999 DYNAMICAL GEOLOGY. [Boox I. 
thickness of the cone in proportion to the diameter of the funnel is” 
often greater, the lava very commonly rises into the crater. Should 
the crater walls be too weak to resist the pressure of the molten mass 
they give way, and the lava rushes out from the breach. This is 
seen to have happened in several of the puys of Auvergne, so well 
figured and described by Scrope (Fig. 41). But if the crater be 
massive enough to withstand the pressure, the lava, if still impelled 
upward by the struggling vapour, will at last flow out from the 
lowest part of the rim. 
As soon as the molten rock reaches the surface the superheated 
water or steam imprisoned within its mass escapes copiously, and 
hangs as a dense white cloud over the moving current. The 
lava streams of Vesuvius sometimes appear with as large and dense 
a steam cloud at their lower ends as that which escapes at the 
same time from the main crater. Even after the molten mass has 




























































































































































































Fic. 41.—View or ONE or THE Turr Cones Or AUVERGNE, BROKEN DOWN ON 
ONE SIDE BY THE Escape or a Struam or Lava. (AFTER SoROPE. 
flowed several miles, steam continues to rise abundantly both from 
its end and from numerous points along its surface, and continues to 
do so for many weeks, months, or it may be for several years. 
Should the point of escape of a lava stream lie well down on the 
cone, far below the summit of the lava-column in the funnel, the 
molten rock, on its first escape, driven by hydrostatic pressure, will 
sometimes spout up high into the air—a fountain of molten rock, 
This was observed in 1794 on Vesuvius, and in 1832 on Etna. In the 
eruption of 1852 at Mauna Loa, an unbroken fountain of lava, from 
200 to 700 feet in height and 1000 feet broad, burst out at the base 
of the cone. Similar “ geysers” of molten rock have subsequently 
been noticed in the same region. Thus, in March and April 1868, 
four fiery fountains, throwing lava to heights varying from 500 to 
1000 feet, continued to play for several weeks. According to Mr. Coan, 
such outbursts take place from the bottom of a column of lava 3000 
feet high. The volcano of Mauna Loa strikingly illustrates another 
feature of yoleanic dynamics in the position and outflow of lava. It 

