DEPOSITION OF SOIL BY RIVERS. 15 



result is that the water of these torrents is often turbid, and loaded 

 with mud, sand, flints, or even blocks of stone ; but when they 

 reach a flat country, or fall into a large basin, their course is much 

 less rapid, and the foreign materials they held in suspension are 

 gradually deposited; the heaviest sink first, and, at length, these 

 materials line the bottom of the river with an earthy bed, whose 

 thickness is continually increasing. 



17. The river Po, which is precipitated from a lofty chain of the 

 Alps, and traverses Lombardy, is a remarkab.e example of this 

 curious phenomenon. This river, and its principal tributaries, 

 have transported, in this way, so much earthy matter from the 

 mountains to the plain, that, since the Roman era, several large 

 lakes and extensive marshes, situated near Parma, Paisance, Cre- 

 mona, &c., have been filled up and become dry : the bed of these 

 rivers is also gradually filled up, so that they have several times 

 changed their course, and poured over the neighbouring plains. 

 It has been necessary to restrain them artificially, by building up 

 a long dyke on each bank ; this has put an end to these disastrous 

 inundations, but has not prevented the bottom of the river from 

 continuing to rise up; every year it is therefore necessary also to raise 

 up the dykes, so that now these rivers flow in a sort of immense 

 aqueduct, and at certain places the surface of their waters is higher 

 than the roofs of the surrounding houses, as at Ferrara, for ex- 

 ample. 



18. The river Rhone descends on the northern side of the Alps, 

 and passes the Valais too impetuously to deposit the rnul and flints 

 with which it is abundantly freighted ; but, when it empties into 

 the lake of Geneva, its course becomes so slow as to be almost 

 imperceptible, and its waters, which were at first turbid and muddv, 

 are limpid and transparent, when they escape from the opposite 

 side of this basin to pass through the town of Geneva: the result 

 is that the Rhone deposits in this basin all the matters which it 

 carried, and gradually raises up its bottom, constituting what is 

 termed lacustrine formation. This progressive elevation of the 

 soil is so marked at the eastern extremity of the lake, that an an- 

 cient town called Port Valais, formerly situated on its margin, is 

 now found about a half a league from it; about eight centuries 

 have been sufficient for the formation of the great earthy bank 

 which now separates this town from the lake, and the deposite 

 which gave rise to it continues to be made at the bottom of that 

 portion of the lake in its vicinity, and continually tends to raie it 

 up more and more, so that in time it may fill the whole of this 

 basin, and transform the lake into a plain'which the Rhone will 

 pass through without spreading itself. In passing through Geneva, 



17. Give an example of change produced by currents. 



18. What has been the effect of the Rhone passing throuf^i the kke of 

 Geneva ? What is meant by lacustrine formation ? 



