Land shell

“ Almost all oceanic islands, even the most isolated and smallest, are inhabited by land-shells, generally by endemic species, but sometimes by species found elsewhere,— striking instances of which have been given by Dr. A.A. Gould in relation to the Pacific. Now it is notorious that land-shells are easily killed by sea-water; their eggs, at least such as I have tried, sink in it and are killed. Yet there must be some unknown, but occasionally efficient means for their transportal. Would the just-hatched young sometimes adhere to the feet of birds roosting on the ground and thus get transported? ”

“ Beds of extinct land-shells.—Land-shells, all of which appear to be species now extinct, occur embedded in earth, in several parts of the island. The greater number have been found at a considerable height on Flagstaff Hill. ”

“ Land-crabs cast their shells annually, and at such periods remain closely concealed in their holes, almost without motion and without food, for about the space of six or eight days, during which time the new shell hardens. ”

“ The true land-shells offer a still more difficult case, for they are exceedingly sensitive to the influence of salt water; they are not likely to be carried by aquatic birds, and yet they are more or less abundant all over the globe, inhabiting the most remote oceanic islands. ”

“ A consideration of the distribution of the families and genera of land-shells shows us, that although they possess some special features, yet they agree in many respects with the higher animals in their limitation by great natural barriers, such as oceans, deserts, mountain ranges, and climatal zones. ”

“ Shell Beds.—What the circumstances are, which favour in a special degree the accumulation of shells on the bed of the sea, may be partly conjectured; but the subject deserves to be considered as one of the most important problems which geology looks to the naturalist to resolve. Very different conditions are known to govern the aggregation of the different tribes: they choose different soils—so to speak love—different depths, bear unequally the influence of currents, fresh water, climate. ”

“ Shells thus cast up by gales, would, during an elevatory period, never again be touched by the sea. Hence, on this view of a slow and gradual rising of the land, interrupted by periods of rest and denudation, we can understand the pebbles being of about the same size over the entire width of the step-like plains,—the occasional thin covering of sandy earth,—and the presence of broken, unrolled fragments of those shells, which now live exclusively near the coast. ”

“ Besides forming the rough outside, the calcareous exuvium, the mucus of the oyster and other mollusca, forms that beautiful substance, so smooth, and polished, and dyed with rainbow tints, and a glorious opalescence, which, be it as common as luxury has made it, still charms the eye. This is the lining of the shell, the mother-of-pearl, nacre. “The inside of the shell,” said old Dampier, that old sailor with a poet’s mind, “is more glorious even than the pearl itself.” ”

“ Wait till a storm from the sea is ended, and then, if the wind is blowing from the land, a rich harvest of live and dead shells will be found on the sandy beaches and amongst the seaweed and wrack that comes ashore. ”

“ On the other hand, shells occurring in sheltered situations, estuaries, narrow straits, or even on open coasts where there is plenty of shelter from the waves, are comparatively of great size, with a well-developed, sometimes produced spire, and a mouth small in proportion to the area of shell surface. ”

“ When, therefore, we find, in various parts of our country, beds of shells in such a state of preservation as to lead us to believe that the animals must have lived and died where their remains are now to be seen, we justly infer that the districts where they occur must at one period have been submerged. If the shells belong to fresh-water species, it is plain that they occur on the site of an old lake. If they are marine, we conclude that the localities where they are found no matter how high above the sea must formerly have stood greatly lower, so as to form the ocean bed. ”

“ Pits, quarries, and mines that cut down still deeper into the earth and lay it bare bring before our eyes most impressive testimony regarding the ancient changes of the land. Suppose, by way of further illustration, that underneath a bed of sand full of oyster shells there lies a dark brown band of peat. This substance, composed of mosses and other water-loving plants, is formed in boggy places by the growth of marshy vegetation. Below the peat there might occur a layer of soft white marl full of lake shells, such as may be observed on the bottom of -37- many lakes at the present time. ”

