In considering the Physical Changes of the Ocean, we find that the relations of sea and land in respect of area, height and depth, interchange of moisture, aria' currents, and many other circumstances which influence mechanical, chemical, and vital phenomena on our globe, are so important, and, within the compass of the few hundreds or thousands of years which belong to history or tradition, appear to hare suffered so little change, that nothing short of the complete proof furnished by geology ought to satisfy our minds that every one of the conditions which make up the now harmonious mutual dependence of land and sea is variable, that the present aspect of the globe is but one term of a long series of successive transformations, the law of which— the great problem of geology—is perhaps not beyond the reach of observation and induction. But as we proceed in this inquiry, we learn the important lesson that the present history of any of the great elements of terrestrial nature is imperfect, cannot in fact be ascer tained, without the light derived from its past history, of which it is the continuous production. The sciences of Geology and Physical Geography are thus found to be parts of each other, alternately, according to the aspect in which they are viewed. Nearly all aqueous geology, or the knowledge of the deposition of the fossiliferous strata, consists in investigating the physical changes of the sea during past ages of the earth's history, of which the changes it is now undergoing are the ca-er-proceeding completion.
Are the relative areas of sea and land constant f To this geology replies, that what is now land was formerly the sea ; that in some of the parts which are now covered by water land anciently existed, so as to pour down rivers, conveying sediments of different sorts through valleys and plains clothed with various vegetation. The land which we behold is the bed of the ancient sea ; or, to speak more precisely, it is composed of the dried indurated sediments and the cooled volcanic products which, during long intervals of time, were accumulated beneath the ocean. Whether, during the process by which the ancient sea-bed was raised to constitute our now dry land, a proportionate area of what was formerly land was depressed to constitute the modern sea-bed, is an unsettled question ; but it is clearly proved that if any proportion of areas between sea and laud is a necessary condition of our globe, all the parts of these areas are displaceable and have been displaced.
Is the relative level of the land and sea constant ? We cannot affirm it. We cannot deny the existence of causes which may change generally and continually this relation, any more than the operation of agencies which locally and at intervals are known to derange it. If there be a general change of temperature in the earth itself, or com municated from the planetary spaces around it, or occasioned by any condition affecting the radiation of heat in the atmosphere above it ; the unequal Influence of this change on the unequally expanding and contracting liquid and solid masses will necessarily occasion variations in the relative level of sea and land. Now, geology appears to have established many facts regarding the fossil organic remains of plants and animals, which admit of no clear general explanation except by supposing extensive, perhaps general changes of climate. There is
nothing positively known which forbids the belief that such changes may be still really, however slowly, in progress ; there is abundant proof of innumerable local derangements of the level of land and sea in comparatively modern geological, historical, and even very recent periods, and a slow upward movement of land is actually traceable and measurable on the coast of Sweden and Norway.
A change of a few degrees of temperature, a change of the relative height of tho land, a change even in the polar distance of the masses of land, would materially affect many secondary phenomena. The atmosphere would be affected in regard to its moisture, translucency, rate of diminishing temperature, prevalent winds, and quantity of rain. Its power of sustaining particular organic structures both on the land and in the sea would be altered ; and thus we see in the variable nature of the relations which unite the land and the sea, elements of continual change in the mechanical, chemical, and vital phenomena of the globe.
The ocean is continually receiving the spoils of the land, by means of the rivers and the action of the waves on the seacoast; and from that cause, theoretically, would be constantly decreasing in depth, and as the quantity of water is always the same, its superficial extent would increase and its level consequently rise. There are, however, counteracting causes to check this tendency ; the secular elevation, or gradual elevation during the course of ages, over extensive tracts in many parts of the world, is one of the most important of these. Volcanos, coral-islands, as well as the coral structures called barrier reefs, show that great changes of level are constantly taking place in the bed of the ocean itself. In addition to these, it has been con clusively evinced by Mr. Darwin, that symmetrical bands in which the bed of the sea is subsiding below and being elevated above its level at a given time, extend alternately over an area (in the Pacific and South Atlantic oceans) equal to half that of the entire globe. From all these facts it may be concluded that the balance is always maintained be tween the land and the sea, although the distribution of each, respec tively, may vary in the lapse of time. But independently of all these counteractions, so immense is the proportion which the magnitude of our planet bears to the power of the agencies which are at work upon its surface, that, as has been shown by Mr. Alfred Tyler, in a com munication to the Geological Society, the action of the sea on the shores, and of the rivers upon the land,—if the matter they all wear from the land, and carry down in the form of sand and mud, together with that, probably still greater in quantity, which they carry down In chemical solution, to he decomposed iu the sea, and in part precipi tated on its bed,—were uniformly distributed over the entire bed of the ocean, the level of the water would not be raised more than three inches in ten thousand years, which is an amount absolutely insignifi cant, and would not be measurable for many times that period, nor otherwise sensible to man for many more.