Chart of the Island of Ascension, with Remarks on its Geognosy. By Captain Robert Campbell, R.N. Communicated by the Author.
This island, situated in the Atlantic Ocean, in South Lat. 7° 55’, West Long. 14° 51’, is about nine miles in length from SE. to NW., and about five or six miles broad*. During the time of Buonaparte’s confinement in St. Helena, it was judged prudent to keep a small force there. For some time I had the command of the party, and employed myself in making a chart of the island, which I now communicate to the public. In the chart, the principal stations which served for its construction, and the more remarkable points, are marked O.
The angles of the chain of triangles which connect the stations, were taken with a sextant ; and, as their sides were therefore not on a horizontal plane, their inclinations were measured, and their horizontal projections found, by reducing the oblique lines in proportion of radius to the cosines of their inclination.
The positions of the intermediate points were determined by observations made at the principal stations ; but it was not thought necessary to apply reduction to the sides of these secondary triangles, on account of their obliquity.
The height of the Green Mountain (one of the stations), was found, by taking its elevation with the sextant and an artificial horizon, above a station on the sea-coast ; and the height of this station above the level of the sea was carefully measured. As the other mountains were too low to be seen from the sea-coast in the artificial horizon, their heights were found by taking, with the sextant, their angles of elevation at the several stations on the coast, above objects on a level with the eye, and in vertical planes passing through the eye and their summits. The level was determined by looking through a tube to which a spirit-level was fixed.
The whole island has a most forbidding and rugged aspect. Its highest mountain, named Green Mountain Peak, is 2818 feet above the level of the sea. The largest portion of the mountain is 2000 feet above the sea ; and at this height there is a space of comparatively level ground, in which the principal garden in the island is situated. From the top of the Peak down to about this level, or a little lower, the surface, excepting where it is precipitous, is covered with a coat of soil, which is nowhere deep, and having under it masses of pumice and lava. The precipices around this height, are, in many instances, formed of slaggy lava ; and, in the lava, are veins filled with opal, containing embedded fragments of vesicular and slaggy lava. In other parts, there are rocks of a felspar or trachyte porphyry. Among the many ridges shooting from the Green Mountain (M of the chart), one of the most remarkable is that composed of black and dark-green perfectly formed obsidian, which, in some places, is disposed in balls and globular concretions, like that found in Kamtschatka ; and, in others, in large globular concretions, like those of basalt and greenstone. Associated with it there are grey varieties of pearl-stone†. This vitreous mineral is there associated with various porphyries, apparently trachytic ; and, in some places, green pitchstone, with imbedded sphærulite and common pumice and pumice conglomerate, occur. Not far from the obsidian ridge, there is a remarkable hill, named by the sailors The Devil’s Riding-School, marked in the chart P. It is about 700 feet above the level of the sea, and between 400 and 500 feet above the level of the surrounding base. It has a circular hollow on the top, which probably was formerly much deeper than at present, it being now filled up to within 30 feet of the edge of the crater. This hill, as far as can be made out from the specimens brought home, appears to be composed of trachytic rocks. In some varieties, the basis is like claystone, and contains imbedded portions of slaggy lava ; in others, the basis is of felspar, with imbedded crystals of glassy felspar, and fragments of slaggy lava ; and the trachyte porphyry sometimes contains, in its cavities, crystals of Vesuvian. Many of the rocks are in an earthy state, owing to the action of the weather ; and occasionally they are observed decaying in globular and concentric lamellar concretions. The upper and middle parts of the hill, marked B in the chart, are composed of vesicular, spumous, and corded lava. Some of the vesicular varieties much resemble the millstone lava of Andernach. The lower part of the hill consists of rocks of a different description, which form, as it were, a foundation on which the vesicular and corded lavas rest. On the SW. side, the rocks are trachyte-porphyry, occasionally including fragments of slaggy lava. On the NE. side is a bluish clinkstone-lava, with numerous imbedded felspar crystals.
It thus appears, that the Green Mountain, and the hill P, are composed of trachyte, and its congenerous rocks ; while B consists of vesicular and slaggy lava, resting upon trachyte. All those parts of the island coloured in the chart reddish-brown, are of the same description. The rugged parts of the island, all of which are coloured bluish-black in the chart, are composed of a greyish-black lava, slightly vesicular, and containing few crystals of glassy felspar. This lava presents a frightfully rugged surface, which forms irregular eminences, varying in height from 20 to 50, and even 100 feet.
In the bays, and on such parts of the coast as are not precipitous, the beach is formed of a sand of comminuted shells, with fragments of echini and of corals. In some places near to the sea, the fragments of shells are conglutinated together by a calcareous cement, and form a pretty solid mass. The solidity of the mass diminishes as the distance from the sea increases. A turtle’s nest, with eggs, was observed imbedded in this conglomerate. The rocks which rise through these calcareous beaches, and which are so near to the sea as to be washed by its spray, are incrusted with a calc-sinter and calc-tuff, formed by the action of the weather on the calcareous matter of the shells and corals.
Lastly, it may be mentioned, that runs of a sand, composed of the materials of the rocks, occur in different parts of the island, and that these are pointed out in the chart by the pale-yellow colour.
Baron Von Buch divides volcanic islands into three classes, which he characterizes in the following manner :
1. Basaltic Islands. Composed of strata of basaltic rocks, in which there is generally a crater of elevation (Erhebungs crater.)
2. Volcanoes. Isolated ; very elevated peaks, and domes of trachyte, and generally with a great crater on the summit.
3. Erupted Islands. These have been formed by single eruptions, and scarcely ever occur without basaltic islands.
The Island of Ascension is, by Von Buch, referred to the third division ; but it now appears, from the facts stated above, that this island belongs not to the third alone, but rather conjoins in it the characters of the second and third classes‡.
*The Latitude was settled by a series of observations of the sun’s altitude, taken in an artificial horizon, when his northerly inclination admitted of this being done. The Longitude was settled by means of numerous lunar observations, agreeing with a series of observations of the eclipses of Jupiter’s moons, some of which were also observed at Greenwich.
Notes about this version of Campbell:
The text was scanned from an original copy. OCR software was used to generate a text file which was carefully proof-read against the original. Image of the plate is courtesy of the History of Science Collections, University of Oklahoma.
Contributed by Barry Weaver.
Details of the original:
Campbell, R. Chart of the Island of Ascension, with Remarks on its Geognosy.
The Edinburgh Philosophical Journal, Volume XIV, No. 27, pages 47-49, 1826.
Plates: One plate; A Plain Chart of the Island of Ascension by Captain R. Campbell R.N.
Imprint; Published by A. Constable & Co. Edinr 1824
British Library shelfmark: 258.d.9-12.
Library of Congress call number: Not in the catalogue
Last updated: 19 December, 2011