
- Pant I. Szor.i§2.] LAVA-STREAMS. 223 
bears upon its fianks at a distance of 20 miles, but 10,000 feet lower, 
_ the huge crater Kilauea. As Dana has pointed out, these orifices 
form part of one mountain, yet the column of lava stands 10,000 
feet higher in one conduit than in the other, On a far smaller scale 
the same independence occurs among the several pipes of some of the 
geysers in the Yellowstone region of North America. 
From the wide extent of basalt-dykes, such as those of Britain 
which rise to the surface at a distance of 200 miles from the main 
voleanic outbursts of their time, and cover an area of perhaps 100,000 
square miles, it is evident that molten lava may sometimes occupy a 
far greater area within the crust than might be inferred from the 
dimensions and outpourings even of the largest voleanic cone. There 
ean be no doubt that vast reservoirs of melted rock impreguated with 
superheated vapours must formerly have existed, if they do not exist 
still beneath extensive tracts of country (p. 256). Yet even in these 
more stupendous manifestations of volcanism the lava should be 
regarded rather as the sign than as the cause of volcanic action. It 
is the pressure of the imprisoned vapour and its struggles to get 
free which produce the subterranean earthquakes, explosions, and out- 
pouring of lava. As soon as the vapour finds relief, the terrestrial 
commotion calms down again, until another accumulation of vapour 
demands a repetition of the same phenomena. | 
Rate of flow of Lava.—the rate of movement is regulated by 
the fluidity of the lava, by its volume, and by the form and inclina- 
tion of the ground. Hence, as a rule, a lava-stream moves faster at 
_ first than afterwards, because it has not had time to stiffen, and its 
slope of descent is usually steeper than further down the mountain. 
One of the most fluid and swiftly flowing lava-streams ever observed 
on Vesuvius was that erupted on 12th August, 1805. It is said to 
have rushed down a space of 3 Italian (82 English) miles in the first 
four minutes, but to have widened out and moved more slowly as it 
descended, yet finally to have reached Torre del Greco in three 
hours. A lava erupted by Mauna Loa in 1852 went as fast as an 
ordinary stage-coach, or fifteen miles in two hours. Long after a 
current has been deeply crusted over with slags and rough slabs of 
lava, it continues to creep slowly forward for weeks or even months. 
It happens sometimes that, as the lava moves along, the still 
molten mass inside bursts through the outer hardened and deeply 
seamed crust, and rushes out with, at first, a motion much more 
rapid than that of the main stream. Any sudden change in the 
form or slope of the ground affects the flow of the lava. Thus, 
reaching the edge of a steep defile or cliff, the molten rock pours 
over in a cataract of glowing molten rock, with clouds of steam, 
showers of fragments, and a noise utterly indescribable. Or on the 
other hand, encountering a ridge or hill across its path, it accumu- 
lates until it either finds egress round the side or actually overrides 
and entombs the obstacle. The hardened crust or shell within which 
_ the still fluid laya moves serves to keep the mass from spreading. 
