366 PACTETC OCH AN, 
an increase of twelve feet over the whole surface. This would give 
near four hundred thousand years for the formation of the part of the 
mountain above the sea. We leave this computation with the simple 
mention of the result, and the remark that it indicates the period to 
have been long, though affording no approximation to the actual 
amount of time. 
The lateral or subordinate cones of the mountain are mostly the 
product of a day, week, or month; that is, they are the immediate 
result of particular eruptions. It is a very little matter for Mount 
Loa to throw up a cone of six or eight hundred feet in height,—very 
much less than we should gather from some writers upon Jorullo and 
Monte Nuovo. ‘These lateral cones are seldom centres of any exten- 
sive streams of lava, as many descriptions would lead us to infer; on 
the contrary, the lavas about them have generally flowed from an 
opened fissure, and the cone is an elevation arising from prolonged 
eruptions over some part of such a fissure. 
Conclusions.—The foregoing discussions have led to the following 
conclusions :* 
I. That Mount Loa and similar summits (among which we would 
include most of the islands of the Pacific examined by us, besides all 
others of the same general character) were formed from successive 
eruptions of molten rock, alternating sometimes with cinder or frag- 
mentary ejections. 
II. That the eruptions are in general the result of a rising or ascent 
of the lavas, owing to the inflation by heat of such vaporizable 
substances as sulphur and water, the overflow or lateral outbreak 
taking place in consequence of the increased pressure from gravity, 
and from the elasticity of the confined vapours; and that the contrac- 
tion of the earth’s surface is no more necessary for an eruption, than 
the contraction of the sides of a boiling pot of water to make it boil 
over. The influence of the earth’s contraction is extremely gradual, 
being inappreciable except at long intervals: as is evident from many 
reasons already stated, it cannot explain the ordinary phases of a 
volcano from quiet to activity and the reverse. 
Farther, that the change from quiet to activity is generally a gra- 
dual progressive change, sometimes appearing paroxysmal when the 
action in its earlier stage is below the surface, and only in its last stage 
* For our previous deductions with regard to volcanic action, see pages 193 and 216. 
