remain in the mud, silt, or sand which the animals may frequent, penetrating into them to various depths, according to their habits, so that such remains are preserved after their death in the position usually occupied by the molluscs, numerous other shells remain on the surface to be acted upon in the manner of any inorganic substance.

That shells are so scattered about, multitudes brought up by the arming of the sounding-lead abundantly attest. Moreover, collections of certain species are found to mark particular portions of soundings off given coasts. Thus off the shores of the British Islands, charts give localities as marked by Hake's teeth, as they are termed; commonly nothing else than a multitude of the shells of Dentalium scattered over particular areas. Other collections of shells are equally well known. While these shells, scattered over the sea bottom, are often either the entire hard parts of univalves, or single and uninjured valves of the bivalves, at other times they are crushed or broken. Whether in the one state or the other, according to their specific gravities, volume, and form, they will be acted upon by streams of tide, by ocean currents sweeping within sufficient depths, or by surface wind-wave action transmitted to the bottom. With respect to specific gravities, though there is apparently much variation in this respect, the floating molluscs being, some of them at least, provided with shells of comparatively minor specific gravity, the range seems something between 2.67 and 2.85*. With equal forms and volumes, fragments or rounded grains of a great proportion of marine shells would apparently be specifically heavier than grains of quartz and rock crystal (2.63 -2.65), of common felspar (2 53-2.60), of albite (2.61-2.68),

*The author obtained the following specific gravities of a few marine shells some years since. Researches in Theoretical Geology, 1834, p. 76.

It is not improbable, that if experiments on this head were much multiplied, individual differences would often be found in the same species. While the shell of the Argonauta tuberculosus is lighter than pure Sussex chalk (2·49), and that of Haliotis tuberculatus is equal in specific gravity to Carrara marble (2·70), the greater numbers exhibit a packing of particles more approaching Arragonite.

M

and of chlorite (2.71), while they would be lighter than mica (2.94).

The forms of shells or their fragments, except they have been ground down to rounded grains by breaker action on beaches, commonly agree little with those of the sedimentary matter among which they lie superficially mixed. When, therefore, we have to regard any movement of water around whole shells or their fragments, their forms become important, as also the mode in which they may be exposed to any moving force employed. Thus the same shell, if a conical univalve, would offer a different resistance, according as it might be placed with its apex or its base to the moving water, when acted upon, though we might expect that the moving water would soon turn such body, so that its apex would be presented to the line of action. With the valve of a bivalve, its hold on a bottom of sand or silt would be very different, whether it were turned with the margin of the valve downwards, or merely rested upon some part of its bombed surface. How far the valve of a shell could be transported along the bottom without being upset, will depend on very obvious conditions. In all cases we have to consider that shells, or their fragments, having a specific gravity rarely, perhaps, exceeding 2.85, and often presenting forms readily moved, are not difficult of transport in a medium of the specific gravity of 1·027—1·028.

Referring to the plan and section above given (figs. 69 and 70), the observer will have to distinguish between the remains of those molluscs which may die amid the mud, silt, or sand, and so have their harder parts preserved in the situations where they live, and the remains on the surface of the sea bottom. How far these may retain their positions relatively to the zones of depth suited to their animals, will depend upon the circumstances above noticed. Looking at the subject generally, they would be liable to be moved at the depth at which surface-wave action could reach, and therefore to be moved shorewards in shallow waters; so that the remains of molluscs accumulated near the coast in the zones b a l, b' a' l' (fig. 69), varying in the depths b d, d e (fig. 70), would, at the proper depths, have surface-wave action added to tidal streams able to transport the shells or their fragments, tending to move them on-shore. In the outer zone e ƒ (fig 69), and at the depths f g (fig. 70), the effects of the tidal movement may not only be little felt, but also any action upon the bottom from surface-waters be inappreciable. Still further out, in the deep waters g (fig. 69), or h (fig. 70),

there may be no movement sufficient to produce transport of loose matters on the bottom. There might, therefore, be movements in the water producing considerable mixtures of the remains of molluscs in shallow situations, extending even to the casting of shells or their fragments upon the shore, from depths depending upon various local modifications of the causes of transport above noticed.

