Part 11

As the earth cooled, beds of concrete substances would necessarily be formed, which, floating at first in isolated masses on the surface of the semi-fluid matter, would in course of time come together, consolidate, and form continuous banks; just as we see with the ice of the present Polar Seas, which, when brought in contact by the agitation of the waves, coalesces and forms icebergs, more or less movable. By extending this phenomenon to the whole surface of the globe, the solidification of its entire surface would be produced. A solid, but still thin and fragile crust, would thus envelop the whole earth, enclosing entirely its still fluid interior. The entire consolidation would necessarily be a much slower process--one which, according to the received hypothesis, is very far from being completed at the present time; for it is estimated that the actual thickness of the earth’s crust does not exceed thirty miles, while the mean radius or distance from the centre of the terrestrial sphere, approaches 4,000 miles, the mean diameter being 7,912·409 miles; so that the portion of our planet, supposed to be solidified, represents only a very small fraction of its total mass.

We say thirty miles, for such is the ordinary estimated thickness of the earth’s crust, usually admitted by savants; and the following is the process by which this result has been obtained.

We know that the temperature of the earth increases one degree Centigrade for every hundred feet of descent. This result has been borne out by a great number of measurements, made in many of the mines of France, in the tin mines of Cornwall, in the mines of the Erzgeberge, of the Ural, of Scotland, and, above all, in the soundings effected in the Artesian wells of Grenelle and Passy, near Paris, of St. André de Iregny, and at a great number of other points.

The greatest depth to which miners have hitherto penetrated is about 973 yards, which has been reached in a boring executed in Monderf, in the Grand Duchy of Luxembourg. At Neusalzwerk, near Minden, in Prussia, another boring has been carried to the depth of 760 yards. In the coal-mines of Monkwearmouth the pits have been sunk 525 yards, and at Dukinfield 717 yards. The mean of the thermometic observations made, at all these points, leads to the conclusion that the temperature increases about one degree Fahrenheit for every sixty feet (English) of descent after the first hundred.

In admitting that this law of temperature exists for all depths of the earth’s crust, we arrive at the conclusion that, at a depth of from twenty-five to thirty-five miles--which is only about five times the height of the highest mountains--the most refractory matter would be in a state of fusion. According to M. Mitscherlich, the flame of hydrogen, burning in free air, acquires a temperature of 1,560° Centigrade. In this flame platinum would be in a state of fusion. Granite melts at a lower temperature than soft iron, that is at about 1,300°; while silver melts at 1,023°. In imagining an increase of temperature equal to one degree for every hundred feet of descent, the temperature at twenty-five miles will be 1,420° C. or 2,925° F.; thirty miles below the surface there will be a probable temperature of 1,584° C. or 3,630° F.; it follows, if these arguments be admitted, and the calculation correct, that the thickness of the solid crust of the globe does not much exceed thirty miles.

This result, which gives to the terrestrial crust a thickness equal to 1/190 of the earth’s diameter, has nothing, it is true, of absolute certainty.

Prof. W. Hopkins, F.R.S., an eminent mathematician, has much insisted upon the fact, that the conductibility of granite rocks, for heat, is much greater than that of sedimentary rocks; and he argues that in the lower stratum of the earth the temperature increases much more slowly than it does nearer the surface. This consideration has led Mr. Hopkins to estimate the probable thickness of the earth’s solid crust at a minimum of 200 miles.

In support of this estimate Mr. Hopkins puts forward another argument, based upon the precession of the equinoxes. We know that the terrestrial axis, instead of always preserving the same direction in space, revolves in a cone round the pole of the ecliptic. Our globe, it is calculated, will accomplish its revolution in about 25,000 years. In about this period it will return to its original position. This balancing, which has been compared to that of a top when about to cease spinning, produces the movement known as the precession of the equinoxes. It is due to the attraction which the sun and moon exercise upon the swelling equatorial of the globe. This attraction would act very differently upon a globe entirely solid, and upon one with a liquid interior, covered by a comparatively thin crust. Mr. Hopkins subjected this curious problem to mathematical analysis, and he calculated that the precession of the equinoxes, observed by astronomers, could only be explained by admitting that the solid shell of the earth could not be less than from about 800 to 1,000 miles in thickness.

In his researches on the rigidity of the earth, Sir William Thomson finds that the phenomena of precession and nutation require that the earth, if not solid to the core, must be nearly so; and that no continuous liquid vesicle at all approaching 6,000 miles in diameter can possibly exist in the earth’s interior, without rendering the phenomena in question very sensibly different from what they are.

