401. We have concluded from the facts stated above, that the level of the sea rises in the Mediterranean, and sinks in the more northern latitudes; and thence some have suspected, that the level of the sea had in general a tendency to rise towards the equator, and to sink towards the poles. This is the notion of Frisi, as has been already remarked, and he suggests, that this rise of the sea may be owing to a slight acceleration in the earth's diurnal motion. But there are facts which show, that between the tropics the relative level of the sea and land has sunk, and is lower at present than it was at some former period, probably not extremely remote. The opinion of Frisi, therefore, is unsupported by observation, and, as has been already shown, cannot be justified from theory.
Between the tropics, islands are formed from the mere accumulation of coral; and it is the peculiarity of those regions, to produce rocks that have not passed through the usual process of mineral consolidation.[208] The islets, however, which are thus formed, must have their bases laid on a solid rock, though perhaps at a great depth; and it is not probable, that after they are once raised above the surface of the sea, they can still rise farther, except by some elevation of the rock which serves as their foundation.[209] Now, at Palmerston island, which comprehends nine or ten low islets, that may be reckoned the heads of a great reef of coral rock, Captain Cook informs us of his having seen, "far beyond the reach of the sea, even in the most violent storms, elevated coral rocks, which, on examination, appeared to have been perforated in the same manner that the rocks are that now compose the outer edge of the reef. This evidently shows," he adds, "that the sea had formerly reached so far; and some of these perforated rocks were almost in the centre of the island."[210]
[208] Dr Foster, in his Voyage round the World, (vol. ii. p. 146,) gives an instance in the South Sea Islands, where the surface of the island, though entirely a coral rock, was raised forty feet above the level of the sea.
[209] A very curious account of the formation of such islands is given by A. Dalrymple, Esq., in the Philosophical Transitions, vol. lvii. p. 394.
[210] Cook's Third Voyage, vol i. p. 221.
The same excellent navigator, giving an account of the peninsula at Cape Denbigh, remarks: "It appeared to me, that this peninsula must have been an island in remote times; for there were marks of the sea having flowed over the isthmus."
402. We are here touching on one of those subjects, where we feel much the want of accurate and ancient observations, and where it is not from the infancy, but the maturity of science that any thing approaching to certainty can be looked for. The utmost that we can expect at present, is an anticipation, which future ages must certainly modify and correct. The best thing, in the mean time, that can be done for the advancement of this branch of geological knowledge, is to ascertain with exactness the relative level of the sea, and of such points upon the land as can be distinctly marked, and pointed out to succeeding ages. This is not so easy as it may at first appear. Where every object changes, it is difficult to find a measure of change, or a fixed point from which the computation may begin. The astronomers already feel this inconvenience, and when they would refer their observations to an immoveable plane, that shall preserve its position the same in all ages, they meet with difficulties, which cannot be removed but by a profound mathematical investigation.
In geology, we cannot hope to be delivered from this embarrassment in the same manner; and we have no resource but to multiply observations of the difference of level; to make them as exact as possible, and to select points of comparison that have a chance of being long distinguished. The improvements in barometrical measurements, which give such facility to the determination of heights, along with so considerable a degree of accuracy, will furnish an accumulation of facts that must one day be of great value to the geologist.
Note xxii. § 123.
Fossil Bones.