710 DYNAMICAL GEOLOGY. 



crater, over its bottom, in streams which cool and become solid lava. 

 Whether the whole rising is due to this cause is not ascertained. The 

 risings and overflowings are repeated from time to time, until the ma- 

 terial within the crater has reached a height and an intensity of action 

 that lead to an eruption. 



At Kilauea (the bottom of which, when at its lowest mark, is 3,000 feet above the 

 sea), the conduit of liquid lava, below the crater, is 3,000 feet long, to the sea-level; 

 and it may extend many miles, or perhaps scoi - es of miles, below this. Nineteen miles 

 would correspond to about 100,000 feet. A rise of the lavas within the crater, for 400 

 to 500 feet, in the manner above explained, is all that, in three cases of eruption at Ki- 

 lauea, preceded the outbreak. Five hundred feet in 100,000 is an average expansion of 

 only a half of one per cent. But it is probable that the vapors which produce this 

 result are comparatively superficial; they may be from the fresh waters of the sur- 

 rounding region, or from those of the ocean adjoining. 



The increase of activity, as the lavas rise in a crater, has two obvi- 

 ous causes: (1) the temperature of the lavas increases with the pres- 

 sure ; and, consequently, a rise of 100 feet would have increased very 

 much the temperature at the bottom of that 100 feet, and so on for 

 greater depths ; (2) the rise exposes a higher column of liquid lava 

 above to the action of external waters. 



(b.) Circulating Movement. — In the lava-conduit, the greatest heat 

 is along the centre, most remote from the cold sides. Hence, as in 

 any cauldron, the ascent from inflation by rising vapors would be 

 greatest at the centre ; there would therefore be at the surface a flow 

 from the centre to the sides, and a system of circulation. This was 

 exhibited on a grand scale at Kilauea, in 1840, where the liquid lava 

 in the great lake (1,000 feet across, a, Fig. 1110, p. 705) seemed like 

 a river that came to the surface for a moment and then disappeared. 



The area of greatest heat was near the northeast side of the lake ; 

 and the stream seemed to flow to the southwest. 



3. Eruptions. — (a.) General Facts. — The lavas within the crater 

 reach such a height, and the activity of the vapors, from one cause or 

 another, becomes so great that an eruption takes place, either over the 

 brim of the crater or through the fractured mountain, and generally 

 the latter. The lavas flow off to a distance sometimes of sixty miles 

 or more. 



The outflow of lavas is attended, in most volcanoes, as at Vesuvius, 

 with the ejection of cinders from the wider parts of fissures; and they 

 often continue to be thrown out, long after the flow has ceased. They 

 thus build up a cinder-cone immediately around the open vent. 



Most of the small cones about volcanic mountains — called often parasitic cones — are 

 formed in this manner about a point in some opened fissure from which lavas were 

 ejected. Cinder and vapor eruptions are the last effects of the subsiding fires of a vol- 

 cano. Mt. Kea is an example of a mountain-cone which finished its career as an erup- 

 tive volcano by the formation of a number of cinder-cones at summit. Their height is 



