,714 DYNAMICAL GEOLOGY. 



and 3,000 feet of lavas correspond to 3,750 pounds of pressure to the square inch. In 

 the eruptions from the summit-crater of Mount Loa, the fountain-head is 10,000 to 

 13,000 feet above the sea; and the first three of the above-mentioned eruptions may also 

 have been mainly a result of hydrostatic pressure. Previous to the last, there had been 

 a season of long-continued rains; and the waters thus furnished to the island, sinking 

 down to the seat of the fires, may have been, as Mr. Coan observes, one occasion of the 

 violence of the eruption. 



In the eruptions of Vesuvius, there are usually earthquakes of more or less power, 

 Ioftj'' ejections of cinders and dark vapors, a breaking of the mountain's summit on one 

 side or the other, or Assures opened in the sides below. In these violent ejections, there 

 may be proof of a sudden evolution of vapors. But pressure also acts as at Mount Loa; 

 for the volcano, during the year or more preceding, had become charged nearly to its 

 brim, ready for the outbreak. 



(c.) Eruptions mostly through Fissures. — Most eruptions take place 

 through fissures in the sides of the mountain, and not by overflows 

 of the crater. The fissures may come to the surface only at intervals, 

 so as to appear like an interrupted series of rents, although continuous 

 deep below ; and they may underlie the erupted lavas as far as the 

 flow extends, although nothing appears to indicate it, owing to their 

 being concealed from view by the lavas. But frequently small cones 

 form over the wider parts of the rent, and stand along the lava-field, 

 marking the courses of the fissures. 



This method of eruption through fissures makes dikes (p. 112) in 

 the mountain ; and all volcanic mountains, when the interior is ex- 

 posed by gorges, contain dikes in great numbers. After the mending 

 of the fracture by a filling of solid lava, the mountain is stronger 

 than before. 



(d.) Eruptions Periodical. — Three eruptions occurred at Kilauea 

 at intervals of eight to nine years, this being the length of time re- 

 quired to fill the crater up to the point of outbreak, or four to five 

 hundred feet. The action was regular in its period, or a result of a 

 systematic series of changes, and not paroxysmal. The crater again 

 filled to within 500 feet of the top, or half its depth, in eight years 

 after 1840. But, for some reason, another great eruption did not take 

 place until April, 1868. There may have been a submarine eruption 

 in the interval. 



Even in the case of Vesuvius, — the other type of volcanoes, — the 

 history may be similarly progressive, although the violent activity 

 excited usually ends in a kind of paroxysmal eruption. There are, 

 however, so many causes of irregularity that the periodicity, if exist- 

 ing, would be distinguishable only after a long period of observation. 



(e.) Difference in Eruptions due to Liquidity of Lavas. — At Mount 

 Loa, the absence of cinders and the low lava-jets prove remarkable 

 liquidity in the lavas at all times. At Vesuvius, the great abundance 

 of cinder-eruptions proves, on the contrary, that the lavas are very 



