Lyell has given a figure ("Principles of Geology" book 3 chapter 17) of some eggs, containing the bones of young turtles, found thus entombed.) One of the most compact varieties, when placed in acid, was entirely dissolved, with the exception of some flocculent animal matter; its specific gravity was 2.63.The specific gravity of ordinary limestone varies from 2.6 to 2.75; pure Carrara marble was found by Sir H.De la Beche to be 2.7.
("Researches in Theoretical Geology" page 12.) It is remarkable 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 informed by Lieutenant Evans, is caused by a change in the prevailing 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 redissolved, when the water is less charged with lime, or more probably is mechanically abraded.Lieutenant Evans has observed 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 quarrymen had lately removed 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, cause 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 Lieutenant Holland, R.N., that this incrustation is formed on many parts of the coast, on most of which, Ibelieve, there are likewise great masses of comminuted shells.
A FRONDESCENT CALCAREOUS INCRUSTATION.
(FIGURE 5.AN INCRUSTATION OF CALCAREOUS AND ANIMAL MATTER, coating the tidal-rocks at Ascension.)In many respects this is a singular deposit; it coats throughout the year the tidal volcanic rocks, that project from the beaches composed of broken shells.Its general appearance is well represented in Figure 5; but the fronds or discs, of which it is composed, are generally so closely crowded together as to touch.These fronds have their sinuous edges finely crenulated, and they project over their pedestals or supports; their upper surfaces are either slightly concave, or slightly convex; they are highly polished, and of a dark grey or jet black colour; their form is irregular, generally circular, and from the tenth of an inch to one inch and a half in diameter; their thickness, or amount of their projection from the rock on which they stand, varies much, about a quarter of an inch being perhaps most usual.The fronds occasionally become more and more convex, until they pass into botryoidal masses with their summits fissured; when in this state, they are glossy and of an intense black, so as to resemble some fused metallic substance.I have shown the incrustation, both in this latter and in its ordinary state to several geologists, but not one could conjecture its origin, except that perhaps it was of volcanic nature!
The substance forming the fronds has a very compact and often almost crystalline fracture; the edges being translucent, and hard enough easily to scratch calcareous spar.Under the blowpipe it immediately becomes white, and emits a strong animal odour, like that from fresh shells.It is chiefly composed of carbonate of lime; when placed in muriatic acid it froths much, leaving a residue of sulphate of lime, and of an oxide of iron, together with a black powder, which is not soluble in heated acids.
This latter substance seems to be carbonaceous, and is evidently the colouring matter.The sulphate of lime is extraneous, and occurs in distinct, excessively minute, lamellar plates, studded on the surface of the fronds, and embedded between the fine layers of which they are composed; when a fragment is heated in the blowpipe, these lamellae are immediately rendered visible.The original outline of the fronds may often be traced, either to a minute particle of shell fixed in a crevice of the rock, or to several cemented together; these first become deeply corroded, by the dissolving power of the waves, into sharp ridges, and then are coated with successive layers of the glossy, grey, calcareous incrustation.