The calculations of Mr. Hennessey are in direct opposition to those of Sir William Thomson, and show that the earth’s crust cannot be less than eighteen miles, or more than 600 miles in thickness.

Admitting, for the present, that the terrestrial crust is only thirty miles in thickness, we can express in a familiar, but very intelligible fashion, the actual relation between the dimensions of the liquid nucleus and the solid crust of the earth. If we imagine the earth to be an orange, a tolerably thick sheet of paper applied to its surface will then represent, approximately, the thickness of the solid crust which now envelopes the globe. Fig. 13 will enable us to appreciate this fact still more correctly. The terrestrial sphere having a mean diameter of 7,912 miles, or a mean radius of 3,956 miles, and a solid crust about thirty miles thick, which is 1/260 of the diameter, or 1/130 of the radius, the engraving may be presumed to represent these proportions with sufficient accuracy.

To determine, even approximately, the time such a vast body would take in cooling, so as to permit of the formation of a solid crust, or to fix the duration of the transformations which we are describing, would be an impossible task.

The first terrestrial crust formed, as indicated, would be incapable of resisting the waves of the ocean of internal fire, which would be depressed and raised up at its daily flux and reflux in obedience to the attraction of the sun and moon. Who can trace, even in imagination, the fearful rendings, the gigantic inundations, which would result from these movements! Who would dare to paint the sublime horrors of these first mysterious convulsions of the globe! Amid torrents of molten matter, mixed with gases, upheaving and piercing the scarcely consolidated crust, large crevices would be opened, and through these gaping cracks waves of liquid granite would be ejected, and then left to cool and consolidate on the surface. Fig. 14 represents the formation of a primitive granitic mountain, by the eruption of the internal granitic matter which forces its way to the surface through a fracture in the crust. In some of these mountains, Ben Nevis for example, three different stages of the eruption can be traced. “Ben Nevis, now the undoubted monarch of the Scottish mountains,” says Nicol, “shows well the diverse age and relations of igneous rocks. The Great Moor from Inverlochy and Fort William to the foot of the hill is gneiss. Breaking through, and partly resting on the gneiss is granite, forming the lower two-thirds of the mountain up to the small tarn on the shoulder of the hill. Higher still is the huge prism of porphyry, rising steep and rugged all around.” In this manner would the first mountains be formed. In this way, also, might some metallic veins be ejected through the smaller openings, true injections of eruptive matter produced from the interior of the globe, traversing the primitive rocks and constituting the precious depository of metals, such as copper, zinc, antimony, and lead. Fig. 15 represents the internal structure of some of these metallic veins. In this case the fracture is only a fissure in the rock, which soon became filled with injected matter, often of different kinds, which in crystallising would completely fill the hollow of this cleft, or crack; but sometimes forming cavities or geodes as a result of the contraction of the mass.

But some eruptions of granitic and other substances, ejected from the interior, never reach the surface at all. In such cases the clefts and crevices--longitudinal or oblique--are filled, but the fissures in the crust do not themselves extend to the surface. Fig. 16 represents an eruption of granite through a mass of sedimentary rock--the granite ejected from the centre fills all the clefts and fractures, but it has not been sufficiently powerful to force its way to the surface.

On the surface of the earth, then, which would be at first smooth and unbroken, there were formed, from the very beginning, swelling eminences, hollows, foldings, corrugations, and crevices, which would materially alter its original aspect; its arid and burning surface bristled with rugged protuberances, or was traversed by enormous fissures and cracks. Nevertheless, as the globe continued to cool, a time arrived when its temperature became insufficient to maintain, in a state of vapour, the vast masses of water which floated in the atmosphere. These vapours would pass into the liquid state, and then the first rain fell upon the earth. Let us here remark that these were veritable rains of boiling water; for in consequence of the very considerable pressure of the atmosphere, water would be condensed and become liquid at a temperature much above 100° Centigrade (212° Fahr.)

The first drop of water, which fell upon the still heated terrestrial sphere, marked a new period in its evolution--a period the mechanical and chemical effects of which it is important to analyse. The contact of the condensed water with the consolidated surface of the globe opens up a series of modifications of which science may undertake the examination with a degree of confidence, or at least with more positive elements of appreciation than any we possess for the period of chaos; some of the features of which we have attempted to represent, leaving of necessity much to the imagination, and for the reader to interpret after his own fashion.

