第50章

Seale has found the bones of the tropic-bird and of the albatross; the former now rarely, and the latter never visiting the island.From the difference between these layers, and the sloping piles of detritus which rest on them, I suspect that they were deposited, when the gorges stood beneath the sea.Mr.Seale, moreover, has shown that some of the fissure-like gorges become, with a concave outline, gradually rather wider at the bottom than at the top; and this peculiar structure was probably caused by the wearing action of the sea, when it entered the lower part of these gorges.(A fissure-like gorge, near Stony-top, is said by Mr.Seale to be 840 feet deep, and only 115 feet in width.) At greater heights, the evidence of the rise of the land is even less clear: nevertheless, in a bay-like depression on the table-land behind Prosperous Bay, at the height of about a thousand feet, there are flat-topped masses of rock, which it is scarcely conceivable, could have been insulated from the surrounding and similar strata, by any other agency than the denuding action of a sea-beach.Much denudation, indeed, has been effected at great elevations, which it would not be easy to explain by any other means: thus, the flat summit of the Barn, which is 2,000 feet high, presents, according to Mr.

Seale, a perfect network of truncated dikes; on hills like the Flagstaff, formed of soft rock, we might suppose that the dikes had been worn down and cut off by meteoric agency, but we can hardly suppose this possible with the hard, basaltic strata of the Barn.

COAST DENUDATION.

The enormous cliffs, in many parts between one and two thousand feet in height, with which this prison-like island is surrounded, with the exception of only a few places, where narrow valleys descend to the coast, is the most striking feature in its scenery.We have seen that portions of the basaltic ring, two or three miles in length by one or two miles in breadth, and from one to two thousand feet in height, have been wholly removed.There are, also, ledges and banks of rock, rising out of profoundly deep water, and distant from the present coast between three and four miles, which, according to Mr.Seale, can be traced to the shore, and are found to be the continuations of certain well-known great dikes.The swell of the Atlantic Ocean has obviously been the active power in forming these cliffs; and it is interesting to observe that the lesser, though still great, height of the cliffs on the leeward and partially protected side of the island (extending from the Sugar-Loaf Hill to South West Point), corresponds with the lesser degree of exposure.When reflecting on the comparatively low coasts of many volcanic islands, which also stand exposed in the open ocean, and are apparently of considerable antiquity, the mind recoils from an attempt to grasp the number of centuries of exposure, necessary to have ground into mud and to have dispersed the enormous cubic mass of hard rock which has been pared off the circumference of this island.The contrast in the superficial state of St.Helena, compared with the nearest island, namely, Ascension, is very striking.At Ascension, the surfaces of the lava-streams are glossy, as if just poured forth, their boundaries are well defined, and they can often be traced to perfect craters, whence they were erupted; in the course of many long walks, I did not observe a single dike; and the coast round nearly the entire circumference is low, and has been eaten back (though too much stress must not be placed on this fact, as the island may have been subsiding) into a little wall only from ten to thirty feet high.Yet during the 340 years, since Ascension has been known, not even the feeblest signs of volcanic action have been recorded.(In the "Nautical Magazine" for 1835page 642, and for 1838 page 361, and in the "Comptes Rendus" April 1838, accounts are given of a series of volcanic phenomena--earthquakes--troubled water--floating scoriae and columns of smoke--which have been observed at intervals since the middle of the last century, in a space of open sea between longitudes 20 degrees and 22 degrees west, about half a degree south of the equator.These facts seem to show, that an island or an archipelago is in process of formation in the middle of the Atlantic: a line joining St.Helena and Ascension, prolonged, intersects this slowly nascent focus of volcanic action.) On the other hand, at St.Helena, the course of no one stream of lava can be traced, either by the state of its boundaries or of its superficies; the mere wreck of one great crater is left; not the valleys only, but the surfaces of some of the highest hills, are interlaced by worn-down dikes, and, in many places, the denuded summits of great cones of injected rock stand exposed and naked; lastly, as we have seen, the entire circuit of the island has been deeply worn back into the grandest precipices.

CRATERS OF ELEVATION.