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been followed by many natural philosophers who have given it all possible extension, and, from physical truths and exact observations, have conducted this theory to a degree of veri similitude of which the others are not capable. I adopt it, not only as it appears to me the most consonant to the theorems of natural philosophy, but as I find it most proper to give the most natural and easy explanation of the facts which we observe in Sicily, and which seem to add additional proofs to those observed in other regions.”*
Bouguer, and many other naturalists, have observed, that Subsidence. in South America the plains have palpably subsided, and left the rocks elevated in many fantastic forms. It is indeed to be conceived that the earth, originally in a fluid state, as appears from the depression of the poles, and many other circumstances well known in natural philosophy, and replete with innumerable vapours and gases, could only acquire its present comparative solidity by prodigious subsidences, arising from the gravitation of the solid and semifluid parts towards the centre. The most prodigious of the subsidences must have been that which sunk two thirds of the globe to make room for the present oceans, sufficient receptacles for the primeval waters, if the idea of this vast subsidence can be supported. Ferrara, arguing only on that subsidence which gave place to the Mediterranean, says that the mountains above Reggio are very sensibly inclined towards the sea, which indicates that their base sunk to form the channel which divides Italy from Sicily. He also observes, that the inclination of the strata towards the sea may be seen in all the mountains which border the southern side of Sicily t. The following passage likewise deserves observation: "Where the mountains are formed of soils in which the lavas are united with the calcareous masses, or, to explain myself more clearly, where a frontier of consolidated lava was filled from the bottom to the top with calcareous masses, the series of these heights is calcareous on the one side, and volcanic on
the other. Such is the mountainous mass, terminating in
Perhaps this doctrine of subsidence might of itself explain the inequalities and other phenomena of the earth's surface, without having recourse to any concussion of a satellite or other body. The summits of basalt, and the caps of lime
+ Ferr. 378.
stone, in the Tyrolese, might perhaps be explained in this manner, and we are at least certain that the cause exists. But it is far from the intention of this work to propose or support any theory; and these remarks must only be regarded as a few scattered hints which may interest the reader.
Pini, in what he calls a new theory of the earth, supposes Pini's system. a nucleus surrounded with a fluid zone, which contained the elements of the various substances; and he imagines the effects and variations to have been very prompt and sudden, owing to the extreme rapidity of the rotation of the earth. He argues for a formation wholly aqueous; but his chief new fact seems to be a granitic mountain at Gana, in Austrian Lombardy, which is throughout full of cavities, a few inches distant from each other, and lined with crystals of quartz and felspar *.
The chief features of De Luc's new system of geology De Luc's. seem to be the following. He supposes that during the deluge the former continents disappeared; but this is clearly contrary to the Mosaic account of paradise, and the whole scriptural narrative, which represents the land as stable and unalterable, That successive catastrophes affected the beds of our continents, even while they were rising under the waters by chemical precipitations, being occasioned by caverns which formed under them. That valleys, lakes, abrupt precipices, existed at the birth of our continents, in consequence of those catastrophes by which the beds were ruined. That stony masses and gravel, which are scattered in such great quantities upon the continents, are also original features, and do not arise from currents; the flints proceeding from beds of chalk dissolved; and the gravel, as well as the large blocks, caused by the attrition of fragments, have been expelled from the interior by expansive fluids, during the subsidence of the beds, and dispersed at the same time at the bottom of the sea. That the precipices towards the sea have not been produced by the sea itself, but are original features,
See the Opuscoli Scelti, tom. xiii. Milan 1790, 4to. p. 369, 379.
resulting from the rupture of the beds, at the time of the vast subsidence which sunk the former continents, and produced the new concavity of the ocean.
These theories may be compared with the Wernerian and Huttonian, and that of Ferrara, founded, as he says, on that of Burnet. The rocks having been hitherto considered as the chief province of the geologist, it is hoped these few cursory remarks will not be found foreign to the purpose. But Petralogy, as already observed, has little more connexion with Geology than its sister sciences Lithology or Metallogy; and, like them, can only be regarded as an introduction. In which point of view these observations may not be found unuseful to the student. But it is time to return to the description of the Accidential Domains, an accurate knowledge of which may be regarded as peculiarly indispensable to any system of geology, such theories having so often confounded the pride of human science. The more humble sage will perhaps be contented with the knowledge of the substances themselves, and prefer what Gibbon calls a LEARNed ignoRANCE to any geological theory.
THIS division comprehends the rocks which consist of different substances blended together, and for which no distinct denominations have been adopted. Many of them have been classed under vague names, particularly that of granite.
Under the division of Aggregated Rocks, Gmelin's plan. Gmelin, in his edition of Linnæus, has ar