782 



GEOLOGY. 



It must be borne in mind as almost certain, that the volcanic chimneys descend much 

 further below the level of the sea than they rise above it perhaps fifty times as much ; 

 so that the actual force pressing upon the lava in its reservoir, and the initial velocity, 

 are much greater than represented in the preceding table. It is obvious that the matter 

 ejected by volcanic action must be brought up from an immense depth, or a single volcano 

 would soon become exhausted, as the lava of one eruption frequently exceeds in amount 

 what the whole mountain melted down could supply. The lava of Etna in 1669 covered 

 eighty-four square miles; and in 1660 the amount disgorged by the mountain was equal 

 to twenty times its entire mass. The most prodigious masses accumulated by volcanic 

 action have, however, transpired in Iceland, of which we have the following record of 

 eruptions, since its first colonisation by the Norwegians at the commencement of the tenth 

 century: 



From Hecla, since the year - - - 1004 



From Kattlagiaa Jokul - 90O 



From Krabla - ] 724 



In different parts of the Guldbringe Syssel - - JOOO 



At sea - - - 1583 2 



From the lake Grimsratn, in - - 1716 1 



From Eyafialla Jokul - - - 1717 1 



From Eyrefa Jokul, in - - 1720 1 



From Skaptar Jokul, in - - 17H3 1 



22 

 7 

 4 

 3 



42 



To the above list must be added the eruption of Hecla during the past year, 1845, which 

 rivalled any of its former displays. On the night of September 2nd, the largest of the 

 Orkney islands was covered with fine ashes, resembling ground pumice stones, during a 

 violent storm. It was at once supposed that Hecla had been in eruption, though 400 

 miles distant, as the wind blew from that quarter, and the ashes were volcanic. The 

 surmise was afterwards confirmed by the statement, that the mountain, after reposing 

 between sixty and seventy years, had again resumed its explosions, bursting asunder in 

 two places at mid- day on September 2nd, and vomiting masses of molten matter. On the 

 night of September 15th, the volcano resumed its activity, with loud subterranean 

 detonations, heard over the whole island, and lava was projected to a distance of from 

 twenty to thirty miles, killing numbers of cattle, and destroying a large extent of the 

 finest pasturage. At the distance of two miles from the mountain, the fiery torrent was 

 a mile in width, and from forty to fifty feet in depth. But though great geolo- 

 gical changes are at present 

 wrought by volcanic action, 

 they are confined compara- 

 tively to very limited areas ; 

 and it has been estimated, 

 that while volcanic eruptions 

 tend to increase the diameter 

 of the earth by adding to the 

 superficies at the expense of 

 the interior, yet the mass of 

 active and extinct mountains 

 cannot be supposed to have 

 augmented its diameter to 

 the extent of two feet. Un- 

 doubtedlv, however, stnk.ng local alterations transpire, as when such a mass as that of 



Volcano of Jorullo. 



