202 SOIL. 



Yellow river. These fluviatile deposites accumulate £t vhe mouths 

 of great rivers, and gradually encroach upon the ocean ; ihis is very 

 conspicuous, for example, at the mouths of the Elbe, where, at the 

 turn of the tide,' when there is an interval of calm, the earthy mat- 

 ters which are held in suspension are precipitated, and a sediment 

 results, which is thrown up by the next waves upon the beach. By 

 these successive deposites, the beach rises gradually, and an extensive 

 alluvium is formed which remains dry at neap and ordinary tides. 

 These new lands, the fertility of which is truly surprising, constitute 

 the polders of whicl ihe Dutch make so much. During spring 

 tides, and storms from particular quarters, these polders would of 

 course be all submerged, had not the active industry of the inhabit- 

 ants raised dykes, which successfully oppose the waters of the 

 ocean. 



Besides the mechanical causes of the destruction of rocks al- 

 ready quoted, there is a chemical action depending upon meteoro- 

 logical influences, which exerts a powerful influence upon the con- 

 stituent elements of crystalline rocks. Felspar, amphibolite, mica, 

 and the protoxide of iron suffer decomposition in certain circum- 

 stances with surprising rapidity, without our being able to foresee, 

 and still less to explain, this singular tendency to destruction. In 

 granite, for example, the felspar and the mica lose their vitreous 

 and crystalline state, they become friable, earthy, and are trans- 

 formed into an argillaceous substance, which is known in the arts 

 under the name of kaoline, and which is extensively used m the 

 manufacture of porcelain ; amphibolite, and pyroxenite, undergo an 

 alteration of the same kind. In these minerals the protoxide of iron 

 passes to the state of the maximum of oxidation. The air and 

 moisture appear to exert a great influence upon this alteration, which 

 frequently extends to a great depth, as we see in the beds of porce- 

 lain earth, which are worked in various granite districts, and as I 

 have myself ascertained, in a bed of decomposed syenitic porphyry 

 where there are very extensive subterraneous works. In these 

 works, which are carried on in auriferous strata, the alteration in 

 the felspar and amphibolite can be followed to a depth of nearly 330 

 feet. In the midst of the rocks so changed, we every here and there 

 meet with masses which have resisted the decomposing action, and 

 still possess all their original hardness and freshness. Historical 

 monuments also show us unalterable granites ; such is that, for in- 

 stance, which now forms the obelisk ir/ the square of San Giovanni 

 di I^aterano at Rome, and which was cut at Siena, under the reign 

 of a king of Thebes, thirteen hundred years before the Christian era. 

 Such is further the obelisk of the Place of St. Peter, which was 

 consecrated to the sun by a son of Sesostris more than three thou- 

 sand years ago. 



The schists, by reason of their structure, wear away with much 

 greater facility. Calcareous rocks resist atmcvpherical agencies 

 somewhat better ; but their softness in general suffers them to be 

 readily attacked by mechanical causes, and water even acts upon 

 hem as a sdveat through the medium of the carbonic acid which 



