REEVE, BENHAM, AND REEVE,
PRINTERS AND PUBLISHERS OF SCIENTIFIC WORKS,
KING WILLIAM STREET, STRAND.

THOUGHTS
ON A
PEBBLE,
OR,
A FIRST LESSON IN GEOLOGY.

BY THE AUTHOR OF "THE WONDERS OF GEOLOGY."

"There is no picking up a pebble by the brook-side, without finding all nature in connexion with it."

Contemplations of Nature.

EIGHTH EDITION; WITH THIRTY-TWO ILLUSTRATIONS.

LONDON:

REEVE, BENHAM, AND REEVE, KING WILLIAM STREET, STRAND.

1849.

TO

MY SON,

Reginald Nebille Mantell, C.E.,

THESE

"THOUGHTS ON A PEBBLE"

ARE MOST AFFECTIONATELY

INSCRIBED.

LONDON,
19, CHESTER SQUARE, PIMLICO.
1849.

"Every grain of sand is an immensity—every leaf a world—every insect an assemblage of incomprehensible effects in which reflection is lost."

Lavater.

"To the natural philosopher there is no natural object that is unimportant or trifling. From the least of Nature's works he may learn the greatest lessons. The fall of an apple to the ground may raise his thoughts to the laws which govern the revolutions of the planets in their orbits; or the situation of a pebble may afford him evidence of the state of the globe he inhabits, myriads of ages before his species became its denizens."

Sir J. F. W. Herschel.

TO THE READER.

Deeply impressed with the conviction that it is of the highest importance the young and inquiring mind should have a correct idea of natural phenomena—that it should not be left to its own unaided efforts to unravel the mysteries of the beautiful world in which this first state of being is destined to be passed—or have its curiosity stifled or misled by unsatisfactory or erroneous conjectures—I have endeavoured in this little work to explain in a simple and attractive guise, some of the grand truths relating to the ancient physical history of our planet, which modern geology has established.

The favourable reception of these desultory "Thoughts" which were originally penned for the amusement and instruction of an intelligent boy, is a gratifying proof that the attempt has not been unsuccessful; and I would fain indulge the hope, that this "First Lesson in Geology" may still be productive of good, by exciting in some youthful minds a desire for the acquisition of natural knowledge; and inculcating the important truth, that He who formed the Universe has created nothing in vain; that His works all harmonize to blessings unbounded by the mightiest or most minute of His creatures; and that the more our knowledge is increased, and our powers of observation are enlarged, the more exalted will be our conception of His wondrous works.

Chester Square,
     Pimlico.

CONTENTS.

LIGNOGRAPHS.

LITHOGRAPHS.

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.[F]

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.

Byron, The Island.

The Ammonites, so called from the supposed resemblance of their shells to the fabled horn of Jupiter Ammon, are only known in a fossil state; but they must have swarmed in the ancient seas, for several hundred species have been discovered in the Chalk and antecedent strata, though none have been found in any deposits of more recent formation; at the termination of the chalk epoch the whole race, therefore, appears to have perished. The Ammonites are commonly termed snake-stones, from the origin ascribed to them by local legends; those of Whitby are well known (see Lign. 7).[G]

Scott's Marmion.

The Nautili were the contemporaries of the Ammonites, and many kinds are found associated with those shells, in strata far more ancient than the Chalk; and several species of both genera, as we have previously shown, were inhabitants of the cretaceous ocean. When the Ammonites became extinct, the Nautili continued to flourish, and numerous examples occur in the strata that were deposited during the vast period which intervened between the close of the Chalk formation, and the dawn of the existing condition of the earth's surface. At the present time two or three kinds only are known in a living state, and these are restricted to the seas of tropical climes, and so seldom approach the shores, that but few specimens of the animals that inhabit the shells have been obtained.

The Nautilus, therefore, is one of those types of animal organization that have survived all the physical revolutions to which the surface of the earth was subjected during the innumerable ages that preceded the creation of the human race.[H] This remarkable fact is portrayed with much force and beauty by Mrs. Howitt, in the following stanzas:

TO THE NAUTILUS.

We have thus acquired satisfactory proof that the flint of which our pebble is composed, was once fluid in an ocean teeming with beings, of genera and species unknown in a living state, and that it consolidated and became imbedded in the chalk, which was then being deposited at the bottom of the sea; hence the shells, corals, and other organic remains, which we now find attached to its surface, and enclosed in its substance. Thus much for the origin of the pebble; let us next inquire by what means it was dislodged from its rocky sepulchre, cast up from the depths of the ocean, and transported to the summit of the hill whence it was dislodged by yonder torrent. If we stroll along the sea-shore, and observe the changes which are there going on, we shall obtain an answer to these questions; for

Byron.

The incessant dashing of the waves against the base of the chalk-cliffs, undermines the strata, and huge masses of rock are constantly giving way and falling into the waters. The chalk then becomes softened and disintegrated, and is quickly reduced to the state of mud, and transported to the tranquil depths of the ocean, where it subsides and forms new deposits; but the flints thus detached, are broken and rolled by attrition into the state of boulders, pebbles, and gravel, and ultimately of sand.