119 VOLCANIC PHENOMENA. ERUPTIONS. 



lances ; and lastly, portions of melted matter torn from the lava 

 filling the crater, and becoming rounded by their motion through 

 the air, form what are called volcanic bombs. From all this we 

 have, amidst violent detonations, immense bundles or masses of 

 various matters projected to great heights, lighted by reflection from 

 the melted lava, part of which fall at greater or less distances, ac- 

 cording to their weight and the force with which they are impelled 

 Ashes, rapilli, or pumice then produce in the vicinity of the volcan, 

 sometimes even at a distance, considerable deposits, which becoming 

 solid by their weight and by water, form what is termed volcanic 

 tufa,pumice tufa, and various conglomerates. 



The vapours and ashes ejected from volcanoes sometimes form enormous 

 clouds, frequently dense enough to intercept the light of day, and shroud 

 the whole neighbourhood in darkness. These clouds, driven by the wind, 

 are sometimes carried to the distance of twenty, fifty, and even two him. 

 dred leagues. This happened in 1-812, when the ashes of Saint Vincent, in 

 the Antilles, were carried to Barbadoes, and so darkened the air that persons 

 could not see their way. The ashes of Vesuvius were carried in 1794 to 

 the end of Calabria; and it was found even in Procopus, that during the 

 eruption of 452 they were conveyed as far as Constantinople. 



What occurs at the bottom of seas during eruptions is not seen ; but it is 

 clear that the ejection of earthy matters, rapilli, and pumice, are not less 

 abundant, because we find at these times on the surface enormous quanti- 

 ties of them, and in land upheaved, there are seen distinctly deposits of 

 volcanic tufa, pumice tufa, and conglomerates, precisely like those formed 

 on land. 



30. Appearance of melted matters. The phenomena mentioned 

 are sometimes the only effects of an eruption ; but most generally 

 they are only the precursors orsequents of the expulsion of melted 

 matter, which soon appears under different forms. Sometimes 

 these matters, most frequently in mass, rise in cones or domes 

 above the very orifice from which they issued, sometimes entire, 

 sometimes vertically perforated in the centre, sometimes suscep- 

 tible of- being pushed further out. This happened at Jorullo, and 

 again and again in the gulf of Santorin, and the same must occur 

 in a great many other localities. 



31. Under other circumstances, the crater first formed at the 

 oummit of a volcan is completely filled with melted matters ; these 

 soon break a passage at a greater or less depth, pouring out tor- 

 rents, which furrow the side of the mountain, and run to the 

 plain, where they spread more or less. 



32. Form of currents. If fissures or cracks of eruption be 

 formed at the foot of a volcano in a flat country, the lava escaping 

 from it at once forms broad horizontal sheets in the middle of the 

 plain. This occurred in Iceland in 1783; crevasses formed in the 

 plain at the foot of Skaptar-Jokul, a high volcanic mountain of the 



30. What is the form of melted matters ejected from volcanoes ? 



31. How are lava-currents formed ? 



32. What i? the form of Lva-currcnts? 