On many exposed ocean-coasts we have even the accumulation of sandy dunes, composed, for the most part, of fragments of mollusc shells ground down to sand, these cast on shore and dealt with by winds in the manner of common sand. The western coasts of Ireland, Scotland, and of part of England, afford many examples of this fact.*

The induration of sands formed of comminuted shells has been previously mentioned (p. 62), and, as may be expected, such indurated sands occasionally include remarkable mixtures of organic remains. The rock in which the human bones were discovered at Guadaloupe would appear to be of this character. Not only corals and shells from the neighbouring sea, but terrestrial shells also, including the Bulimus guadaloupensis (Ferussac), are preserved in it. Teeth of the caiman, with stone hatchets and other remains of human art, are mentioned as having been found in this consolidated sand.

The study of the manner in which the shells of molluscs, and the harder parts of marine animals generally, are thrown on shore, of the depths from which they may be borne by the action of on-shore waves and breakers, and of the various arrangements of whole, broken, or comminuted shells in layers, from their accumulation like ordinary detrital matter at various depths in the sea to their rejection upon the land, is one which will amply reward the observer anxious to compare the manner in which these remains are now distributed and arranged with that of the organic remains found in fossiliferous rocks. He may at times see the

*This shell sand is often employed as manure; it is known to have been so employed in Cornwall in the reign of Henry III. A charter of Richard, King of the Romans, granting the liberty of taking this sand for manure, was confirmed by Henry III. (Lysons, "Mag. Brit.," Cornwall, p. ccciii, who cites Rot. Chart., 45 Hen. III.) Carew notices the use of it in his Survey of Cornwall (1602), and it is largely employed for agricultural purposes to the present day. Mr. Worgan, in 1811, estimated the cost of the land carriage of this sand in Cornwall at more than 30,0007. per annum. Large quantities are obtained at the Dunbar Sands, in Padstow Harbour, the annual amount estimated at 100,000 tons. It has been calculated that 5,600,000 cubic feet of sand, chiefly composed of comminuted sea-shells, are annually taken from the coasts of Cornwall and Devon, and spread over the land in the interior as a mineral manure. Report on the Geology of Cornwall, Devon, and West Somerset (1839), p. 479.

pushing action of the small wash of the sea driving the larger shells and their fragments before it into convenient localities, there accumulating in a mass those which may have been distributed by breaker action along a line of coast, while at others he will find the shells jammed in amid the joints and crevices of rocks so firmly that they become difficult to remove.

CHAPTER X.

CORAL REEFS AND ISLANDS.-DISTRIBUTION OF CORAL ANIMALS. CHEMICAL COMPOSITION OF CORALS.-KEELING ATOLL.-FORM OF CORAL ISLANDS.BARRIER REEFS.-LAGOON ISLANDS.-ISLE OF BOURBON.

GREAT as the accumulations of the harder remains of molluscs may be in the sea or on its shores (and regarding the amount of matter, chiefly calcareous, abstracted from the sea or contained in their food the volume of these harder remains added annually to common detrital and chemical deposits must be very considerable), the coral accumulations of tropical regions present us with the most striking additions, by means of animal life, to the mineral deposits now in progress. They have for many years attracted the attention of navigators and naturalists, so that much information has been obtained respecting them.*

With regard to the distribution of corals, Mr. Dana states, that the Astræacea, Madreporacea, and Gemmiporide among the Caryophyllacea, are, with few exceptions, confined to the coralreef seas, a region included between the parellels of 28° north and south of the equator, † these corals forming the principal portion of the reefs, and being confined to depths within 120 feet from the surface. Other corals, as is well known, extend to far greater depths, and into colder regions. Sir James Ross, in his voyage to the South Polar Regions, obtained live corals from a

* We would more especially call attention to the labours of Mr. Darwin, who has not only been personally engaged in the investigation of coral reefs and islands, but has also carefully studied the works of navigators and naturalists relating to the subject. The results of his investigations are contained in his work, entitled, "Structure and Distribution of Coral Islands," London, 1842. We would also refer to the labours of Mr. Dana, contained in his "Structure and Classification of Zoophytes," Philadelphia, 1846. Mr. Dana's views are also founded on the personal examination of coral reefs and islands.

+ Locally, coral reefs are found further north and south than 28°. They extend in the Bermuda Islands to lat. 32° 15′ N., the greatest distance from the equator, as Mr. Darwin observes, at which they are known to exist, and to lat. 30° N. in the Red Sea. Houtman's Abrolhos, on the western shores of Australia, in lat. 29° S., are of coral formation.