MINERALOGY. 



401 



fceu of wa 



By thi* agency of water, the height of limestone and 

 mountain* is gradually diminished, caves are 

 excavated in them, and the water of such countries is 

 much impregnated with gypseous and calcareous mat- 

 ters. The rock-salt which occurs in hills of gypsum, 

 is often dissolved by the water, and thus cavities of con- 

 siderable magnitude are formed ; and by the continued 

 action of the water on the gypsum, the cavities increase 

 in size, until the superincumbent pressure becomes too 

 great, and then the roof falls in and forms those re- 

 markable funnel-shaped hollows so often observed in 

 gypsum countries. 



Sometime*, a* in felspar rocks, the percolating water 

 washes away the alkaline ingredient ; in other cases, 

 the moisture combines with iron, and forms hydrate, 

 or by its decomposition oxidates the metallic substances 

 in a greater or less degree. By it* action on sulphu- 

 rous compounds, as on pyrites, it gives rise to sulphates 

 or vitriols. As iron is the most general and abundant 

 snetal in the mineral kingdom, and is easily acted on by 

 air and moisture, it follows that it must be one of the 

 most active agents in the disintegration of mineral sub- 

 stance*. 



Forming Effect* of Walcr. 



We shall next consider the firmmg effects of wa- 

 Ur, which are, a* already mentioned, either meckanieml 

 or chmtical. 



Mcchan il It M evident, that every mechanical destruction will 

 forming cf- be followed by a mechanical formation ; for the maste* 

 which are separated bv the water will be again deposi- 

 ted on the surface of tfie lan.l. in lakes, rivers, on coasts, 

 or on the bottom of the sea. During land floods, the water 

 does not always convey its mechanically mixed part* to 

 rivers ; on the contrary, it often depo*ites them in hol- 

 low place*. Those particles that reach rivers, form 

 sand-hanks, particularly in slow-flowing riven, 

 extensive mechanical formations are daily taking place 

 on the coasts, and even in some places at a 

 distance from them, by the water* of the ocean. In the 

 Baltic or East Sea, many appearance* of this kind are 

 Formation to be observed. Thus the Bay of Fulbaka, which was 

 fne navigated with boat* within the memory of man, is mow 

 "" ** filled sip, and covered with grass. Several harbours in 

 Lapland that formerly admitted vesseb, are now three 

 or four thousand paces from the sea ; and at Helsingor 

 there are iron-work* in place* which were covered by 

 the sea about eighty years ago. The whole of the an- 

 cient kingdom of I'ruxia appears to have been formed 

 in this manner ; it is said that the sea reached at far as 

 Culm within the period of human history. The city of 

 Dantsic, several hundred year* ago, was close on the 

 sea-shore. 



Similar appearances occur on other coast*. Between 

 the coasts of Norfolk and Zealand in Holland, there i* 

 a great sand-bank where opposite current* meet, and it 

 i* probable that this bank will in time form an island, 

 and probably even an isthmus. Much of the country 

 of the L'intt <1 Province* hat been produced by the form- 

 ing action of the sea. 



A great portion of the flat country from the mouth 

 of the Rhone to the Pyrenees, is said to be the work of 

 the ocean ; and the whole tract of country from Pisa 

 to Leghorn, is a formation of the same nature. 



In those parts of the sea where its waters are but lit- 

 tie agitated, similar forming effects are to be observed. 



Where marine currents How rapidly, and near the 

 they exert a destroying power, but when they 

 act at distance, a forming power. 



VOL. XIV. FAHT II. 



The effects produced by the sea alone, without the Gaogmxr. 

 aid of rivers, are far less beneficial. When the fee,- * ^J 

 coast is low, and the bottom consists of sand, the waves Formation 

 push this sand towards the shore, where, at every reflux ^ down - 

 of the tide, it becomes partially dried ; and the wind*, 

 which almost always Mow from the sea, drift up some 

 portion of it upon the beach. By this means don-ns, or 

 ranges of low sand hills are formed along the coast. 

 These, if not fixed by the growth of suitable plants, either 

 denominated by nature, or propagated by human indus- 

 try, would be gradually, but certainly carried towards 

 the interior, covering up the fertile plains with their 

 sterile particles, and rendering them unfit for the habita- 

 tion of mankind, because the same winds which carried 

 the loose dry sand from the shore to form the downs, 

 would necessarily continue to drift that which is at the 

 summit further towards the land. On the east coast of 

 Scotland, and in many of the islands, there are striking 

 effects of this kind. De Luc, the brother, in the Mer- 

 cure de France, communicate* the following interesting 

 statement in regard to the progress of the blowing sand, 

 termed the tmnd flood in Egypt : 



: he tandt of the Lybian desert," he says, " driven Sand- flood. 

 by the west wind*, have left no land* capable of tillage 

 on any parts of the western bank* of the Nile not shel- 

 tered by mountain*. The encroachment of these sandt 

 on soil* which were formerly habited and cultivated is 

 evidently seen. M. Denon inform* us, in the account 

 of hi* Travel* in /Mirer ami Upper Egypt, that summit* 

 of the ruins of ancient cities buried under these tandt 

 still appear externally ; and that, but for a ridge of 

 mountain* called the Lybian chain, which borders the 

 left bank of the Nile, and forms, in the parts where it 

 rise*, a barrier against the invasion of these tandf, the 

 shore* of the rivtr, on that side, would long since have 

 ceased to be habitable. Nothing can be more melan- 

 choly," says this traveller, " than to walk over villages 

 wallowed up by the sand of the desert, to trample un- 

 der foot their roofs, to strike against the summits of 

 their minarets, to reflect that yonder were cultivated 

 fields, that there grew trees, that here were even the 

 dwellings of men, and that all has vanished. 



I f then our continents were as ancient as has been Progress ( 

 pretended, no trace* of the habitation of men would ap- the uad- 

 pear on any part of the western bank of the Nile, which (ood '" 

 I* exposed to this scourge of the tandt of the desert. The **T* 

 existence, therefore, of such monuments, attests the sue- |^ 

 ceaaive progress of thai encroachments of the sand ; and ,h e ' 

 these parts of the bank, formerly inhabited, will for ever surface of 

 remain arid and waste. Thus the great population of our conti- 

 Egypt, announced by the vast and numerous ruins of n*n<- 

 its cities, was in great part due to a cause of fertility 

 which no longer exist*, and to which sufficient attention 

 has not been given. The tundt of the desert were for- 

 merly remote from Egypt; the Oases, or habitable 

 spots (till appearing in the midst of the sands, being 

 the remains of the soils formerly extending the whole 

 way to the Nile; but these tandi, transported hither 

 by the western winds, have overwhelmed and buried 

 this extensive tract, and doomed to sterility a land which 

 wa* once remarkable for its fruitful: 



" It is therefore not solely to her revolutions and 

 changes of sovereign* that Egypt owe* the loss of her 

 asjsjtssjt splendour ; it is also to her having been thus 

 irrecoverably deprived of a tract of land, by which, be- 

 fore the tandt of the desert had covered it and caused 

 it to disappear, her wants had been abundantly sup- 

 plied. Now, if we Ax our attention on this fact, and 

 reflect on the consequence* which would have attended 

 3 K 



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