Essay on the Theory of the Earth

by baron Georges Cuvier

The occurrence of mica, which forms a predominant constituent part of our primitive mountains, as an artificial production, gave rise to the following geological speculations.

The artificial production by fusion, of the minerals which compose our primitive rocks, appears, according to Mitscherlich, to place beyond doubt the theory that our primitive mountains were formerly a melted mass. Such a state of fluidity, he continues, affords an easy explanation of the figure of the Earth, of the increase of temperature as we proceed into its interior, of hot springs, and of many other phenomena. With respect to this theory, we may refer to M. Laplace, who is convinced of its plausibility, without grounding his belief upon the reasons which chemistry presents. I propose, however, to make mention of a few facts, in order to shew with what facility many chemical phenomena in geology may be explained by following this theory.

Primitive mountains are generally distributed over the surface of the earth: it necessarily follows that the bodies which have composed the surface of the earth have participated of the temperature which the primitive mountains have had at the period when they were in a fluid state. The temperature at which water boils depends upon the pressure of the atmosphere; and if the temperature of the earth increases, we only require to diminish the mean height of the sea 32 feet, in order to have a pressure of an atmosphere more; and it is by this pressure that the degree of temperature at which water boils will also be raised higher. M. Laplace judges from the height of the sea during flowing and ebbing, that the mean depth of the sea is about 96,000 feet. Supposing three-fourths of this mass of water were converted into vapour, the pressure of this vapour would be nearly equal to 2250 atmospheres; and this pressure would so augment the degree of heat at which water enters into ebullition, that the primitive mountains might be in a state of fusion, without the water with which they are covered being heated to the boiling point; for the water which is not converted into vapour, and whose quantity is a fourth of the whole mass of vapour, according to the supposition which we have made, would cover the whole earth, because water expands in increasing proportion if the temperature be raised, and because the expansion of water is much greater than that of the mass of our primitive mountains; and, consequently, according to this supposition, our primitive mountains are formed, covered with red hot water. The great pressure of so many atmospheres necessarily modifies the reciprocal affinities of the substances which compose the primitive mountains.

Primitive mountains are distinguished from volcanic productions in this, that the lime and magnesia, which in them are combined with carbonic acid, form with the silex silicates and bisilicates. It is necessary that the silex, which, under the ordinary pressure, and at an elevated temperature, expels the carbonic acid, exercise no influence under the pressure of so many atmospheres; and it is not surprising that crystals of quartz occur in Carrara marble. In volcanic productions, this pressure no longer exists, and we should find among these the same phenomena which our laboratories and metallurgic operations present. Following this theory, the circumstances that primitive mountains contain gypsum and carbonates, and that water occurs in quartz, very readily admit of explanation. And with regard to this latter phenomenon, the observations detailed by Sir Humphry Davy afford an additional confirmation of the theory in question.

We may explain in the same manner another phenomenon, which is more in connection with the present state of our globe. Many observations shew that the sea stood formerly at a much higher level than it does at present. The water of the sea expands, if the temperature be elevated more than the land. Admitting that the surface of the earth has a temperature of 80° of Reaumur, and that the mean depth of the sea may be 96,000 feet, the height of the sea would then be 4000 feet higher than it is at present. If we suppose, as may be done without committing any great error, that the expansion of the primitive mountains is equal to that of glass, and that they have been at a temperature of 200°, and even at a much lower one, the water of the sea would cover the secondary mountains, in which we find the remains of marine animals. This explanation of the former height of the sea appears very simple, because the elevated temperature of the earth may have resulted either from its original state of fluidity, or from a geological revolution, which has destroyed, at the same time, the organic beings of a former period.

If primitive mountains and volcanic formations have been fluid, and have crystallised on cooling, it is necessary that we should retrace in them the same phenomena and the same laws which we still observe at the present time. If a fluid body become solid by cooling, these phenomena are differently modified, according to the chemical nature of the bodies, and according to the crystalline forms which they acquire on cooling; but the laws remain always the same. Mitscherlich says, I am in possession of some specimens which explain several of the phenomena so often shewn by basalt and volcanic formations. I do not possess artificial basalt resembling the natural columnar kind; yet the slags obtained at the furnaces of Sahla resemble basalt so perfectly, as to deceive the most experienced eye, especially as their cavities contain crystals of augite. But I have found at Fahlun a bisilicate of protoxide of iron, which has in consequence a composition analogous to that of basalt, and which has distinct joints. In this slag we perceive that the joints, which are parallel to the axis of the prism and to the lateral planes of the crystals, are always perpendicular to the plane of cooling. This is particularly observable in a specimen which was obtained by melting the slag in a mould; on crystallizing it had several planes of cooling, and the joints are parallel to each of these planes. The planes of separation in basalt present exactly the same phenomenon as this slag.

The phenomena which take place when a fluid body crystallizes may be observed in sulphur, better than in any other body. All fluid bodies, however, and even water, on freezing, present the same phenomena.

If a fluid body has cooled to the point at which it begins to become solid, for example, sulphur, in a round vessel, a crust of sulphur is not formed upon the surface of the cooled vessel, and another crust upon the surface of the sulphur itself, as might be expected; on the contrary, if a crystal be formed upon a point of the inner surface of the vessel, the crystal enlarges by growing in the direction of its axis, and the mass which surrounds the crystal remains liquid, and sometimes cools, without the molecules arranging themselves in the same manner as the crystal already formed. On examining the cooled mass, we observe that it shews a lamellar texture where the crystal was formed, and that the mass which surrounded it does not shew this texture in the same degree. This explains how veins of large-granular granite traverse a small-granular granite, as well as other phenomena of the same nature.

This observation also affords an explanation of another phenomenon. If the half of the liquid mass has become solid, and if the fluid part be poured off, we obtain isolated crystals, which have been formed in the fluid mass. If the fluid part be not poured off, and be permitted to cool slowly, it contracts, as is the case with most bodies, and the contraction produces the same effect as the decantation; small cavities will be formed, and these will be traversed and covered over with distinct crystals. We also observe this phenomenon in the geodes of primitive and volcanic mountains, in which the crystals they contain are of the same minerals as those of which the mountains themselves are composed.