Illustrations of the Huttonian Theory of the Earth

by John Playfair

391. The shores of the Low Countries, and of Holland, have been often instanced in proof of the same kind of changes, and it has been supposed, that, independently of those artificial barriers which at present exclude the waters of the ocean from overflowing a great part of this tract, nature herself has brought it nearer to the surface than it had formerly been. It is indeed certain, that those countries, to a very great extent inland, have either been under the sea at some period, by no means remote if compared with the great revolutions of the globe, or that they are entirely alluvial, and of the same sort with the Deltas formed at the mouths of rivers. The relative changes, however, of the sea and land on this tract, have been differently represented, and I am unwilling, on that account, to found any argument on them.

392. If we proceed farther to the north, to the shores of the Baltic for instance, we have undoubted evidence of a change of level in the same direction as on our own shores. The level of this sea has been represented as lowering at so great a rate as 40 inches in a century. Celsius observed, that several rocks which are now above water, were not long ago sunken rocks, and dangerous to navigators; and he particularly took notice of one, which, in the year 1680, was on the surface of the water, and in the year 1791 was 20½ Swedish inches above it. From an inscription near Aspô, in the lake Melar, which communicates with the Baltic, engraved, as is supposed, about five centuries ago, the level of the sea appears to have sunk in that time no less than 13 Swedish feet.[202] All these facts, with many more which it is unnecessary to enumerate, make the gradual depression, not only of the Baltic, but of the whole northern ocean, a matter of certainty.

[202] Frisii Opera, tom. iii. p. 274.

393. Supposing these changes of level between the sea and land to be sufficiently ascertained, the supposition which at first occurs is, that the motion has been in the sea rather than in the land, and that the former has actually descended to a lower level. The imagination naturally feels less difficulty in conceiving, that an unstable fluid like the sea, which changes its level twice every day, has undergone a permanent depression in its surface, than that the land, the terra firma itself, has admitted of an equal elevation. In all this, however, we are guided much more by fancy than reason; for, in order to depress or elevate the absolute level of the sea, by a given quantity, in any one place, we must depress or elevate it by the same quantity over the whole surface of the earth; whereas no such necessity exists with respect to the elevation or depression of the land. To make the sea subside 30 feet all round the coast of Great Britain, it is necessary to displace a body of water 30 feet deep over the whole surface of the ocean. The quantity of matter to be moved in that way is incomparably greater than if the land itself were to be elevated; for though it is nearly three times less in specific gravity, it is as much greater in bulk, as the surface of the ocean is greater than that of this island.

394. Besides, the sea cannot change its level, without a proportional change in the solid bottom on which it rests. Though there be reason to suppose that such changes in the bottom do actually take place, yet they are probably much slower and more imperceptible than those which we are here considering. It is evident, therefore, that the simplest hypothesis for explaining those changes of level, is, that they proceed from the motion, upwards or downwards, of the land itself, and not from that of the sea. As no elevation or depression of the sea can take place, but over the whole, its level cannot be affected by local causes, and is probably as little subject to variation as any thing to be met with on the surface of the globe.

395. Other observations, however, made on different shores from the preceding, give greater certainty to this conclusion, and make it clear, that the motion or change which we are now treating of is not to be ascribed to the sea itself.

The observations just mentioned prove, that the level of the North Sea is lower now than it was heretofore; but it appears, that in the Mediterranean, the opposite takes place. Very accurate observations made by Manfredi, render it certain, that the superficies of the Hadriatic was higher about the middle of the last century, than toward the beginning of the Christian era.

Some repairs that were carrying on in the cathedral church of Ravenna, in the year 1731, afforded him an opportunity of observing, that the ancient, and probably original, pavement, was four feet and a half below the present, and nearly a foot under the level of the sea at high water.[203] Now, when the church was built, this cannot have been the position of the pavement, relatively to the level of the sea, for it would have subjected the floor to be under water twice in twenty-four hours, and must have done so the more unavoidably, because at that time (the beginning of the fifth century) the walls of Ravenna were washed by the sea. The fact that this pavement is under the high-water mark, by the quantity just mentioned, was ascertained by actual levelling. This result was confirmed by similar facts, observed by Zendrini at Venice.

[203] Commentarii Academiæ Bononiensis, tom. ii. pars 1ma, p. 237, &c. and pars 2da, p. 1. &c.

396. Manfredi himself attributes all this to the elevation of the surface of the sea, and has entered into a long calculation to ascertain at what rate that surface may be supposed to rise, on account of the earth and sand brought down by the rivers, and spread out over the bottom of the sea. But as the fact of the rise of the level of the sea is not general, and as the contrary is observed in the north seas, as already proved, this hypothesis will not explain the apparent rise in the level of the Hadriatic.