Thoughts on a Pebble, or, a First Lesson in Geology

PART I.

Well might our immortal Shakspeare talk of "Sermons in stones;" and Lavater exclaim, that "Every grain of sand is an immensity" and the author of 'Contemplations of Nature' remark, that "there is no picking up a pebble by the brook-side without finding all nature in connexion with it."

I shall confine my remarks to a flint pebble, as being the kind of stone familiar to every one. The pebble I hold in my hand was picked up in the bed of the torrent which is dashing down the side of yonder hill, and winding its way through that beautiful valley, and over those

Huge rocks and mounds confus'dly hurl'd. The fragments of an earlier world,

which partially filling up the chasm, and obstructing the course of the rushing waters, give rise to those gentle murmurings that are so inexpressibly soothing and delightful to the soul.

Upon examining this stone I discover that it is but the fragment of a much larger mass, and has evidently been transported from a distance, for its surface is smooth and rounded, the angles having been worn away by friction against other pebbles, produced by the agency of running water. I trace the stream to its source, half way up the hill, and find that it gushes out from a bed of gravel lying on a stratum of clay, which forms the eminence where I am standing, and is nearly 300 feet above the level of the British Channel. From this accumulation of water-worn materials the pebble must have been removed by the torrent, and carried down to the spot where it first attracted our notice; but we are still very far from having ascertained its origin. The bed of stones on the summit of this hill is clearly but a heap of transported gravel--an ancient sea-beach or shingle--formed of chalk-flints, that at some remote period were detached from their parent rock, and broken, rolled, and thrown together, by the action of the waves. We are certain of this because we know that flints cannot grow; that they were originally formed in the hollows or fissures of other stones; and upon inspecting the pebble more attentively, we perceive, not only that such was the case, but also that it has been moulded in Chalk, for it contains the remains of certain species of extinct shells and corals, which are found exclusively in that rock. Here then a remarkable phenomenon presents itself for our consideration; this flint, now so hard and unyielding, must once have been in a soft or fluid state, for the delicate markings of the case and spine of an Echinus, or Sea-Urchin, are deeply impressed on its surface; and a fragile shell with its spines, is partially imbedded in its substance. Nay more, upon breaking off one end of the pebble, we find that a sponge, or some analogous marine zoophyte, is entirely enveloped by the flint; and also that there are here and there portions of minute corals, and scales of fishes. What a "Medal of Creation" is here--what a page of nature's volume to interpret--what interesting reflections crowd upon the mind!

"Flints cannot grow."--Here I would digress for a moment to notice an opinion so generally prevalent, that perhaps some of my young readers will not be prepared at once to answer the question--Do stones grow? The farmer who annually ploughs the same land, and observes a fresh crop of stones every season, will probably reply in the affirmative; and the general observer who has for successive years noticed his gardens and plantations strewn with stones, notwithstanding their frequent removal, may possibly entertain the same opinion; but a little reflection will show that stones cannot be said to grow or increase, in the proper acceptation of the term. Animals and plants grow, because they are provided with vessels and organs by winch they are capable of taking up particles of matter and converting them into their own substance; but an inorganic body can only increase in bulk by the addition of some extraneous material; hence stones may become incrusted, or they may be cemented together and form a solid conglomerate, but they possess no inherent power by which they can increase either in size or number--they cannot grow.

Plate I, a.

Plate I, b.

Plate I, c.

(See 'Medals of Creation', p. 340.)]

To avoid confusion, we will reverse the order of our inquiry, and first contemplate the formation of the flint in its native rock. The Chalk, that beautiful white stone, which (as an American friend, who saw it for the first time, observed), is so like an artificial production, abounds in marine shells and corals, and in the remains of fishes, crabs, lobsters, and reptiles, all of which differ essentially from living species; although a few of the corals and shells resemble, in some particulars, certain kinds that inhabit the seas of hot climates. These remains are found in so perfect a state--the shells with all their spines and delicate processes (Lign. 3), and the fishes with their teeth (Lign, 4), scales, and fins, entire--that no doubt can be entertained of the animals having been surrounded by the chalk while living in their native sea, and that many of them were entombed in their stony sepulchres suddenly, when the rock was in the state of mud, or like liquid plaster of Paris.

See Note I. Shells in the Chalk.

But besides the fossils which are obvious to the unassisted eye, the Chalk teems with myriads of minute forms that may readily be detected with a lens of moderate power; and even when these have been extracted, the residue, which appears to be merely white calcareous earth, is found, when examined under the microscope, to consist almost wholly of bodies yet more infinitesimal--of perfect shells and corals, so minute, that a cubic inch of chalk may contain upwards of a million of these organic remains (see Lign. 5)!

a, a, Shells called Rotalia.

b, ------------- Textularia.

(See 'Medals of Creation,' p. 232.)

The Chalk is stratified--that is, divided into strata or layers--as if a certain quantity of mud had sunk to the bottom of the sea, and enveloped the shells, corals, &c., which fell in its way, and had become somewhat solid before another layer was deposited upon it.

The mineral substance termed silex or flint, is variously distributed in the chalk. It most commonly occurs in the state of nodules of an irregular or spheroidal, globular figure, which are arranged in rows parallel and alternating with, the cretaceous strata; it is likewise disposed in continuous thin layers, which are spread over considerable areas; and it often forms horizontal, vertical, and oblique veins, that fill up the fissures and interstices of the chalk. The siliceous nodules frequently enclose corals, shells, sponges, and other organic remains, as in the pebble before us; and in many instances these fossils are found partly imbedded in the chalk and partly invested with flint. But though flints contain in abundance relics of the same species of marine animals as the chalk, they are not like that rock composed of an aggregation of fossil remains; on the contrary, the siliceous earth, which is their constituent substance, was evidently once in a state of complete solution in water, and precipitated into the chalk before the latter was consolidated, the organic bodies serving as nuclei or centres around which the silex concreted; for the deposition of the flint, like that of the chalk, appears to have taken place periodically.

Note II. Wood in flint.

1, 2, 3, 6, Rotaliæ;

4, Portion of a Nautilus;

5, Rotalia composed of flint.

(See 'Medals of Creation,' p. 232.)

The composition of the Chalk, and the prevalence throughout that rock of the relics of animals that can only live in salt-water, prove incontestably that the chalk and flint were deposited in the sea; and that our beautiful South Downs, now so smooth and verdant, and supporting thousands of flocks and herds, and the rich plains and fertile valleys spread around their flanks, were once the bed of an ocean. It is also evident not only that such must have been the case, but also that the Chalk was deposited in the basin of a very deep sea--in the profound abyss of an ocean as vast as the Atlantic.

From the absence of gravel, shingle, and sea-beach, it is certain that the white chalk-strata were formed at a great distance from sea-shores and cliffs; and this inference is confirmed by the swarms of shells termed Ammonites and Nautili, which we know from their peculiar structure were, like the recent pearly Nautilus, inhabitants of deep waters only. For these are chambered shells; that is, are divided internally by thin transverse shelly septa or plates, into numerous cells; the body of the animal occupied only the outer compartment, but was connected with the entire series of chambers by a tube or siphuncle, which passed through each partition. This mechanism constituted an apparatus which contributed to the buoyancy of these animals when afloat on the waves; for the Ammonites and Nautili were able to swim on the surface, or sink to the depths of the ocean at pleasure.

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