第16章

Under Signal Post Hill, the white stratum dips into the sea in a remarkable manner.This hill is conical, 450 feet in height, and retains some traces of having had a crateriform structure; it is composed chiefly of matter erupted posteriorly to the elevation of the great basaltic plain, but partly of lava of apparently submarine origin and of considerable antiquity.The surrounding plain, as well as the eastern flank of this hill, has been worn into steep precipices, overhanging the sea.In these precipices, the white calcareous stratum may be seen, at the height of about seventy feet above the beach, running for some miles both northward and southward of the hill, in a line appearing to be perfectly horizontal;but for a space of a quarter of a mile directly under the hill, it dips into the sea and disappears.On the south side the dip is gradual, on the north side it is more abrupt, as is shown in Figure 2.As neither the calcareous stratum, nor the superincumbent basaltic lava (as far as the latter can be distinguished from the more modern ejections), appears to thicken as it dips, I infer that these strata were not originally accumulated in a trough, the centre of which afterwards became a point of eruption; but that they have subsequently been disturbed and bent.We may suppose either that Signal Post Hill subsided after its elevation with the surrounding country, or that it never was uplifted to the same height with it.This latter seems to me the most probable alternative, for during the slow and equable elevation of this portion of the island, the subterranean motive power, from expending part of its force in repeatedly erupting volcanic matter from beneath this point, would, it is likely, have less force to uplift it.Something of the same kind seems to have occurred near Red Hill, for when tracing upwards the naked streams of lava from near Porto Praya towards the interior of the island, I was strongly induced to suspect, that since the lava had flowed, the slope of the land had been slightly modified, either by a small subsidence near Red Hill, or by that portion of the plain having been uplifted to a less height during the elevation of the whole area.

THE BASALTIC LAVA, SUPERINCUMBENT ON THE CALCAREOUS DEPOSIT.

This lava is of a pale grey colour, fusing into a black enamel; its fracture is rather earthy and concretionary; it contains olivine in small grains.The central parts of the mass are compact, or at most crenulated with a few minute cavities, and are often columnar.At Quail Island this structure was assumed in a striking manner; the lava in one part being divided into horizontal laminae, which became in another part split by vertical fissures into five-sided plates; and these again, being piled on each other, insensibly became soldered together, forming fine symmetrical columns.The lower surface of the lava is vesicular, but sometimes only to the thickness of a few inches; the upper surface, which is likewise vesicular, is divided into balls, frequently as much as three feet in diameter, made up of concentric layers.The mass is composed of more than one stream; its total thickness being, on an average, about eighty feet:

the lower portion has certainly flowed beneath the sea, and probably likewise the upper portion.The chief part of this lava has flowed from the central districts, between the hills marked A, B, C, etc., in the woodcut-map.The surface of the country, near the coast, is level and barren;towards the interior, the land rises by successive terraces, of which four, when viewed from a distance, could be distinctly counted.

VOLCANIC ERUPTIONS SUBSEQUENT TO THE ELEVATION OF THE COASTLAND; THEEJECTED MATTER ASSOCIATED WITH EARTHY LIME.

These recent lavas have proceeded from those scattered, conical, reddish-coloured hills, which rise abruptly from the plain-country near the coast.

I ascended some of them, but will describe only one, namely, RED HILL, which may serve as a type of its class, and is remarkable in some especial respects.Its height is about six hundred feet; it is composed of bright red, highly scoriaceous rock of a basaltic nature; on one side of its summit there is a hollow, probably the last remnant of a crater.Several of the other hills of this class, judging from their external forms, are surmounted by much more perfect craters.When sailing along the coast, it was evident that a considerable body of lava had flowed from Red Hill, over a line of cliff about one hundred and twenty feet in height, into the sea: