CHAPTER III

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ARRANGEMENT OF FOSSILS IN STRATA.—FRESH-WATER AND MARINE FOSSILS.

Successive Deposition indicated by Fossils. — Limestones formed of Corals and Shells. — Proofs of gradual Increase of Strata derived from Fossils. — Serpula attached to Spatangus. — Wood bored by Teredina. — Tripoli formed of Infusoria. — Chalk derived principally from Organic Bodies. — Distinction of Fresh-water from Marine Formations. — Genera of Fresh-water and Land Shells. — Rules for recognising Marine Testacea. — Gyrogonite and Chara. — Fresh-water Fishes. — Alternation of Marine and Fresh-water Deposits. — Lym-Fiord.

Having in the last chapter considered the forms of stratification so far as they are determined by the arrangement of inorganic matter, we may now turn our attention to the manner in which organic remains are distributed through stratified deposits. We should often be unable to detect any signs of stratification or of successive deposition, if

## particular kinds of fossils did not occur here and there at certain

depths in the mass. 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. He must never lose sight of the fact that, during the process of deposition, each separate layer was once the uppermost, and immediately in contact with the water in which aquatic animals lived. Each stratum, in fact, however far it may now lie beneath the surface, was once in the state of shingle, or loose sand or soft mud at the bottom of the sea, in which shells and other bodies easily became enveloped.

Rate of Deposition indicated by Fossils.—By attending to the nature of these remains, we are often enabled to determine whether the deposition was slow or rapid, whether it took place in a deep or shallow sea, near the shore or far from land, and whether the water was salt, brackish, or fresh. Some limestones consist almost exclusively of corals, and in many cases it is evident that the present position of each fossil zoophyte has been determined by the manner in which it grew originally. The axis of the coral, for example, if its natural growth is erect, still remains at right angles to the plane of stratification. If the stratum be now horizontal, the round spherical heads of certain species continue uppermost, and their points of attachment are directed downward. This arrangement is sometimes repeated throughout a great succession of strata. From what we know of the growth of similar zoophytes in modern reefs, we infer that the rate of increase was extremely slow, and some of the fossils must have flourished for ages like forest-trees, before they attained so large a size. During these ages, the water must have been clear and transparent, for such corals cannot live in turbid water.

Fossil Gryphæ, covered both on the outside and inside with fossil serpulæ. In like manner, when we see thousands of full-grown shells dispersed everywhere throughout a long series of strata, we cannot doubt that time was required for the multiplication of successive generations; and the evidence of slow accumulation is rendered more striking from the proofs, so often discovered, of fossil bodies having lain for a time on the floor of the ocean after death before they were imbedded in sediment. Nothing, for example, is more common than to see fossil oysters in clay, with Serpulæ, or barnacles (acorn-shells), or corals, and other creatures, attached to the inside of the valves, so that the mollusk was certainly not buried in argillaceous mud the moment it died. There must have been an interval during which it was still surrounded with clear water, when the creatures whose remains now adhere to it grew from an embryonic to a mature state. Attached shells which are merely external, like some of the Serpulæ (_a_) in Fig. 9, may often have grown upon an oyster or other shell while the animal within was still living; but if they are found on the inside, it could only happen after the death of the inhabitant of the shell which affords the support. Thus, in Fig. 9, it will be seen that two Serpulæ have grown on the interior, one of them exactly on the place where the adductor muscle of the _Gryphæa_ (a kind of oyster) was fixed.

Fig. 10: Serpula attached to a fossil. Fig. 11: Recent Spatangus with spines removed from one side. Some fossil shells, even if simply attached to the _ outside_ of others, bear full testimony to the conclusion above alluded to, namely, that an interval elapsed between the death of the creature to whose shell they adhere, and the burial of the same in mud or sand. The sea-urchins, or _Echini_, so abundant in white chalk, afford a good illustration. It is well known that these animals, when living, are invariably covered with spines supported by rows of tubercles. These last are only seen after the death of the sea-urchin, when the spines have dropped off. In Fig. 11 a living species of _Spatangus_, common on our coast, is represented with one half of its shell stripped of the spines. In Fig. 10 a fossil of a similar and allied genus from the white chalk of England shows the naked surface which the individuals of this family exhibit when denuded of their bristles. The full-grown _ Serpula_, therefore, which now adheres externally, could not have begun to grow till the _Micraster_ had died, and the spines became detached.

Fig. 12: Ananchytes from the chalk. Now the series of events here attested by a single fossil may be carried a step farther. Thus, for example, we often meet with a sea-urchin (_Ananchytes_) in the chalk (see Fig. 12) which has fixed to it the lower valve of a _Crania_, a genus of bivalve mollusca. The upper valve (_b_, Fig. 12) is almost invariably wanting, though occasionally found in a perfect state of preservation in white chalk at some distance. In this case, we see clearly that the sea-urchin first lived from youth to age, then died and lost its spines, which were carried away. Then the young _Crania_ adhered to the bared shell, grew and perished in its turn; after which the upper valve was separated from the lower before the _Ananchytes_ became enveloped in chalky mud.

