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THE WONDERS OF GEOLOGY. Lect. VIII. 



and lava -streams, by occasionally acting the part of huge 

 dams across valleys and ravines, cause immense accumu- 

 lations of water ; it is true these barriers are more fragile 

 in the case of glaciers, and the consequences are therefore 

 the more destructive."* 



The effects produced by lava-currents, and their rate of 

 progress, depend of course on their degree of incandescence 

 and fluidity. When the molten mass first issues, it appears 

 like a stream of fire, but the surface quickly acquires a 

 rough scum, or crust, which soon thickens, and is broken 

 into angular pieces by the onward motion of the fluid 

 beneath. In this condition it appears like melted iron or 

 copper, and if a stick be thrust in, large semifluid masses 

 adhere, and may be removed ; and coins or other articles 

 may be plunged in, and will remain permanently imbedded 

 when the lava cools, f 



Lava-currents from Vesuvius have flowed a mile and a 

 half in fourteen minutes ; others have reached the sea in 

 three hours from the summit of the mountain, a distance 

 of 3,200 yards. The stream which destroyed Catania in 

 1669, was fourteen miles long and five wide. In Etna, 

 currents have been traced forty miles in length; and a 

 stream that issued from Mount Hecla, in Iceland, is com- 

 puted at ninety-four miles in length, and fifty in its greatest 

 breadth ; and its depth, where there were obstacles to 

 its progress, was in some places several hundred feet. 



Lava currents retain a high temperature for a long period; 

 some have been observed to flow slowly ten years after 

 their eruption. A mass of lava on the flanks of Vesuvius 

 ignited wood thrust into it four years after its motion had 

 ceased. 



The cooled lavas, and other mineral products which form 



* Patchwork, by Captain Basil Hall, vol. iii. p. 153. 

 f Persons visiting Vesuvius generally bring away such mementos 

 of their ascent to its crater. 



