ON GEOLOGY. 



on the wings of the breeze, or casually dropped with the excretions of 

 birds or other animals that pass over them. In several parts, observes M. 

 Cuvier, these proceed with a frightful rapidity, overwhelming forests, 

 houses, and cultivated fields, in their irresistible progress. Those on the 

 coast of the Bay of Biscay have actually buried a coDsiderable number of 

 villages whose existence is noticed in the records of the middle ages. And 

 even in the present day they are threatening not fewer than ten distinct 

 hamlets with almost inevitable destruction ; one of which, named Mimigan, 

 has been in perpetual danger for upwards of twenty years, from a sand-hill 

 of more than sixty feet in perpendicular height, produced by the cause We 

 are now contemplating, and which is very obviously augmenting.''^ 



There are various forelands on the coasts of the North Sea, and parti- 

 cularly on those of the counties of Sleswigh 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 Lybian 

 Desert, and in Lower Egypt. M. Denon informs us, in his travels over 

 this part of the world, that the sumrpits 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 Lybian 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 habit- 

 able. " 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, m 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. "J 



The various islands that spot the surface of the sea have arisen from 

 different causes. Many of them have been merely separated from the ad- 

 joining continent by the inroad of the sea itself upon the main-land ; others 

 have been thrown up by volcanoes, which have at times disgorged prodi- 

 gious blocks of granite among the mixt materials, such as are frequently 

 found in the Danish archipelago, in the midst of the geest^ or alluvial mat- 

 ter, 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 astonishmg 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 Linnseus. 



In so large an abundance, and with so much facility, is calcareous mat- 

 ter elaborated by these, as well as by various other animals, and especially 

 the testaceous worms, that M. Cuvier is inclined to ascribe all the calca- 

 reous rocks that enter into the soUd crust of the earth to an animal origin. § 



* Report concerning the downs of the Gulf of Gascony, or Bay of Biscay, by M. Tassin, 

 Mont de Marsan. an. x. Cuvier, Theory of the Earth, § 31. 

 t De Luc, Voyages Geologiques, torn. i. 



I Jameson's Notes on Cuvier's Theory, &c. p. 217. Compare Dolomieu's Mtnaoif ou 

 Egypt, in Journ. de Physique, torn. xlii. 



§ 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 ail 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. Tellia- 

 mid, p. 160, Fhilosoptiie Zoologique, passim. 



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