Tongataboo, already mentioned, is ten feet above the high water-mark, which is a greater elevation than can be produced by the action of the sea ; supposing that all that space consisted of fragments, such as have been de scribed, and not of perfect corals, which cannot raise them selves to the least distance above the sea. But Captain Cook observed in many islands, that the corals, with all their characters as perfect as if they had been alive, were found at elevations of even 100 feet above this. It is very certain that the ocean could not have been depressed by this quantity, or rather, that it never could have stood i00 feet higher than it does at present, so that we must con clude that the island has been elevated. It is not difficult to trace the causes to which this is owing, which have produced in the bottom of the ocean the changes neces sary for the production of these effects, and it will be seen that they must have depended on the action of volcanic powers. We shall be at no loss in discovering the ex istence of this cause in many places, but the following isl ands will afford as convenient and satisfactory proofs of it as any other.
If we take the two islands of Tongatahoo and Eeooa, we shall find that they form the first link in this chain, and one which is peculiarly valuable, from the proximity of these two tracts of coral land. Eeooa is separated from the former island by a distance of only twenty miles. This island consists of a hill of considerable elevation, although its height is unfortunately not given in Cook's narrative. This, however, is not of any moment for the present pur pose, as the essential circumstance is, that coral was ob served on it at 300 feet above the level of the sea, continu ing to near the summit. The soil above the coral is de scribed as consisting of a soft yellow sandstone and a red dish clay. Now the position of the coral is here, such as, even in a greater degree than in the preceding instances, to indicate the former existence of a force which must have raised it to that height above the level of the sea. From the proximity of these two islands, it is also proba ble that both of them were raised at the same time, by the same force, and that the chief power was exerted under Eeooa, while the much lower island of Tongataboo was raised to so inconsiderable a height, comparatively, be cause it lay on the verge, or towards the evanescent mar gin of the expansive and elevating power. No other cause is adequate to the production of these effects, and it is evident that the action which produced the greatest is also capable of accounting for the least of these.
Now, although it may be said that no volcano exists in Eeooa, and that such a cause cannot therefore be admitted, it will be sufficient to show that volcanoes have, in other instances, and in this sea, exerted that very action. and in
such a manner, that the coral rises upon the sides of the volcanic mountain, proving in these cases, what may safely be inferred for the others, that it is not only capable of producing the required effects, but that, in these in stances, it has actually produced them. That force, there fore, which has exerted its action so as entirely to erupt the volcanic matter, may well be allowed to have also ex erted that much less one, which was sufficient, as in the case of earthquakes, to alter the level of the submarine land.
The elevation of volcanic islands from these and other seas, where no coral rocks are present, is too common a phenomena to require mention, and it justifies this expla nation equally. But•it is placed in even a stronger point of view by analogous phenomena, visible in the volcanic islands of the African coast, where limestone, of a charac ter quite distinct from any that are known elsewhere, are found at great elevations, and incumbent on•the volcanic socks, which seem, in the same manner, to have lifted them up from the bottom of the ocean. Indeed, this is such a necessary consequence of submarine volcanoes, that it must have been expected, though no instance of it should have been produced. The analogy of Italy con firms the same views; and by this are explained the pecu liar circumstances of Monte Bolca, as well as of the gene ral Suhappenine deposits mentioned in other parts of this essay and in their proper places.
Of Fossil Shells.
The number of the fossil shells in the different genera is so great, amounting to many hundreds, that we cannot pretend to notice more than a few oc the most remarkable examples. The description of this department alone, would occupy a large book. They are found in all the strata, of whatever nature, and in all parts of the series, and are consequently changed in all the various modes that have formerly been described. They often abound so much where they do occur, as to form by themselves strata of enormous dimensions.
Chiton octovalvis, one of the rarest fossil shells, occurs at Grignon. The fossil species of the genus Patella are numerous. In the genus Murex, Lamarck has found se venteen species near Paris, and thirty-three in that of Fu sus. In the same district, so fertile in these remains, the genus Pleurotoma contains twenty-five species. Cerithiuni is equally remarkable for its numbers, sixty species oc curring in the same district. This is a very ornamental tribe, owing to the great variety in the disposition of the tubercles. The Giganteum is very remarkable from its great length. Planorbis and Helix are chiefly fresh-water shells, and remarkable for their tender and delicate struc ture. They are often preserved in the surrounding mat ter without petrifaction.