THEORY OF THE OCEAN'S LEVEL,

AS AFFECTED BY THE RISING OR SINKING OF THE LAND.

THE mean level of the sea cannot be regarded as a fixed line, unless, during the geologic changes of the past, it has invariably maintained the same distance from the earth's centre. If the earth, in consequence of the expansive influence of a vastly higher temperature than that which in the present era it possesses, was once greatly bulkier than it is now, the line, in proportion to the bulk, would be farther removed than it is now from the centre. The sea would stand greatly higher than at its present line. And who that has surveyed the contortions, the bends, the inflections, the ever-recurring rises and falls, of the more ancient stratified rocks, such as our Scotch grauwacke for instance, bends and inflections that forcibly remind the geologist of the foldings of a loose robe, grown greatly. too large for the shrunken body which it covers, or that has weighed the yet farther evidence furnished by the carboniferous vegetation, extra-tropical in character even in Greenland, who, I say, that has considered this evidence will venture to decide that the earth's temperature was not higher, nor the earth's radius greater, in the days of the Silurian period, or of the Coal Measures, than it is now? And, of course, if the earth's radius was greater,

the level line of the sea must have stood higher, — vastly higher, it seems not impossible, than the line now touched by the summits of our highest mountains. Had there been a graduated pole of adamant, equal in length to the radius of the globe, placed in that ocean of the Silurian period in which our Scotch graptolites lived,-a pole with its lower end fixed immovably at the earth's centre, and its upper end level with the medium surface of the sea, where, I marvel, would that upper end be now? High, I suspect in the clouds; nay, in an attenuated atmosphere, to which cloud never now ascends. The graduated markings of the pole, indicatory not merely of how the tide, but also of how the land, has fallen, would, I doubt not, be found more conveniently summable in leagues than in fathoms.

But even setting aside all this as fanciful and extravagant, even taking it as a given fact (what, I suspect, is no fact at all) that the earth's bulk has not very materially altered, the line of the sea-level may have, notwithstanding, been considerably affected simply by the rise of the land. It is estimated that about one-fourth part of the surface of the globe is occupied, according to the present distribution of oceans and continents, by land, and the remaining three-fourths by water; or, more correctly, that the land is as one, and the water as 2.76. Let us suppose this fourth part of land annihilated to the mean depth of the ocean. Of course, the effect would be, that the ocean, having then to cover four parts, instead of three, would sink, all over the globe, exactly one-fourth part of its mean depth. If the mean depth of the ocean be, as has been estimated, four miles, the fall in its level that would take place, in consequence of this annihilation of the land, would be just a single mile. And, of course, a creation

of land at the bottom of the sea, which would rise to its surface, would, on the same principle, and in exactly the same ratio, have the effect of elevating the ocean level. It would do on a large scale what the pebbles dropped by the crow in the fable into the pitcher did on a small one. Nor must it be forgotten, that though creation and annihilation are terms which may seem suggestive of the fanciful and the extravagant, there are phenomena exceedingly common in nature which, for all the purposes of my argument, would have exactly the effect of the things which these terms signify. In intense cold, the mercury in a thermometer is confined to the bulb of the instrument; plunged into boiling water, it straightway rises two hundred and twelve degrees in the tube; and, when a second time subjected to the intense cold, it sinks again into the bulb, as at first. So far as mere bulk is concerned, there takes place what is analogous to a creation and annihilation of the quantity of mercury in the tube. Again, if a rod of lead a mile in length be raised in temperature from the freezing point to the point at which water boils, it lengthens rather more than five yards;what is equal to a creation of five yards of lead-rod has been effected. Cooled down again, however, the five yards are annihilated. A rod of flint-glass of the same length, raised to the same temperature, would stretch out only four feet, two inches, and rather more than seven lines. All the metals-even platinum expand more than glass; but were there some deep-lying stratum, five miles in thickness, of that portion of the earth's crust on which Great Britain rests, to be heated two hundred and twelve degrees above its present temperature, it would at even this comparatively low rate of expansion elevate the island more than twenty feet higher than now over the

existing sea-level, a height fully equal to that of by far the best marked of our ancient coast lines. And if this increase in temperature took place, not in a stratum of the the earth's crust five miles in thickness, underlying Great Britain, but in a stratum twenty miles in thickness, underlying one-fourth the area of the bed of the ocean, the effect would of course be of a reverse character. This creation of land at the bottom of the sea would raise the ocean level nearly twenty feet all over the globe, and send the waves dashing around our own shores, against the ancient coast line, as of cld.

Nor do I see that the bearing of these consequences on the sea-line consequences that would render its level dependent on the elevation or submergence of every continent that has existed, or shall yet ex.st can be set aside, save on the supposition that for every tract of land that rises, another tract of the same area and cubic contents sinks; or, to state the case in other words, and more definitely, that for every protuberance formed within the sea, there is a corresponding hollow formed also within it elsewhere. Now, even were it to be granted that for every protuberance which rises on the earth's crust there is a corresponding depression of the surface, which takes place somewhere else (though on what principle this should be granted is not in the least obvious), I do not at all perceive why that depression should always take place within the sea. It may take place, not on any of the parts of the earth's surface covered by water, but on that fourth part occupied by land. It may take place on the table-land of a continent. Or, vice versa, a hollow formed in the sea, considerable enough to lower the sea's level, may find its counterbalancing protuberance in the further elevation of the interior of some vast tract, such as Asia or New Hol

land, already raised over the ocean. The submerged continent of the Pacific, which now exists but as a wilderness of scattered atolls, may have been the contemporary with that of South America, existing at the time as a flat tract, which simply occupied a certain area in the sea; and the hollow which the submergence of the Polynesian land occasioned may possibly have been balanced by the rise of those enormous table-lands of Mexico and the adjacent countries that give to the entire continent in which they are included a mean elevation of more than a thousand feet; or the submergence of that Atlantis which was drained by the great rivers of the Wealden period may have been balanced, in like manner, by the rise of the still more extensive table-land of Asia: and in both cases the level of the sea could not fail to be very sensibly lowered. It would have in each instance the area of the submerged continent to occupy; and there would be no corresponding elevation within its bed, to balance against the waste by the space which it filled. But why, I repeat, the balancing theory at all? If elevations or depressions can, as has been shown, be mere results of changes of temperature in portions of the earth's crust, why deem it more necessary to hold that there is a refrigerating process taking place under one area, in the exact proportion in which there is a heating process taking place under another, than to hold that when the mercury is rising in the tube of a thermometer, it is sinking in some other tube attached to the instrument, but not visible? The argument, however, is one of those which can be reasoned out more conclusively by lines than by words. It will be found, too, that the lines make out not only a more conclusive, but also a stronger case.