698 DYNAMICAL GEOLOGY. 



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 

 required 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 

 filled up again in eight years after 1840. But, for some reason, the 

 fires then began to decline (perhaps after a submarine eruption), 

 and another eruption has not taken place. 



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 existing, 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 equally the viscidity of the lavas. In the 

 latter case, the escape of vapors would be more likely to be 

 repressed until violent paroxysmal effects became a consequence of 

 the accumulation ; and this may be one reason of the earthquakes 

 attending the eruptions of such volcanoes. 



4. Origin of the forms of volcanic cones. 



The general form of the growing mountain has been stated to 

 depend on the nature of the material ejected, whether lava, tufa, 

 or cinders, or combinations of these. But there are modifications 

 arising from other causes. The principal one is the following : — 



The angle of declivity in a growing cone depends on the part of 

 the cone from which the eruptions take place. Overflows at top, 

 if descending but part of the way to the base, increase the height 

 and steepness ; but descending all the way to the base they add 

 to the magnitude of the cone without varying the general slope. 

 In fissure-eruptions, fissures at the summit widen the top and 

 increase the slope, for it is like driving in a wedge ; but fissures 

 and outflow about the base spread the base and diminish the ave- 

 rage slope: the southeastern slope of Mount Loa spreads out 

 for a score of miles at an angle of one to three degrees, owing to 

 this flattening process. The slope, then, of a cone depends on 

 the concomitant action of the force causing eruption (this force 



