REVIEW OF VOLCANIC ACTION. 359 
presupposes a comparatively small area to the vent; the cinders are 
thrown up, and fall directly around it or back again, and the summit 
crater has limits, therefore, which cannot be exceeded. 
Another consequence of the two conditions, is the following. 
In the case where the vent is large, and the molten rock very liquid, 
the overflow of lavas will be far more copious ; the streams will spread 
over wider areas, may flow of greater depths, and descend steeper 
slopes. Suppose an overflow to continue for several days in continu- 
ous action; such lavas will spread and run down a declivity even of 
thirty degrees in a continuous stream; for assuredly it could not 
choose to run or leave its traces in narrow lines.* If the flood is four 
hundred feet deep, (the depth of the lower pit of Kilauea,) and such a 
flood is no extravagant assumption, may it not form a bed of great 
thickness on a slope of ten degrees, especially if the flow be incessant 
or rapidly intermittent for some days? These ejections succeeding 
one another rapidly, may accumulate upon one another, even though 
the declivity be rapid, and thus produce a layer of great thickness, 
just as, on a smaller scale, steep cones accumulate in Kilauea. 
These are no extravagant assumptions. ‘There is the same evi- 
dence that the deeper streams have actually flowed, as the thinner; 
for both exist in the structure of these mountains under the same cir- 
cumstances. It is unreasonable to draw conclusions that are to be 
deemed general laws of igneous action from vents whose sluggish 
lavas scarcely have motion, except such as come up to the surface 
through some deep fissure opening from hotter depths. The condition 
of the lavas of Vesuvius affords us no data for judging of those of 
Kilauea. The former are so viscid that vapours cannot make their 
way through till they have accumulated into a vast bubble, and ac- 
quired force sufficient to carry the fragments of lava to a height of a 
thousand feet or more; while in the latter, the vapours pass freely, 
and thirty feet is the usual height: the action is constant and intense. 
A third consequence of the two conditions, is the frequency of cinder 
or fragmentary eruptions, of great violence, from the less fluid vents, 
* Streams. of cooled lava that descended a slope of fifty or sixty degrees, have been 
alluded to as occurring on the walls of Kilauea, and as being continuous for three or four 
hundred feet. They are narrow; but if the source had been more generous, it is difficult 
to see that they would not have had a greater breadth ; and by a succession of ejections 
upon each cooled layer, even a considerable thickness might have been attained, 
t See on this point the memoir by M. C. Prevost, Bulletin de la Soc. Geol. de France, 
xi. 194, 198. 