“ As the localization of marine plants and animals is in a considerable degree determined by the nature of the rocks and their detritus, it follows that each part of this coast will have its more or less distinct Flora and Fauna. What now must result from the action of the waves in the course of a geologic epoch? As the sea makes slow inroads on the land, the place at which each group of strata crops out on the beach will gradually move towards the west: its distinctive fish, mollusks, crustaceans, and sea-weeds, migrating with it. ”

“ Along the shining rim of cape and cove
The shells in fair, unplanned mosaic lie;
And there the children, keen of heart and eye,
Gather their harvest in of treasure-trove.
Yet this is one of ocean’s mysteries—
That, while the humbler shells the breakers brave,
The fairest are most fragile, and the wave,
Ruthless, has crushed and mutilated these!
Ah, sea of life, we, too, like children, stand
Through youth and age, expectant, at thy rim,
To pray for golden argosies from Him
Who holds thee in the hollow of His hand. ”

“ A million leaves fall and decay to enrich the soil wherefrom two million more may spring. An infinity of little shell-fish die, and the ages grind their shells to powder to make the sands and the chalk cliffs. ”

“ We cannot speak confidently as to the time needed to form each thin layer of vegetable soil, marsh peat, or estuarine silt. On comparing the submerged land-surfaces, however, with similar accumulations formed within known periods, such as marsh soils grown behind ancient embankments, or forest-growth over flats silted up at known dates, we can learn something. ”

“ Think of continuous piles of these shells, many hundreds of feet in thickness, all built out of the dead shells of dying millions of tiny creatures. Deep, deep below the surface, where waves had no power, where currents were sluggish, fell a ceaseless gentle rain of these minute shells. Life had fled from each jelly inhabitant; its brief day was over; and the empty shell went quietly down, whether from near the surface or from lower depths, till it reached the ocean-bed. ”

“ In other places again extensive layers of coral, sea-shells, marine plants, and other products of the sea, imbedded in the firm soil, prove these tracts of land to have been an ancient bottom of the sea. ”

“ The frost and rain of the past winter have washed away the sand and light soil which had hidden them. Why! these are shells, not pebbles. Stone shells, fossil shells! See how perfect every delicate fluting is, how sharply the hinge of the long-perished bivalve is defined. Some are of a delicate pale gray, others almost white; but long ago in the Silurian seas perhaps they shone in brown and yellow, red, pink, buff, and maroon, as gaily as the scallop shells of to-day, which in shape they much resemble.
What is this lump of stone covered with little star-shaped knobs? ”

“ Owing to the conditions of their growth, the deposits formed in marine marshes contain no distinct peat, the nearest approach to that substance being the tangle of wirelike roots which covers the upper foot or so of the accumulation. The greater part of the mass is composed of fine silt, brought in by the streams of land water which discharge into the basin, and by the remains of animals which dwelt upon the bottom or between the stalks of the plants that occupy the surface of the marshes. These interspaces afford admirable shelter to a host of small [Pg 337] marine forms. ”

“ When they are upheaved, therefore, the waves on the beach will bear down and disperse the loose materials; whereas if the bed of the sea subsides slowly, a mass of strata containing abundance of such species as live at moderate depths may increase in thickness to any amount, and may extend over a broad area, as the water gradually encroaches on the land. ”

“ But in protected bays, such as in that of Bahia Blanca, wherever the sea is accumulating extensive mud-banks, or where the winds quietly heap up sand-dunes, beds of shells might assuredly be preserved buried in the positions in which they had lived, even whilst the land retained the same level; any, the smallest, amount of elevation would directly aid in their preservation. ”

“ It lays millions of eggs and hatches countless baby mollusks. It lives its life in its shell, lugging it around, snuggling into it when alarmed, burrowing into mud, fastening itself to a rock and creating ingenious camouflage. It builds its calcareous house with a great instinctive talent for color and sculpture. ”

