An Expository Outline of the "Vestiges of the Natural History of Creation" / With a Notice of the Author's "Explanations:" A Sequel to the Vestiges

by S. Laing

EARTH AND ITS GEOLOGICAL HISTORY.

Our globe is somewhat less than 8,000 miles in diameter; it is of a spheroidal form, the equatorial exceeding the polar axis in the proportion of 300 to 299, and which slight inequality, in consequence of its diurnal revolution, is necessary to preserve the land near the equator from inundation by the sea. The mean density or average weight of the earth is, in proportion to that of distilled water, as 5.66 to 1. So that its specific gravity is considerably less than that of tin, the lightest of the metals, but exceeds that of granite, which is three times heavier than water.

Descending below the surface, the first sensation that strikes is the increase of temperature. This is so rapid, that for every one hundred feet of sinking we obtain an increase of more than one degree of Fahrenheit's thermometer. If there be no interruption to this law, and no reason exists to conclude there is, it is manifest that at the depth of a few miles we must reach an intensity of heat utterly unbearable. Hence it follows that by no improvements in machinery can mining operations be carried down to a great depth below the surface. The greatest depth yet penetrated does not exceed three thousand feet, and forms a very small advance towards the earth's centre, distant 4,000 miles.

Geologists, however, without penetrating far into the earth, have found means for obtaining an insight for several miles into its interior structure, and armed with hammer, chisel, and climbing hook, they explore the beetling sea-cliff, traverse the deepest valleys, and scale the highest mountains, carefully examining their formation, disposition, and substance, and are thus enabled to obtain some knowledge of the earth's stomach, as it were, by scrutinising the deposits and eruptive ejectments on its surface. For example, we come to a mountain composed of a particular substance with strata or beds of other rock lying against its sloped sides; we, of course, infer that the substance of the mountain dips away under the strata that we see lying against it. Suppose that we walk away from the mountain across the turned-up edges of the stratified rocks, and that for many miles we continue to pass over other stratified rocks, all disposed in the same way, till we begin to cross the opposite edges of the same beds; after which we pass over these rocks all in reverse order, till we come to another extensive mountain composed of similar materials to the first, and shelving away under the strata in the same way; we should then infer that the stratified rocks occupied a basin formed by the rocks of these two mountains, and by calculating the thickness right through these strata could say to what depths the rock of the mountain extended below. In this way has the interior of the globe been examined, and its contents and arrangement, for several miles below the surface, ascertained. The result of such inspection we leave the author of the Vestiges to describe:—

"It appears that the basis rock of the earth, as it may be called, is of hard texture, and crystalline in its constitution. Of this rock, granite may be said to be the type, though it runs into many varieties. Over this, except in the comparatively few places where it projects above the general level in mountains, other rocks are disposed in sheets or strata, with the appearance of having been deposited originally from water. But these last rocks have nowhere been allowed to rest in their original arrangement. Uneasy movements from below have broken them up in great inclined masses, while in many cases there has been projected through the rents rocky matter more or less resembling the great inferior crystalline mass. This rocky matter must have been in a state of fusion from heat at the time of its projection, for it is often found to have run into and filled up lateral chinks in these rents. There are even instances where it has been rent again, and a newer melted matter of the same character sent through the opening. Finally, in the crust as thus arranged, there are, in many places, chinks containing veins of metal. Thus, there is first a great inferior mass, composed of crystalline rock, and probably resting immediately on the fused and expanded matter of the interior: next, layers or strata of aqueous origin; next, irregular masses of melted inferior rock that have been sent up volcanically and confusedly at various times amongst the aqueous rocks, breaking up these into masses, and tossing them out of their original levels."

This, we believe, is a correct outline of the crust of the earth, so far as it has been possible to observe it. It exhibits extraordinary signs of commotion and vicissitude; the lowest rocks indicating a previous condition of igneous fusion; those above them of aqueous solution. Fire and water have thus been the chief tellurian anarchists, and the shaking of continents and the constant shifting of level in sea and land still continue to attest their restless energies. That igneous matter has, during many periods, been protruded from below—that mountains have risen in succession from the sea, and injected their molten substance through cracks and fissures of superincumbent strata—are facts resting on indubitable evidence. Many masses of granite became the solid bottom of some portions of the sea before the secondary strata were laid gradually upon them. The granite of Mont Blanc rose during a recent tertiary period. "We can prove," says Professor SEDGWICK, "more than mere shiftings of level, and that many portions of sea and land have entirely changed their places. The rocks at the top of Snowdon are full of petrified sea-shells; the same may be said of some high crests of the Alps, Pyrenees, and Andes. We have proof demonstrative that many parts of Scotland, and that all England, formed, during many ages, the solid bottom of the sea. It may be true that the antagonist powers of nature during the human period have reached a kind of balance. But during all geological periods there have been such long intervals of repose, or of such gradual movements, that we may trace the history of the earth in the successive deposits formed in the waters of the sea." This is the great business of geology.

Although at first sight the interior of the earth appears a confused scene, after careful observation we readily detect in it a regularity and order from which much instructive light is thrown on its past vicissitudes. The deposition of the aqueous rocks and the projection of the volcanic have unquestionably taken place since the settlement of the earth in its present form. They are, indeed, of an order of events which are going on under the agency of intelligible causes, down to the present day. We may therefore consider these generally as recent transactions. But advancing to the far distant antecedent era of its existence, we may consider it to have been a globe of its present size enveloped in the crystalline rock already described, with the waters of the present seas and the present atmosphere around it, though these were probably in considerably different conditions, both as to temperature and their constituent materials, from what they now are. We may thus presume that, without this primitive case of granitic texture, the great bulk of the matters of our earth were agglomerated, whether in a fluid or solid state is uncertain; but there cannot be any doubt that they continue to exist in a condition of great heat and compression, having a mean density of more than double that of the minerals on the surface.

Judging from the results and still observable conditions, it may be inferred that the heat retained in the interior of the globe was more intense, or had greater freedom to act, in some places than in others. These become the scenes of volcanic operations, and in time marked their situations by the extrusion from below of trap and basalts—rocks composed of the crystalline matter, fused by intense heat, and developed on the surface in various conditions, according to the particular circumstances under which it was sent up; some, for example, being thrown up under water, and some in the open air, which contingencies would make considerable difference in its texture and appearance. It would, however, be a mistake to infer that, previous to these eruptions, the earth was a smooth ball, with air and water playing round it. Geology tells us plainly that there were great irregularities—lofty mountains, interspersed with deep seas—and by which, perhaps, the mountains were wholly or partially covered. But it is a fact worthy of observation that the solids of our globe cannot for a moment be exposed to water or the atmosphere without becoming liable to change. They instantly begin to wear down. The matter so worn off being carried into the neighbouring depths and there deposited, became the components of the successive series of stratified rocks, extending from the basal envelope of granite to the earth's surface, and which it will be proper briefly to describe.