Fig. 13: Fossil wood bored by Teredina.

Fig. 14: Recent wood bored by Teredo.

It may be well to mention one more illustration of the manner in which single fossils may sometimes throw light on a former state of things, both in the bed of the ocean and on some adjoining land. We meet with many fragments of wood bored by ship-worms at various depths in the clay on which London is built. Entire branches and stems of trees, several feet in length, are sometimes found drilled all over by the holes of these borers, the tubes and shells of the mollusk still remaining in the cylindrical hollows. In Fig. 14, _ e_, a representation is given of a piece of recent wood pierced by the _Teredo navalis_, or common ship-worm, which destroys wooden piles and ships. When the cylindrical tube _d_ has been extracted from the wood, the valves are seen at the larger or anterior extremity, as shown at _c._ In like manner, a piece of fossil wood (_a_, Fig. 13) has been perforated by a kindred but extinct genus, the _Teredina_ of Lamarck. The calcareous tube of this mollusk was united and, as it were, soldered on to the valves of the shell (_b_), which therefore cannot be detached from the tube, like the valves of the recent _Teredo._ The wood in this fossil specimen is now converted into a stony mass, a mixture of clay and lime; but it must once have been buoyant and floating in the sea, when the _ Teredinæ_ lived upon, and perforated it. Again, before the infant colony settled upon the drift wood, part of a tree must have been floated down to the sea by a river, uprooted, perhaps, by a flood, or torn off and cast into the waves by the wind: and thus our thoughts are carried back to a prior period, when the tree grew for years on dry land, enjoying a fit soil and climate.

Strata of Organic Origin.—It has been already remarked that there are rocks in the interior of continents, at various depths in the earth, and at great heights above the sea, almost entirely made up of the remains of zoophytes and testacea. Such masses may be compared to modern oyster-beds and coral-reefs; and, like them, the rate of increase must have been extremely gradual. But there are a variety of stone deposits in the earth’s crust, now proved to have been derived from plants and animals of which the organic origin was not suspected until of late years, even by naturalists. Great surprise was therefore created some years since by the discovery of Professor Ehrenberg, of Berlin, that a certain kind of siliceous stone, called tripoli, was entirely composed of millions of the remains of organic beings, which were formerly referred to microscopic Infusoria, but which are now admitted to be plants. They abound in rivulets, lakes, and ponds in England and other countries, and are termed Diatomaceæ by those naturalists who believe in their vegetable origin. The subject alluded to has long been well-known in the arts, under the name of infusorial earth or mountain meal, and is used in the form of powder for polishing stones and metals. It has been procured, among other places, from the mud of a lake at Dolgelly, in North Wales, and from Bilin, in Bohemia, in which latter place a single stratum, extending over a wide area, is no less than fourteen feet thick. This stone, when examined with a powerful microscope, is found to consist of the siliceous plates or frustules of the above-figured Diatomaceæ, united together without any visible cement. It is difficult to convey an idea of their extreme minuteness; but Ehrenberg estimates that in the Bilin tripoli there are 41,000 millions of individuals of the _Gaillonella distans_ (see Fig. 16) in every cubic inch (which weighs about 220 grains), or about 187 millions in a single grain. At every stroke, therefore, that we make with this polishing powder, several millions, perhaps tens of millions, of perfect fossils are crushed to atoms.

Figs 15 and 16: Gaillonella; Fig. 17: Bacillaria parodoxa A well-known substance, called bog-iron ore, often met with in peat-mosses, has often been shown by Ehrenberg to consist of innumerable articulated threads, of a yellow ochre colour, composed of silica, argillaceous matter, and peroxide of iron. These threads are the cases of a minute microscopic body, called _Gaillonella ferruginea_ (Fig. 15), associated with the siliceous frustules of other fresh-water algæ. Layers of this iron ore occurring in Scotch peat bogs are often called “the pan,” and are sometimes of economical value.

It is clear much time must have been required for the accumulation of strata to which countless generations of Diatomaceæ have contributed their remains; and these discoveries lead us naturally to suspect that other deposits, of which the materials have been supposed to be inorganic, may in reality be composed chiefly of microscopic organic bodies. That this is the case with the white chalk, has often been imagined, and is now proved to be the fact. It has, moreover, been lately discovered that the chambers into which these Foraminifera are divided are actually often filled with thousands of well-preserved organic bodies, which abound in every minute grain of chalk, and are especially apparent in the white coating of flints, often accompanied by innumerable needle-shaped spiculæ of sponges (see