72 ON GEOLOGY. 



tains, and in that of the almost infinite varieties of the c. familiarfs or domes- 

 tic dog, in the bosom of our own country, in the form of the c. aureus, 

 chacal or jackal, we meet with it in the warmest parts of Asia and Barbary, 

 prowling at night in flocks of one or two hundred individuals. 



The extensive TURBARIES or PEAT-FIELDS, which are so common to many 

 parts of Europe, are produced by an accumulation of the remains of sphag- 

 num and other aquatic mosses. These surround and cover up the small 

 knolls upon which they are formed ; or, in many places, descend along the 

 valleys after the manner of the glaciers of Switzerland; but, while the latter 

 melt away every year at their lower edges, the mosses are not checked by 

 any obstacle in their regular increase ; and as such increase takes place in 

 determinate proportions, by sounding their depth to the solid ground we may 

 form some estimate of 'their antiquity. 



The ordinary rise of those extensive ranges of DOWNS which are seen 

 skirting the coasts of many countries, and especially where the shore is not 

 very bold, is a mixed effort of sea and wind. To produce this, however, the 

 soil that the sea washes over must consist of sand. This is first pushed in 

 successive tides towards the shore ; it next becomes dry, by being left there 

 at every reflux of the sea ; and is then drifted up the beach, and to a consi- 

 derable distance from the beach, by the winds which are almost always blow- 

 ing from the sea, and often in whirls or eddies ; and are at length fixed by 

 the growth of wild plants, whose seeds are in like manner wafted about 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 considerable number of villages whose 

 existence is noticed in the records of the middle ages. And even in the pre- 

 sent 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 particularly 

 on those of the counties of Sleswigh and Holstein, which are formed in the 

 same manner.f 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 moun- 

 tains 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. 

 " 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 no>v vanished."! 



The various ISLANDS that spot the surface of the sea have arisen from differ- 

 ent causes. Many of them have been merely separated from the adjoining 

 continent by the inroad of the sea itself upon the mainland ; others have 

 been thrown up by volcanoes, which have at times disgorged prodigious 

 blocks of granite among 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 madre- 



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

 san. an. x. Cuvier, Theory of the Earth, $ 31. t De Luc, Voyages Ge ologiques, torn. i. 



J Jameson's Notes on Guvier's Theory, &c. p. 217. Compare Dolomieu's Memoir on Egypt, in Journ. d 

 Physique, torn. xlii. 