“ Species, it is well known, both of plants and animals, are limited to particular localities of [325] variable, and often of no great, extent. If marine, an alteration in the sea bottom will prove fatal to many. If terrestrial, an increase of altitude, the conversion of dry land into marshes and lakes, or of lakes and marshes into meadow and arid loamy soil, will completely alter the flora and fauna of the district in question. Look into any estuary or rocky pool along the shore of the ocean, swarming with testacea, and crustacea; every bowlder incrusted with corallines ”

“ Supporting this evidence, we have freshwater or littoral shells found at great depths off our coasts. Mr. Godwin Austen records the dredging up of a freshwater shell (Unio pictorum) off the mouth of the English Channel between the fifty fathom and 100 fathom lines, while in the same locality gravel banks with littoral shells now lie under sixty or seventy fathoms water. ”

“ They entered four, six, and even ten fathoms of water, for the shell is larger the deeper they go, and if at times the larger ones come in shallow water it is through storms, or because they go from one place to another in search for food, and having found their pasture they stay there until they have finished it. They perceive those who search for them, and stick so close to the rocks or ground, or one to another, that much strength is needed to detach them, and many times the fishermen cannot raise them and leave them, thinking they are stones. ”

“ The poverty in number and the frequent small size of the shells in the passage-beds indicate a change of conditions in the nature of the waters in which they lived; and the plants alluded to clearly point to the close neighborhood of a land, of which we have no direct signs, in the vast development of a purely marine fauna in lower portions of the Ludlow strata. ”

“ As the Carboniferous group of rocks exhibits the remains of ocean-bed, lake-bottom, and land-surface, so we find in it shells of marine, fresh-water and (though rarely) terrestrial mollusca. The marine genera greatly predominate, just as the shells of the sea at the present day vastly outnumber those either of lakes or of the land. In England they occur chiefly in the lower part of the formation, giving a characteristic stamp to the deep series of beds known as the mountain limestone. There they are associated with the corals and stone-lilies already described—all productions of the sea. ”

“ At one level, for example, univalve shells of some one or more species predominate; at another, bivalve shells; and at a third, corals; while in some formations we find layers of vegetable matter, commonly derived from land plants, separating strata.
It may appear inconceivable to a beginner how mountains, several thousand feet thick, can have become full of fossils from top to bottom; but the difficulty is removed, when he reflects on the origin of stratification, as explained in the last chapter, and allows sufficient time for the accumulation of sediment. ”

“ Our shells, whether of marine, lake, river, or land production, deserve a very critical examination, more especially as the fossil remains of this extensive tribe of animals, both of living and extinct species, are considered as affording the most certain criteria for determining the priority of existing geological formations in the order of time. There is no department of our natural history which, for scientific purposes, requires more careful investigation. ”

“ They will, and this is another source of calcareous earth. The shells of all animals, both land and sea, as oysters, mussels, cockles, crabs, lobsters, snails, and the like, and also egg-shells of all kinds, consist of this earth; and so does coral, which is formed by insects under the sea, and is very abundant in some countries. Vast quantities of shells are often found deep in the earth, in the midst of chalk and lime-stone beds; whence some have supposed that all calcareous earth is originally an animal production. ”

“ It is not uncommon to meet with layers of vegetable matter, impressions of leaves, and branches of trees, in strata containing fresh-water shells; and we also find occasionally the teeth and bones of land quadrupeds, of species now unknown. The manner in which such remains are occasionally carried by rivers into lakes, especially during floods, has been fully treated of in the "Principles of Geology." ”

“ In the beds of mud and sand, we should probably find the shells of some of the molluscs inhabiting the waters, and also those of terrestrial species, washed in from the inundated land surface, or brought down from the banks of the tributary rivulets; while mixed among the larger pebbles we might expect to find any animal bones that had been lying on the land contiguous to the stream, or any of the larger and heavier objects of human workmanship, that would have been carried off by such an inundation, had mankind been living on the banks of the river. ”