16 ALLUVILM. DELTAS. 



this beaut.ful river, as we have already said, is clear and limpid ; 

 but a little beyond the town it receives new tributaries, such as the 

 Arve, which pour into it their muddy waters, and little by Hide it 

 is again loaded with sand and mud, which it rolls on impetuously 

 to the sea ; but at its mouth, its course being slow, these foreign 

 materials, the debris of Mont Blanc, of the Alps, of Dauphiny, 

 and the central regions of France, are in their turn deposited, and 

 gradually elevate the soil they cover ; the result is new land which 

 advances more and more on the sea. 



19. We give the name of alluvium (from the Latin, alluvio, an 

 inundation, or alluo, I wash) to formations caused in this way by 

 the deposite of materials carried by waters, and as these alluvial 

 formations, when deposited at the mouth of a river, often assume 

 the form of the Greek letter A delta, we designate the new-made 

 land, which in a manner encroaches on the domain of the sea, 

 under the name of delta. 



20. The delta of the Rhone, to which we alluded above, and 

 that which is found at the mouth of the Po, are very inconsider- 

 able ; but, in certain parts of the globe, several are found of very 

 much greater geological importance. One of the most celebrated 

 is the Delta of the Nile, which, according to the calculations of 

 some authors, must have grown nearly half a league since the 

 time of Herodotus; and according to the commonly received 

 opinion, its formation began at the foot of the rocks upon which 

 were built the pyramids of Memphis ; but the deltas at the mouth 

 of the Mississippi, and the mouth of the Ganges, increase more 

 rapidly, and possess greater interest for the naturalist. 



21. Other formations are also produced, so to speak, under our 

 eyes, by the deposite of materials which the waters of certain 

 springs hold in solution, and throw down when they reach the sur- 

 face of the earth. In different parts of France, near a spring 

 situated at the north of Clermont Ferrand, for instance, we see 

 examples on a small scale, and in many parts of Italy, enormous 

 masses of calcareous stone, known under the name of Travertin 

 (from the Italian, travertine), are formed. 



22. We often behold issuing from the craters of volcanoes, a 

 burning, serni-liquid matter, which spreads over the surface of the 

 neighbouring country, and, on cooling, is converted into a hard 

 compact rock, called lava. Etna has furnished a great number of 

 irruptions of lava, one of which was six leagues in length, and, in 

 1783, Hecla, a volcano of Iceland, gave origin to a similar cur- 

 rent, which extended twenty leagues in length, and twelve in 

 breadth. 



19. What is alluvium ? What is a Delta ? 

 ,20. Mention some examples of Deltas. 

 21. What is Travertin ? 

 22 What is lava? 



