterly direction by the bank of Newfoundland, towards the Azores. We learn from the posthumous work of Rennell on this subject, that the Lagullas current, so called from the cape and bank of that name, is formed by the junction of two streams, flowing from the Indian Ocean; the one from the channel of Mozambique, down the south-east coast of Africa; the other, from the ocean at large. The collective stream is from ninety to one hundred miles in breadth, and runs at the rate of from two and a half to more than four miles per hour. It is at length turned westward by the Lagullas bank, which rises from a sea of great depth to within one hundred fathoms of the surface. It must, therefore, be inferred, says Rennell, that the current here is more than one hundred fathoms deep, otherwise the main body of it would pass across the bank, instead of being deflected eastward, so as to flow round the Cape of Good Hope. From this cape it flows northward, along the western coast of Africa, taking the name of the South Atlantic current. It then enters the Bight, or Bay of Benin, and is turned westward, partly by the form of the coast there, and partly, perhaps, by the Guinea current, which runs from the north into the same great bay. From the centre of this bay proceeds the Equatorial current, holding a westerly direction across the Atlantic, which it traverses, from the coast of Guinea to that of Brazil, flowing afterwards by the shores of Guiana to the West Indies. The breadth of this current varies from 160 to 450 geographical miles, and its velocity is from twenty-five to seventy-nine |