There are various forelands on the coasts of the North Sea, and particularly on those of the counties of Sleswig and Holstein, which are formed in the same manner. * But the most extraordinary inroads of sand-storms and sand-floods are, perhaps, those which have taken place in the Libyan Desert, and in Lower Egypt. M. Denon informs us, in his travels over this part of the world, that the summits of the ruins of ancient cities buried under mountains of drifted sands still appear externally; and that but for a ridge of mountains, called the Libyan Chain, which borders the left bank of the Nile, and forms a barrier against the invasion of these sands, the shores of the river, on that side, would long since have ceased to be habitable. 66 Nothing," says M. Denon, " can be more melancholy, than to walk over villages swallowed by the sand of the desert, to trample under foot the roofs of their houses, to strike against the tops of their minarets, and to reflect, that yonder, in days of yore, were cultivated fields, that hard by were groves of flourishing trees, and the dwellings of men close at hand; —and that all has now vanished."+

The various ISLANDS that spot the surface of the sea have arisen from different causes. Many of them have been merely separated from the adjoining continent by the inroad of the sea itself upon the main-land; others have been thrown up by

Bay of Biscay, by M. Tassin, Mont de Marsan. an. x. Cuvier, Theory of the Earth, § 31.

* De Luc, Voyages Géologiques, tom. i.

Jameson's Notes on Cuvier's Theory, &c. p. 217. Compare Dolomieu's Memoir on Egypt, in Journ. de Physique,

volcanoes, which have at times disgorged prodigious blocks of granite amongst the mixed materials, such as are frequently found in the Danish archipelago, in the midst of the geest, or alluvial matter, which has collected around them. Other islands are altogether the masonry of madrepores, and other coral zoophytes of wonderful industry and perseverance, of which the South Sea furnishes us with the largest and most astonishing specimens. These islands are for the most part flat and low, and surrounded by enormous belts of coral reefs. Most of the calcareous zoophytes are employed in their construction, but the principal worm is the madrepora lubricata of Linnæus.

In so large an abundance, and with so much facility, is calcareous matter elaborated by these, as well as by various other animals, and especially the testaceous worms, that M. Cuvier is inclined to ascribe all the calcareous rocks that enter into the solid crust of the earth to an animal origin. * But this is to suppose the earth of a far higher antiquity, and to have been the subject of more numerous general deluges, and inversions of sea and land, than are called for by the Wernerian system, or appear reconcilable with the Mosaic narrative.

* Some writers have proceeded much farther than this, for they have resolved all the solid materials of the earth's crust into an organic origin. Such was the opinion of Demaillet and Lamarck, who suppose that every thing was originally fluid; that this universal fluid gave rise to plants and animals; that all clay or argillaceous earth is the produce of the former; all calcareous earth of the latter; and that siliceous earth has been the result of the two. Telliamid, p. 169. Philosophie Zoologique, passim.,

M. Cuvier apprehends, indeed, that such catastrophes may have occurred five or six times in succession, at a distance of four, five, or six thousand years from each other; and that even the chalk formation found in the basin of Paris originated in a revolution of this kind that occurred antecedently to that which is usually regarded as the flood of Noah. And, following up this idea, he conceives, towards the close of his Introductory Theory of the Earth, that, if the science of fossil organic productions could be carried to a much higher degree of perfection, we should be able to obtain far fuller information upon this subject; "and man, to whom only a short space of time is allotted upon the earth, would have the glory of restoring the history of thousands of ages which preceded the existence of the human race, and of thousands of animals that never were contemporaneous with his species." But this is to abandon the cool and steady path of philosophic research, according to the principles of Bacon and Newton, and to yield the reins to mere speculation.

130

LECTURE VII.

:

ON GEOLOGY.

(The subject continued.)

In our last lecture I attempted a brief sketch of the chief phænomena that occur to the eye of the geologist upon a survey of the solid crust of the earth, as far as he is able to penetrate into it. The conclusion to which such phænomena lead us is the following that the rudimental materials of the globe, to the utmost depth we are able to trace them, existed, at its earliest period, in one confused and liquid mass; that they were afterwards separated, and arranged by a progressive series of operations, and an uniform system of laws, the more obvious of which appear to be those of gravity and crystallization; and that they have since been convulsed and dislocated by some dreadful commotion and inundation that have extended to every region, and again thrown a great part of the organic and inorganic creation into a promiscuous jumble.

Now the only two causes that have been adduced as competent to the fluidity that appears at first to have existed throughout the whole crust of the earth are FIRE, or a peculiar SOLVENT. But, if a solvent, that solvent seems of necessity to have been WATER: for there is no other liquid in nature in sufficient abundance to act the part of a solvent upon a scale so extensive.

And hence enquiries into this subject have become in some degree limited, and have been chiefly confined to what are called the PLUTONIC and the NEPTUNIAN hypotheses; the origin of the world in its present state from igneous fusion, and from aqueous solution. Both these theories are of very early date, and both of them have been agitated in ancient as well as in modern times with a considerable degree of warmth as well as of plausible argument.

Among the ancients, Heraclitus seems to have headed the advocates for the former theory, and Thales, or rather Epicurus, the supporters of the latter. In what may be regarded as modern times, Hooke may, perhaps, be held the reviver of the Plutonic system, which has since, as I have already observed, been supported by the cosmological doctrines of Buffon and Dr. Herschel. Its principal champions, however, in the present day, are Dr. James Hutton, Professor Playfair*, and Sir James Hall; names, unquestionably, of high literary rank, and entitled to the utmost deference, but most powerfully opposed by the distinguished authorities of Werner, whose system I have just glanced at, Saussure, Kirwan, Cuvier, and Jameson, not to mention that the general voice of geologists is very considerably in favour of the latter class of philosophers, and consequently of the Neptunian or aqueous hypothesis. Let us, however, take a brief view of each of these theories in their order.

According to the former, or the Plutonic conjecture, heat is the great source, not only of the

* Illustrations of the Huttonian Theory of the Earth. Edinb. 1802.