Geological observations on the volcanic islands, visited during the voyage of H.M.S. Beagle, together with some brief notices on the geology of Australia and the Cape of Good Hope. Being the second part of the Geology of the voyage of the Beagle under the command of Capt. Fitzroy, R.N. during the years 1832 to 1836 / [Charles Darwin].

  • Darwin, Charles, 1809-1882.
Date:
1844
Catalogue details

Licence: Public Domain Mark

Credit: Geological observations on the volcanic islands, visited during the voyage of H.M.S. Beagle, together with some brief notices on the geology of Australia and the Cape of Good Hope. Being the second part of the Geology of the voyage of the Beagle under the command of Capt. Fitzroy, R.N. during the years 1832 to 1836 / [Charles Darwin]. Source: Wellcome Collection.

    64/234 (page 50)
    that these rocks of Ascension, formed close to the surface, should be nearly as compact as marble, which has undergone the action of heat and pressure in the plutonic regions. The great accumulation of loose calcareous particles, lying on the beach near the Settlement, commences in the month of October, moving towards the S.W., which, as I was in- formed by Lieut. Evans, is caused by a change in the pre- vailing direction of the currents. At this period the tidal rocks, at the S.W. end of the beach, where the calcareous sand is accumulating, and round which the currents sweep, become gradually coated with a calcareous incrustation, half- an-inch in thickness. It is quite white, compact, with some parts slightly spathose, and is firmly attached to the rock. After a short time it gradually disappears, being either re- dissolved, when the water is less charged with lime, or more probably is mechanically abraded. Lieut. Evans has ob- served these facts, during the six years he has resided at Ascension. The incrustation varies in thickness in different years: in 1831 it was unusually thick. When I was there in July, there was no remnant of the incrustation ; but on a point of basalt, from which the qnarrymen had lately re- moved a mass of the calcareous freestone, the incrustation was perfectly preserved. Considering' the position of the tidal rocks, and the period at which they become coated, there can be no doubt, that the movement and disturbance of the vast accumulation of calcareous particles, many of them being partially agglutinated together, causes the waves of the sea to be so highly charged with carbonate of lime, that they deposit it on the first objects against which they impinge. I have been informed by Lieut. Holland, R.N., that this incrustation is formed on many parts of the coast, on most of which, I believe, there are likewise great masses of comminuted shells. A frondescent calcareous incrustation.—In many respects this is a singular deposit; it coats throughout the year the tidal volcanic rocks, that project from the beaches com- posed of broken shells. Its general appearance is well re-