ASCENSION.
Basaltic lavas.
Numerous craters truncated on the same side.
Singular structure of volcanic bombs.
Aeriform explosions.
Ejected granitic fragments.
Trachytic rocks.
Singular veins.
Jasper, its manner of formation.
Concretions in pumiceous tuff.
Calcareous deposits and frondescent incrustations on the coast.
Remarkable laminated beds, alternating with, and passing into, obsidian.
Origin of obsidian.
Lamination of volcanic rocks.
(MAP 2: THE ISLAND OF ASCENSION.)
This island is situated in the Atlantic Ocean, in latitude 8 degrees S., longitude 14 degrees W.It has the form of an irregular triangle (see Map 2), each side being about six miles in length.Its highest point is 2,870feet ("Geographical Journal" volume 5 page 243.) above the level of the sea.The whole is volcanic, and, from the absence of proofs to the contrary, I believe of subaerial origin.The fundamental rock is everywhere of a pale colour, generally compact, and of a feldspathic nature.In the S.E.portion of the island, where the highest land is situated, well characterised trachyte, and other congenerous rocks of that varying family, occur.Nearly the entire circumference is covered up by black and rugged streams of basaltic lava, with here and there a hill or single point of rock (one of which near the sea-coast, north of the Fort, is only two or three yards across) of the trachyte still remaining exposed.
BASALTIC ROCKS.
The overlying basaltic lava is in some parts extremely vesicular, in others little so; it is of a black colour, but sometimes contains crystals of glassy feldspar, and seldom much olivine.These streams appear to have possessed singularly little fluidity; their side walls and lower ends being very steep, and even as much as between twenty and thirty feet in height.
Their surface is extraordinarily rugged, and from a short distance appears as if studded with small craters.These projections consist of broad, irregularly conical, hillocks, traversed by fissures, and composed of the same unequally scoriaceous basalt with the surrounding streams, but having an obscure tendency to a columnar structure; they rise to a height between ten and thirty feet above the general surface, and have been formed, as Ipresume, by the heaping up of the viscid lava at points of greater resistance.At the base of several of these hillocks, and occasionally likewise on more level parts, solid ribs, composed of angulo-globular masses of basalt, resembling in size and outline arched sewers or gutters of brickwork, but not being hollow, project between two or three feet above the surface of the streams; what their origin may have been, I do not know.
Many of the superficial fragments from these basaltic streams present singularly convoluted forms; and some specimens could hardly be distinguished from logs of dark-coloured wood without their bark.
Many of the basaltic streams can be traced, either to points of eruption at the base of the great central mass of trachyte, or to separate, conical, red-coloured hills, which are scattered over the northern and western borders of the island.Standing on the central eminence, I counted between twenty and thirty of these cones of eruption.The greater number of them had their truncated summits cut off obliquely, and they all sloped towards the S.E., whence the trade-wind blows.(M.Lesson in the "Zoology of the Voyage of the 'Coquille'" page 490 has observed this fact.Mr.Hennah ("Geolog.Proceedings" 1835 page 189) further remarks that the most extensive beds of ashes at Ascension invariably occur on the leeward side of the island.) This structure no doubt has been caused by the ejected fragments and ashes being always blown, during eruptions, in greater quantity towards one side than towards the other.M.Moreau de Jonnes has made a similar observation with respect to the volcanic orifices in the West Indian Islands.
VOLCANIC BOMBS.
(FIGURE 3: FRAGMENT OF A SPHERICAL VOLCANIC BOMB, with the interior parts coarsely cellular, coated by a concentric layer of compact lava, and this again by a crust of finely cellular rock.