The first water which fell, in the liquid state, upon the slightly cooled surface of the earth would be rapidly converted into steam by the elevation of its temperature. Thus, rendered much lighter than the surrounding atmosphere, these vapours would rise to the utmost limits of the atmosphere, where they would become condensed afresh, in consequence of their radiation towards the glacial regions of space; condensing again, they would re-descend to the earth in a liquid state, to re-ascend as vapour and fall in a state of condensation. But all these changes, in the physical condition of the water, could only be maintained by withdrawing a very considerable amount of heat from the surface of the globe, whose cooling would be greatly hastened by these continual alternations of heat and cold; its heat would thus become gradually dissipated and lost in the regions of celestial space.

This phenomenon extending itself by degrees to the whole mass of watery vapour existing in the atmosphere, the waters covered the earth in increasing quantities; and as the conversion of all liquids into vapour is provocative of a notable disengagement of electricity, a vast quantity of electric fluid necessarily resulted from the conversion of such large masses of water into vapour. Bursts of thunder, and bright flashes of lightning were the necessary accompaniments of this extraordinary struggle of the elements--a state of things which M. Maurando has attempted to represent on the opposite page (PLATE VII.).

How long did this struggle for supremacy between fire and water, with the incessant noise of thunder, continue? All that can be said in reply is, that a time came when water was triumphant. After having covered vast areas on the surface of the earth, it finally occupied and entirely covered the whole surface; for there is good reason to believe that at a certain epoch, at the commencement, so to speak, of its evolution; the earth was covered by water over its whole extent. The ocean was universal. From this moment our globe entered on a regular series of revolutions, interrupted only by the outbreaks of the internal fires which were concealed beneath its still imperfectly consolidated crust.

“At the early periods in which the materials of the ancient crystalline schists were accumulated, it cannot be doubted that the chemical processes which generated silicates were much more active than in more recent times. The heat of the earth’s crust was probably then far greater than at present, while a high temperature prevailed at comparatively small depths, and thermal waters abounded. A denser atmosphere, charged with carbonic acid gas, must also have contributed to maintain, at the earth’s surface, a greater degree of heat, though one not incompatible with the existence of organic life.

“These conditions must have favoured many chemical processes, which in later times have nearly ceased to operate. Hence we find that subsequently to the eozoic times, silicated rocks of clearly marked chemical origin are comparatively rare.”

“Address to the American Association for the Advancement of Science,” by Thomas Sterry Hunt, LL.D., p. 56. 1871.

In order to comprehend the complex action, now mechanical, now chemical, which the waters, still in a heated state, exercised on the solid crust, let us consider what were the components of this crust. The rocks which formed its first stratum--the framework of the earth, the foundation upon which all others repose--may be presumed to have been a compound which, in varying proportions, forms granite and gneiss, and has latterly been designated by geologists Laurentian.

What is this gneiss, this granite, speaking of it with reference to its mineralogical character? It is a combination of silicates, with a base of alumina, potash, soda, and sometimes lime--quartz, felspar, and mica form, by their simple aggregation, granite--it is thus a ternary combination, or composed of three minerals.

Quartz, the most abundant of all minerals, is silica more or less pure and often crystallised. Felspar is a crystalline or crystallised mineral, composed of silicate of alumina, potash, soda, or lime; potash-felspar is called orthoclase, soda-felspar albite, lime-felspar anorthite. Mica is a silicate of alumina and potash, containing magnesia and oxide of iron; it takes its name from the Latin micare, to shine or glitter.

Granite (from the Italian grano, being granular in its structure) is, then, a compound rock, formed of felspar, quartz, and mica, and the three constituent minerals are more or less crystalline. Gneiss is a schistose variety of granite, and composed of the same minerals; the only difference between the two rocks (whatever may be their difference of origin) being that the constituent minerals, instead of being confusedly aggregated, as in granite, assume a foliated texture in gneiss. This foliated structure leads sometimes to gneiss being called stratified granite. “The term gneiss originated with the Freiberg miners, who from ancient times have used it to designate the rock in which their veins of silver-ore were found.”

Cotta’s “Rocks Classified and Described,” by P. H. Lawrence, p. 232.

The felspar, which enters into the composition of granite, is a mineral that is easily decomposed by water, either cold or boiling, or by the water of springs rich in carbonic acid. The chemical action of carbonic acid and water, and the action (at once chemical and mechanical) of the hot water in the primitive seas, powerfully modified the granitic rocks which lay beneath them. The warm rains which fell upon the mountain-peaks and granitic pinnacles, the torrents of rain which fell upon the slopes or in the valleys, dissolved the several alkaline silicates which constitute felspar and mica, and swept them away to form elsewhere strata of clay and sand; thus were the first modifications in the primitive rocks produced by the united action of air and water, and thus were the first sedimentary rocks deposited from the oceanic waters.